the R-390A Receivers
The Main Frame - The RF Module - The IF Module
The AF Module - The Power Supply Module - The PTO
Front Panel Restoration - Alignment - Performance Today
on Variants and Accessories,
|The R-390A receivers have always been considered the paragon of electro-mechanical complexity. However, a methodical approach to the restoration of these ultimate performers will ease an otherwise difficult and time-consuming project. This web-article will go through the steps necessary for complete disassembly of the receiver to the individual module level. The thorough inspection and possible problems to look for in the Main Frame and each of the modules will be covered with individual sections. Additionally, photographs of typical problems I've found are included in each module section to illustrate what to be on the lookout for. With careful attention to detail and a thorough approach to the rebuild and alignment the technician-enthusiast will be rewarded with being able to operate one of the best performing vacuum tube receivers ever designed. - Henry Rogers - Feb. 8, 2012|
A Brief History of the Design
R-390/URR - Arguably, the R-390/URR and its later kin, the R-390A/URR, are the ultimate tube-type receivers. The first version of this incredible receiver was the R-390 featuring 33 tubes (includes the 3TF7 ballast tube,) double or triple conversion, two RF stages, six IF stages, modular construction, three audio filter settings, six selectivity bandwidths and frequency coverage from 500 kc. to 32.0 mc. in 32 - one megacycle wide - bands. It is a high performance receiver that really "shows its stuff" when conditions are poor but will also provide fairly nice audio quality when receiving conditions allow for it. The most common complaint is the cumbersome tuning that, while "parked" on one frequency is not apparent, shows up when spanning an entire band or changing ranges. Most of the "stiff tuning" complaints can be traced to an over accumulation of grease and dirt in the gear train. When clean and properly (lightly oiled) lubed, the tuning is very light and easy to manipulate. Only Collins or Motorola built the R-390 contracts which ran from 1951 through 1953. The military complained that the R-390 was very difficult to maintain and too expensive. Some of the maintenance issues involve the R-390's elaborate electronically regulated B+ circuit that uses two 6082 tubes along with two 5651 voltage reference tubes and a 6BH6 DC Voltage Amplifier tube. This circuit runs quite hot and accounts for many of the problems that develop in the audio module (where the regulator circuit is located.) Additionally, the R-390's gear train has a moveable "locking gear" that must be installed prior to removing the RF module (if you want to keep everything synchronized.) This gear was painted green and usually mounted with a screw on the front of the gear box. Each time the RF Module is removed and then replaced on an R-390, the KC and MC drive shaft split gears have to be reset for backlash, the Crystal Oscillator module's bandswitch has to be synchronized and the oldham coupler installed. Removal of any of the crystals in the Crystal Oscillator module requires removal of the hard-wired crystal oven. When the military complained about complex maintenance issues, they weren't exaggerating.
- Collins designed a replacement receiver that was introduced in 1954
with the designation of R-390A/URR. Though the new receiver looked very similar externally to
the R-390, inside numerous changes were made to improve
cost-to-performance and ease of maintenance. The new receiver's gear box
was removable as a unit and synchronization would be maintained, the
crystal oven just plugged into the Crystal Oscillator module (it is
secured by screws though,) the B+ voltage regulator circuit became a
standard 0A2 tube, the crystal calibrator was combined into the RF
module (eliminating the separate Crystal Calibrator module of the R-390)
and the Crystal Oscillator module was mounted to the RF module so
removal of the entire RF deck kept everything synchronized together
except the PTO. All of the maintenance "quirks" of the R-390 were
corrected in the R-390A which made the receiver easy for the military to
maintain. The major performance change involved the
installation of four mechanical filters in the IF section of the
receiver. The steep slopes of the mechanical filters gave the R-390A
excellent selectivity on 16KC, 8KC (really about 11KC,) 4KC and 2KC
bandwidths. The 1KC and .1KC bandwidths are crystal filter derived from
the 2KC wide setting.
The R-390A uses 26 tubes (including the 3TF7 ballast tube) with one RF stage, four IF stages, mechanical filters on four of the six selectivity positions, plus an 800Hz audio filter. When properly set-up, an operator can dig right through the QRM while maintaining fantastic sensitivity making the R-390A one of the finest tube-type receivers ever built. The R-390A was produced in yearly contracts from 1954 up through 1967 (and a very small contract in 1984) with many different contractors building the receivers during those years. Though the R-390A's six modules and redesigned maintenance approach made field repairs much easier, it was still a complex receiver that required experienced technicians to maintain. Though the military wanted a less expensive receiver, it certainly wasn't that either.
The R-390 and R-390A receivers have provided reliable communications under adverse conditions for years and even though the design and nearly all of the receivers are over 50 years old, they are still one of the best tube-type receivers around. Many R-390 and R-390A receivers are still being used today, some in professional applications, but also for serious SWLing and, of course, in vintage ham stations around the world. However, many of the R-390 and R-390A receiver in operation today are being used with all original parts and have not been serviced with the attention necessary for a receiver that is half of a century old.
Although most R-390As will probably operate pretty well on all original parts, many early versions will have some paper capacitors exhibiting leakage current. These "leaky" capacitors can alter original bias voltages, cause excessive current flow in some resistors and inhibit the great performance that the R-390A is capable of. Replacement of the molded capacitors and the "Vitamin-Q" style capacitors is recommended for top performance and reliability. >>>
| >>> The only
module in which the replacement of these capacitors is a challenge is
the IF module. This is due to the compactness of the unit and resulting
component density. The AF module and the RF module also have capacitors
that should be replaced but these are very easy to access and replace.
The Power Supply module and the PTO don't have any paper capacitors in
their circuits. Late versions of the R-390A
have ceramic disk and film capacitors that generally are trouble-free.
The AF module on all versions will have two multi-section electrolytic capacitors. On all versions of the R-390A, these two multi-section caps are fifty or more years old. They may reform but don't be surprised if one section doesn't. For best performance you need a stable power supply, so these two multi-sections should be rebuilt with new electrolytic capacitors.
After the rework, a thorough
check-out to find all of the component related operational problems and
a complete examination of all of the vacuum tubes that includes
replacing any tubes that test less than "almost like new" should be
performed. The receiver will then need a full IF/RF alignment.
Afterwards, the R-390A will be functioning at or better than its
specified performance level.
The R-390 and R-390A receivers have earned a reputation of "the best vacuum-tube receivers ever built" but to achieve this level of performance you will need to "dig into" your receiver. It's a time-consuming project but it really is necessary if the legendary R-390A performance is to be attained.
Assessing Your R-390A - What Really Needs to be Done
|How Far to Go with Your
Rebuild - There are at least two types of rebuilding
approaches that can be taken for this
question. First, there are the users that believe you should operate the
receiver with all original parts. This certainly comes for the fact the
most "well-cared-for" R-390As will function pretty well with all of the
original capacitors. After all, cost was no object when the receiver was
built and the best capacitors available at the time were used in its
construction. The second group of rebuilders believe that even though the capacitors
were the best available, they are now over half-a-century old and
must be exhibiting some leakage current since most of the early
receivers used paper
dielectric capacitors. These rebuilders point to the brown-body,
molded capacitors (likely Sprague manufacture) used in the early RF modules and the IF modules as examples to
be wary off. These molded capacitors are similar to "Black Beauty" types
but seem to be higher quality.
RF modules and IF modules built from the early-1960s and later will probably not have the molded capacitors. These modules have capacitors that appear to be film-type caps are installed instead. The latest versions built in the mid-to-late-1960s will have ceramic disk and film capacitors that generally don't need to be replaced. However, on the receivers built in the 1950s, nearly all of the capacitors are paper-dielectric. The more diligent rebuilders also point out that even the Sprague "Vitamin-Q" capacitors are also paper dielectric types. Sprague did produce the best capacitors at the time, however all contracts didn't use the same manufacturer for capacitors (probably due to cost) so later modules will have similar style capacitors built by Sangamo or General Instruments. It doesn't really matter because all of that style capacitor will have paper as the dielectric material which is "the problem." Today's capacitors that are used for rebuilding are "self-healing," metalized-film types that use a plastic dielectric material such as polystyrene or polypropylene. These new capacitors will function with no problems for many decades.
When considering the type of rebuild, of course, the condition of the R-390A itself has to be taken into account. A poorly stored 1955 contact Collins version is certainly going to have more problems than a never issued 1967 contract EAC version. However, most of the R-390A receivers we run into are from the former category - poorly stored receivers that have many obvious and also many latent problems. I have to admit that for many years I believed that any R-390A receiver could be operated with all original capacitors and performance was at the receiver's design level. The acquisition of two poorly stored examples, a 1961 Capehart and a 1956 Motorola, and my subsequent rebuilds of those receivers changed my mind. I rebuilt the 1961 Capehart and didn't replace the paper capacitors. It worked and seemed at first listening (after alignment) to be a pretty good performer. The 1956 Motorola was carefully and thoroughly rebuilt including replacement of all the paper capacitors. With the Motorola I was able to reduce the IF GAIN down to 50% while maintaining sensitivity that allowed copy of SSB stations on 40M with the RF GAIN at 5. Its performance is "light-years" ahead of the Capehart. The dramatic difference between the two rebuilds leaves me with no doubt - if the receiver has seen a lot of use and was poorly stored for years, the best performance can only be achieved with the replacement of all the paper capacitors along with a thorough and meticulous rebuild followed by a careful receiver alignment performed with quality laboratory type test equipment.
One note on the multi-section electrolytic capacitors that are found in the AF module. As these units age it becomes more and more unlikely that they will successfully reform. I recently (2016) tested all of my spare R-390A multi-sections and found that ALL of them had problems and were not useable. These originals are all over fifty years old and the failure is that one section out of the two or three sections will be defective while the remaining capacitors seem to reform. At any rate, the units are not usable and have to be rebuilt. I now recommend that the two multi-section capacitors be rebuilt with new electrolytics as part of any R-390A rebuild. There is a section further down this page that describes the electrolytic capacitor rebuilding process.
|Experience and Rework Ability - The R-390A is a complex receiver that will require a fairly high level of experience to successfully finish a complete tear-down, rebuild, reassembly and alignment. Though the receivers used the best parts available and were built by some of the best contractor companies over the years, they aren't impervious to damage from poor storage, abuse or incompetent (or indifferent) technicians. If you choose a candidate that is in pretty good shape, you won't run into too many problems and a thorough inspection of all of the modules will probably find all of the operational issues. To complete a project that involves a "storage challenged" receiver you should be fully experienced in complete disassembly and reassembly of electronic equipment. You should be fully experienced in troubleshooting, repair and alignment of complex receivers. You should be able to keep track of multiple assemblies and parts, along with the project's progress over a fairly long time period. You should have first-class soldering equipment, use real SnPb solder and possess good technician skills and habits. You should have laboratory quality test equipment for troubleshooting and final alignment. Most of the people that rebuild and recondition R-390 and R-390A receivers are experienced from the military or from professional commercial electronics work. If you are a reasonably experienced technician and are thorough, careful, methodical and take your time, then you can easily take on the reconditioning or restoration of one of these great performing receivers.||Disassembly will be Required - The only way to inspect everything is to disassemble the receiver down to the main frame. This means you will have to remove the Power Supply module, the Audio Module and the PTO from the bottom of the Main Frame. The IF module and the RF module have to be removed from the top of the Main Frame. Before removing the RF module you must remove the front panel. When inspecting and cleaning the individual modules more disassembly will be necessary. On the RF module, each of the 18 plug-in RF coil assemblies need to be removed to check the condition of the pins and pin sockets. Corrosion is common in these areas in receivers that were stored in humid areas. There are also six plug-in coil assemblies in the Variable IF conversion section that also need to be removed for inspection. The more thorough and meticulous your inspection is, the more actual and potential problems will be found.|
|Thorough Cleaning Required
- This is not only cleaning the obvious dirt and grime but elimination
of the corrosion that turns up in R-390A receivers that have been stored
in areas that are exposed to the temperature fluctuations and humidity
of garages or sheds in some areas of the country. Corrosion in the many
plug-in sockets in the receiver (not only the 26 tube sockets but the 24
plug-in coil sockets and the 15 crystal sockets not to mention all of
the interconnecting cables'
cause anything from non-functionability to erratic changes in
sensitivity or signal level. De-Oxit applied with various kinds of tools is just
about the only way to start the corrosion removal process. Usually, some rubbing
of the De-Oxit with Q-tips, wire inserts, small brushes and even
plugging tubes in and out of the sockets will be required to remove the
corrosion. Be careful not to remove the gold plating that is on the pins
and sockets by using wire brushes or other brutal methods. This was the corrosion protection but gold plating is thin
and somewhat porous and over time corrosion will appear in humid
environments. Sometimes corrosion removal will require the use of wire
brushes but only as a last resort. Most of the underneath of the various module chassis are
in good condition since they are well protected but a thorough
inspection is still required of every part of the circuitry, especially
in receivers that show obvious corrosion problems above the chassis.
Remove all Modules for Access to the Main Frame - The R-390A must be stripped down to the Main Frame. This will require removal of all of the modules. The modules are mounted using "captive screws" that have the screw heads painted green. There are also various interconnecting cables that need to be disconnected. The small coax cables with BNC Jr. connectors are all marked with metal tags for identification. When removing the RF module you will find that the front panel has to be dismounted. Also, there are four captive screws and seven other screws that mount this module that are not captive screws but they will (or should) have the screw heads painted green. Be sure when removing the PTO to set the receiver to XX.000 on the Veeder-Root counter (MC doesn't matter but KC does.) After removal don't move the PTO shaft unless you have marked a reference line on the shaft. Otherwise, you'll have to synchronize the PTO to the RF module when reassembling. This isn't difficult and if you're going to be working on the PTO, it doesn't matter because you'll have to synchronize it anyway. Once all of the modules are removed you'll have complete access to the Main Frame to begin the rebuilding process. Removing all modules except the RF module can be accomplished without dismounting the front panel.
Front Panel Dismount and/or Removal - If you don't need to do any cosmetic work to the front panel and the Main Frame of the R-390A is also in good condition but you want to work on the RF Module, you'll have to dismount the front panel. Remove the Kilocycle and Megacycle knobs and the Ant Trim knob. Remove the Dial Lock knob and loosen the mounting nut so you can turn the clamp off the the locking disk. If you haven't removed the IF module, then loosen the shaft clamps on the BFO and BANDWIDTH controls and pull the knobs and shafts forward. Remove the eight 10-32 FH screws that mount the panel to the side panels. Remove the five 6-32 FH screws on the front panel that have conical lock washers underneath. Remove the mounting nuts for the two large shaft bushings for the MC and KC tunings and push the rear bushing back so it isn't though the front panel. Do the same on the ANT TRIM shaft bushing. You should now be able to pull the front panel forward off of the shafts and lower it face-down in front of the Main Frame. If the front panel can't be pulled forward enough to clear the shafts, loosen the green head screw behind the front panel that holds the clamp for the main cable harness. This should allow you to pull the front panel further forward to clear the shafts. You can now remove the six remaining green head screws and loosen the four green head captive screws on the RF Module. Disconnect all of the coax cables and the PTO cable from the RF Module. Lift the RF Module out of the Main Frame.
Front panel removal will require the steps mentioned in the paragraph above plus removing all of the remaining knobs and dismounting all of the controls. The phone jack will also have to be dismounted. The PC board above the dial cover has to be dismounted. You might have to remove the data plate since the screws for the PC board are behind the data plate. Remove the 6-32 FH screws that mount the clamps for the wiring harnesses. You'll have to unsolder the leads to the Carrier Level meter and the Line Level meter. Now the front panel can be entirely separated from the Main Frame. This operation would be required if serious cosmetic work was necessary on the front. If that's the case, you'll also have to dismount the two meters, the dial cover and the two large handles from the front panel.
Restoring the Main Frame
|Though this really doesn't seem like it would need much
attention several important functions are located on the Main Frame.
First, all of the chassis ground connections are through the main frame.
Good, clean connections are necessary for all of the modules to work
together. Unless the Main Frame is in really good condition, I recommend that the Main Frame be totally disassembled for
cleaning of all of the panel to bed plate contact areas. It's surprising
how much dirt and corrosion can get into these areas. Check for the
proper screw lengths during disassembly. It's common to find different
length screws installed for the side panel to bed plate mountings due to
indifferent reassembly in the past. All bed plate screws should be 1/2"
long and mounted with lock washers. The smaller panels on the underside
are mounted with screws that are the correct length to not protrude
through the pem-nuts on the topside. Be careful on reassembly to use the
correct screws and washers. Look at the side panels and make sure no damage has
occurred to the rear part. It's common to find this area of the side
panels bent from placing the receiver "face up" on its back. While the
side panels are off is a good time to straighten any mechanical
problems. The back panel will need a lot of attention because of the
terminal strips, the AC power cord input cover, fuse holders and other
parts mounted there. Check all of the connections to the rear of the
terminal strips for broken wires or other problems. Reassemble the Main
Frame using the proper length screws with lock washers. Don't
over-torque the screws but make sure they are fully compressing the lock
washers. Be sure to install all of the screws required - they're
there for a reason.
photo right: This shows a completed Main Frame for the 1956 Motorola contract. Note the orange twisted wire in the photo. These wire ties are installed just to keep the harness from "flopping around" until the front panel is re-installed. For minor cleanup of the Main Frame the front panel doesn't have to be removed, it can be dismounted and will lay down flat with the harness and controls mounted.
|Checking the Antenna Relays
- Some users of the R-390A aren't aware that there are two antenna
relays that disconnect the Antenna Input from the receiver (and connect
the Antenna Input to chassis) while in Stand-By and when Break-in is in
operation. The Antenna Relay assembly is mounted to the rear panel on
the inside. There is a gusset that adds strength to the mounting and
protects the BNC connectors and the relay coils on the inside. The relay
coils operate on approximately +20vdc. Even though the Antenna Relay
Assembly is well protected, they are silver and gold plated and seem to
be subject to a lot of oxidation. Normally, silver oxide is quite
conductive and not an electrical problem but occasionally enough
moisture gets into the relay arm area and causes some sort of
non-conductive oxidation. You should check the DC resistance from the
Balanced Input to the two BNC outputs and from the Unbalanced Input to its single BNC output. Without the
relays energized, you should have no resistance (zero ohms.) When the relays are
energized, these points will show an open circuit. Relays energized
should also now
show the Antenna Balanced and Unbalanced Inputs connected to chassis. If you show some
resistance then oxidation has formed inside the relay arm area. Remove
the side covers to expose the relay arm on each side of the assembly.
These screws have green Lok-tite applied during assembly so you'll have
to use a soldering iron to heat the screw to weaken the Lok-tite and then loosen the screws.
See the photo to the left. It shows a relay arm that reads 50 ohms of DC resistance when in NC. You can see that the contact on the arm is black and the brass (gold-plated over brass) barrel of the BNC is also discolored somewhat. I cleaned the contact area with DeOxit applied with a saturated paper pulled through the contacts. This wasn't enough. I had to clean the oxidation using 400 grit aluminum oxide paper. I then used a small paint brush to further clean the area with DeOxit. After this treatment the contact resistance was zero ohms for both relay states. Although the Twin-ax connections did measure zero ohms in both relay states, I gave those contacts the same treatment as a problem preventative measure.
Restoring the RF Module
|Certainly the most complex and difficult to work on
module is the RF module. All of the gear drive is located here along with
the slug racks, the RF coil assemblies, the Crystal Oscillator and the
Calibration Oscillator. However, be patient. It's unlikely you can
completely finish the RF deck in one day. It will take several days
since there are always so many problems lurking about in this module.
There are several subsections to follow due to the numerous functions that this
Cleaning the Gear Box - If the RF Deck is in pretty good shape and the mechanism does move without binding then usually just a good flush will clean up all of the years of spray on grease and other abuses that were typically applied to this robust gear train. Use a full can of WD-40 along with a long handled paint brush with a 1/2" wide brush and an acid brush that the handle is slightly bent. I perform the flush outside since an entire can of WD/40 is going to be flushed through the gearbox. You probably will want to put something like newspaper down so clean-up is easier afterward. I place the RF module on a 3/4" piece of plywood board to protect the underneath and make the unit easy to carry around. This also allows the WD/40 to just flush through the gearbox on its way to the ground (newspapers.) Spray and brush - that's all you need to do. Work the WD/40 with the long handle brush through the gear teeth and flush. Don't worry about the Veeder-Root counter, it's impervious to WD/40. After the entire can has been flushed through the gear box, it will look like the photo to the right. Let it set outside for an hour or so to drain the residual WD/40. Bring it inside to the workbench and using cotton cloth or paper towels remove as much of the remaining WD/40 as you can. You will also have to dry the gearbox front and back plates and remove any stubborn grease during this detailing.
photo above: The gear box after a "flush clean" as described. This is the RF module from a 1956 Motorola contract receiver. Note the Motorola ID in the upper right corner of the front gear plate.
photo above: One of the variable IF slug racks showing that one of the slugs has detached from the adjuster screw. To reattach it is necessary to heat the screw socket up with a soldering iron and when the solder is molten, reinsert the spring-shaft into the socket. When the solder sets-up, the slug will be firmly held in place.
|When finished the gearbox will operate very easily and smoothly.
Now, don't apply a lot of grease (or anything like the original green
grease the military used) to the gear box. It
should only be lightly oiled where the oil-lite bearings are. These are
oil-impregnated bronze bearings but a little light oil won't hurt them.
The anti-backlash gears don't need anything on them except light weight
machine oil. Any grease or heavy oil will only
act as a "dirt magnet" and attract the stuff that ends up hardening into
what you have just cleaned off. Some of the straight-tooth gears can be
lightly greased using modern, non-hardening grease. The surface edges of
the cams can have a very light coating of the same modern grease. The conical gears on
the Vedeer-Root counter can be also greased to reduce noise. The caveat
is use the grease sparingly and use only modern, non-hardening grease.
The lubrication guide in the military manuals should be used as a guide
for application but use the grease and oil sparingly.
If you have serious problems with the gear box that is going to require disassembly to clear up, the go to www.r-390a.com and download the PDF file there on rebuilding the gear box. This PDF is very detailed and includes large photos of each reassembly step. It is the best guide available for gear box rework.
Removal of the Slug Racks - The slug racks are driven up and down via various cams that are part of the gear box. A long spring that is attached to holes in the chassis on one end and to the slug rack on the other end provide a downward pull that the cams work against. Each of the slug racks have three slugs that enter and retract from the RF and Variable IF coil barrels. To remove the slug racks merely detach the springs and the slug rack will lift off easily. Be careful as the slugs are somewhat prone to damage when handled roughly. Tag each slug rack so you know where it should be returned to. The slug racks each have two roller bearings that ride on the lifter cams. These roller bearings are almost always gummed up to the point where they won't turn. That means that the bearing was "skidding" along the cam surface instead of rolling like it should. Usually pressured oil such as WD-40 from the spray tube directed into the bearing end will clear the gunk out. Then oil the bearing with a good grade of light machine oil. The slug racks will probably need cleaning and carefully wipe off the slugs so they are clean.
IMPORTANT NOTE : Check all of the slugs to be sure they are the correct type. They should have a spline socket drive screw head (R-390 used slotted screw drive) and their color should be brownish-gray. Sometimes you'll run into R-390 slugs that have been installed as replacements however the R-390 slugs have a different permeability and shouldn't be used (as mentioned, R-390 slugs have a slotted drive adjuster screw.) Also, be careful to not interchange the Variable IF slugs with the RF slugs. Variable IF slugs have a slightly greenish tint. All RF slugs have the same permeability and are colored brownish-gray and all Variable IF slugs have the same permeability and are colored greenish-gray. The RF slugs and the Variable IF slugs are different from each other and do not the same permeability.
|Slug Rack Spring Removal and Installation Tool - After years of messing around using long needle nose pliers to remove and install the Slug Rack springs I decided there has to be a better, faster way to do it. A very simple tool can be made from 16 gauge wire. I just used about six inches of wire with a very small open hook on one end and a loop on the other end for a handle. The loop can have shrink tubing installed to look more "professional." You'll have to experiment with the hook. It should be fairly open and be sure to use a jeweler's file to round the end so it will slide out from under the spring hook easily. To use the tool to remove a spring just slip the hook under the spring end and lift the spring end out of the hole. When installing a spring use the hook to lift and guide the spring end into the proper hole on the slug rack. With this tool I can remove the springs very quickly but re-installation is where this tool really shows its stuff. Installing the slug rack springs is now fast and easy. Photo to the right shows the tool next to a scale for size reference.|
photo above: This is showing the sockets for the plug-in RF coils. It's obvious that a lot of dirt has made its way under the coils housings in this RF Module. Some of the gold plated sockets have corrosion beginning. Each socket has to be cleaned as described above and then inspected.
|Removal of the RF Coil Assemblies - Each RF coil on
the RF module plugs into small chassis mounted sockets. The coil
assemblies are held in place with a small screw that is accessed down
the coil barrel. When this screw is loosened, the coil can be unplugged
easily by pulling it straight up. Keep track of the coils because you
will have to remove all 24 of them to have access to all of the sockets
for cleaning. The chassis is marked as to which coils go where but keep
track of where they reinstall anyway.
The Coil Assemblies themselves will need to be inspected for corrosion, problems with the trimmer capacitor and broken wires. The coil shield is removed by pressing the two tabs in and then pulling the shield off. Once the coil is out of the shield do your inspection and cleaning and then replace the shield. I clean the coil pins with De-Oxit and a Q-tip. I also clean the sockets using De-Oxit applied into the socket with a bare wire that fits into the sockets. This puts the De-Oxit right where it needs to be where a spray would be messy and waste a lot of De-Oxit.
When inspecting the Ant/RF amp coil assemblies, T-102, T-202, T-203, etc., be sure to check the primary windings for evidence of burning due to excessive RF power being applied to the receiver antenna input. This will generally be found on the 2-4mc, 4-8mc, 8-16mc and 16-32mc coils since these are the coils that are used for ham bands coverage. The coil primary wire should appear white or beige color. If you see the wire is black or has bubbles in the coating then excessive RF was applied. This will require finding another RF coil assembly for that particular function as the coil can't be repaired. Even though the R-390A's "BREAK IN" function disconnects the antenna input from the receiver, it is possible that a defective external antenna relay might allow RF into the antenna while not allowing "BREAK IN" to function.
|Carefully check the trimmer capacitor on each RF coil assembly.
On early style RF coil assemblies, it's not
find some of these that have deformed rubber gaskets. Though the problem
appears to be with the gasket, the actual problem is that the rotor and
stator of the trimmer capacitor are stuck together. When the trimmer is
adjusted, both rotor and stator turn and if the gasket isn't also stuck
it will be forced out by the stator contact fingers. Examine the trimmer
capacitor closely and note if both ceramic pieces move together - they shouldn't. Only the rotor should move (the
top ceramic piece.) The stator should be held in place by friction from
the gasket and the tension provided by the rotor contact clip (later
versions are glued in place.) If both
ceramic pieces move together then the rotor and stator are stuck
together and the trimmer must be disassembled to repair it. The
procedure is below in "Repairing RF Coil Trimmer Capacitors." It is also
possible to just replace the entire RF coil assembly with a known
operational one. If you reinstall the RF coil assembly with the defective
trimmer capacitor still in place, you'll find that when trying to align
the RF section of the receiver, you won't be able to adjust the high
frequency end of the particular coil because the capacitance doesn't
The "stuck rotor-stator" problem shows up only on the early version RF
coil assemblies. By the sixties, the mounting technique changed and the
stator and gasket are glued together and then the gasket is glued to the
fiber board. Also, the gasket is made out of a different type of
material. This mounting technique practically eliminated the problem
since the stator and gasket can't move.
When you have the RF coil assembly apart, if the trimmer looks okay, then place a small drop of De-Oxit on the bottom connection of the trimmer rotor as this does tend to oxidize more than other parts. The photos below show close-ups of the typical R-390A RF coil assembly.
|Photo 1 - This shows the
complete RF coil assembly after it's removed from the RF module. Looking
at the underside. Note the "dish shaped" dimple for the mounting screw.
Photo 2 - Close-up of the underside of the assembly showing the contact pins.
Photo 3 - A shot inside showing the mica capacitors and the coil. This coil is an 18mc to 32mc coil. The ceramic trimmer capacitor is on top of the fiber board.
Photo 4 - Another shot inside showing the contact of the trimmer capacitor rotor and how it connects to the coil and fixed capacitors. This contact should be given a drop of De-Oxit since it always seems to look oxidized. The longer tab to the left is the trimmer capacitor stator connection.
|Repairing Early Style RF Coil Trimmer
Capacitors - When working on early RF decks you are
sure to be working on early style RF coil assemblies. When you remove the RF Coil assembly from
its housing you might find that the trimmer capacitor looks like the one
shown in the photo to the right, photo 1. This is a sure indication that the
rotor and stator of the trimmer are stuck together. When the trimmer is
adjusted, since the stator also turns that action usually forces the
gasket out due to the gasket opening pushing against the two stator contact fingers. Closely observe the
side of the trimmer capacitor while it is being rotated and you'll note that both ceramic
pieces rotate together indicating that they are stuck together.
Disassembly is necessary to un-stick the rotor and stator, clean the
pieces and then reassemble. First, note the underside connections to the
trimmer capacitor and that the rotor is secured in place with a spring clip
- see photo 2. Using
small needle-nose pliers working against the side rail of the RF coil assembly,
gently move the spring clip to disengage the rotor pin. Looking at photo 2 the
spring clip would be moved to the right to disengage the rotor pin.
Important Note: Be very
careful and note whether the spring clip is connected to the coil return
(it will be on some RF coils.) If it is, unsolder the coil return wire
off of the spring clip connection before trying to disengage it. Otherwise you might
break the wire from the coil as you move the rotor clip.
Now the ceramic pieces and the gasket should easily be removable, in fact, they'll probably fall off of the fiber board. If they aren't easily removed you might have a later version coil assembly that has the gasket and stator glued to the fiber board. DO NOT try to force the stator and gasket off of the fiber board or you'll surely break the delicate ceramic stator. If it is the later style trimmer then there should be no problem with stator contact or with the gasket and most of the issues would be with rotor contact and dirt between the rotor and stator causing them to stick.
Now on early style trimmers, note that there are three pieces, the rotor, the stator and the gasket. Also note, that there are two contact fingers that protrude thru the fiber board and also form a connection terminal below the fiber board. These are the stator contact fingers. The next step is to un-stick the rotor and stator. Usually an Exact-o knife can be use to separate the rotor and stator. Usually, the "gunk" that is sticking the two pieces together isn't very strong and separating the two pieces is easy. Now clean both ceramic pieces with denatured alcohol. You can also use a pencil eraser for stubborn dirt. Note how the metal parts are embedded in the ceramic. The stator's metal piece faces down on the gasket and the metal piece makes contact with the stator contact fingers. Then the rotor is mounted with the metal plate down. Note that the trimmer capacitor is using the ceramic spacing as the dielectric and that rotating the top piece (rotor) actually varies the capacitance if the stator stays in a fixed position. With the pieces clean first place the gasket as shown in photo 3. Then place the stator in position as shown in the photo 4 making sure that the stator contact pins are touching the stator plate. Now place the rotor in position and hold in place with your fingers until you can re-install the rotor contact clip using the small needle-nose pliers. Test the trimmer by rotating it. The stator should stay in position while the rotor is moved. You might have to use a very tiny amount of super glue to hold the gasket to the stator piece to keep it in the proper position and a small amount of super glue to hold the gasket in place on the fiber board. This is how later RF coil trimmer capacitors are mounted and this all but eliminated the problem. However, you can mount it "as original" and just the gasket friction against the fiber board should be enough to keep the stator from rotating. Reassemble the RF coil, install in the receiver and test. Realignment will, of course, be necessary after this procedure but you really couldn't proceed with any alignment until this type of problem was repaired anyway.
Photo 2 - The rotor clip secures the assembly
Photo 3 - Gasket and the stator contact pins
Photo 4 - Stator placement over gasket
|Other Common Problems - Check the band
switch for corrosion and that it is correctly synchronized. Clean
with De-Oxit applied with a small paint brush. Inspect and clean with
De-Oxit the two
Crystal Oscillator switches and all 15 crystal sockets and crystal pins.
Check the mechanical alignment of the RF and Variable IF slugs as they enter and retract from their respective coils. All slugs should enter directly into the barrel and not be entering the barrel misaligned where the slug rubs against the inside of the barrel excessively. The two screws that hold the adjusting screw assembly to the slug rack can be loosened and then you can move the slug and the adjusting screw assembly to mechanically align the entry of the slug into the coil barrel. Then retighten the screws. If the slugs to coil barrels alignment is correct, the slug rack will drop down with its own weight (without return springs installed) as the MC or KC tuning is adjusted.
If you want to replace the three paper capacitors, on early decks they are the brown body molded caps with the color code banding. These are pretty good quality for molded paper caps but some rebuilders do find the molded body is cracked on these capacitors (I haven't.) It's probably a good idea to replace them anyway. Some RF modules built from the 1960s-on will use later style, non-molded capacitors. These appear to be film capacitors that shouldn't require replacement. Also, very late modules will have ceramic disks that won't need replacing.
Use 400 volt 716P type SBE Orange Drops as replacement caps. There is another paper cap (.047uf at 100wvdc) that is a metal body, oil-filled, chassis mount unit mounted behind the RF amplifier tube. This capacitor normally isn't replaced since it's an oil-filled type and looks like it was specifically made for its application.
Clean the tube sockets using De-Oxit. A short spray followed by actually applying some De-Oxit with a bare wire to each tube pin. Then a short spray on a tube's pins and then plug the tube in and out of the socket several times. This usually will clean the tube sockets pretty well. I've also used a small but stiff paint brush to "work" De-Oxit into the tube socket. This results is a lot less De-Oxit going into the socket pins and better cleaning if there isn't too much oxidation or dirt.
Be sure to repaint the screw heads green on the captive screws and also repaint the heads of the seven other screws that secure the RF Module. Be sure to use #6 flat washers on these seven screws. The holes for these seven screws are over-size making the flat washer necessary. The captive screws should already have split-ring lock washers installed but check to make sure.
Checking Cam Synchronization - Set the Veeder-Root counter to 07+000. This is actually 8.000mc on the 7.0mc band. You want the "+" indicator to have just moved into place. Now look at the slug rack lifter cams on the front plate. You'll note that there are indicator lines marked by each cam. At this setting of the counter, each of the cam "points" should be pointing exacting to the indicator line. This shows that all of the lifter cams are synchronized with the Veeder-Root counter and the gear train. If you find one or two cams that are not pointing to the indicator line, check your setting of the Veeder-Root counter to make sure you are at 07+000. If correct, then perhaps one of the shaft clamps has loosened. To set the cam, loosed the shaft clamp and move the cam so that it exactly points to the indicator line, then tighten the clamp. Be careful! Don't over-tighten or you'll break the clamp. If you break the clamp it's a major disassembly of the gearbox to replace. This applies to almost any of the clamps but not all. Just be careful and don't over-tighten the clamps. I've only run into a broken cam shaft clamp once. To replace the clamp required 27 steps into the gearbox disassembly procedure. Once the clamp is reasonably tightened, adjust the MC knob and check for rotation of the cam. Place your finger with a slight pressure exerted on the specific slug rack to make sure the cam clamp doesn't slip. Tighten slightly if it does. Perform this procedure to any cam that doesn't point exactly at the indicator line when the counter is at 07+000.
Oscillator Unit - Mounted to the left side of the RF
Module is the Crystal Oscillator Unit. There are some things that need
to be checked in this module and in its mechanical connection to the RF
module. There are two large 31 position switches inside the module that
control crystal selection and oscillator trimmer selection. Under the
metal cover are 17 crystals that are used in both the double or triple
conversion scheme (triple conversion uses an additional fixed-frequency
crystal-controlled oscillator running at 17mc) and as either
fundamentals or harmonic oscillators. As a result, 17 crystals
can cover 31 tuning ranges. The metal cover is also the oven and
contains the heater element in the upper inside part of the cover. Two
wires provide connection to the oven voltage (if its operation is
selected by a switch on the rear panel of the receiver.)
During a rebuild, each crystal should be removed from its socket and the socket checked for corrosion. The crystal pins should also be checked for corrosion. Remove corrosion if found with a very small amount of De-Oxit applied with a small stiff paint brush for the sockets and with a De-Oxit dampened paper towel for the crystal pins. Re-install the crystals when the operation is completed. The contacts of the two 31 position switches can be cleaned with a small paint brush dampened with De-Oxit. >>>
|>>> Check the relationship of the RF module MC range selected
versus the Crystal Oscillator selection. There is a small round opening
on top of the Crystal Oscillator Unit just in front of the adjustment
trimmers. You'll notice that inside the opening you can see a numbered
wheel. The number in the opening has to agree with the MC selected by
the RF Module. If 12.XXX is selected, then 12 will show in the opening.
There are no "odd" numbers indicated, so if 11.XXX is selected, the
wheel will be on the line between 10 and 12. This checks the
synchronization of the Crystal Oscillator to the RF Module. If it
doesn't agree, you have to loosen the clamp on the Crystal Oscillator
drive gear and rotate the shaft until the number in the opening agrees
with the MC selected.
When the R-390A is rebuilt and basically operational but before alignment, you'll have to check the output level of the Crystal Oscillator. With the receiver in the Stand By position, check test point E-210 for voltage. The DC voltage should be between -3.5vdc and about -8.0vdc. Start at 8mc by selecting the 08.000 range and adjust the marked trimmer for maximum indication on the voltmeter (use a VTVM.) Work up in succession from 8mc up to 31mc. The voltage should be indicated on the detent of each band selected. The bands lower than 8mc operate on triple conversion and use harmonics of the crystals that were already adjusted on the higher frequencies that are double conversion. Crystal trimmer adjustment isn't necessary on the lower frequencies. When complete, check that there is voltage indicated on all MC selected. This completes pre-testing of the Crystal Oscillator.
|Slug Problem on a 1951 Collins R-390 Receiver - I recently noticed that my R-390 receiver didn't seem to have the sensitivity that it used to have. I had the RF Gain at max with the Local Gain up pretty high and the Carrier Level meter still wasn't moving very much. After participating in a couple of nets, I decided that something was going bad or had gone wrong. It had been a couple of years since I checked all of the tubes, so that was the first task. I turned up three marginal tubes - not bad, just going bad - gassy or on the weak side. I didn't expect this to solve the low gain issue but the receiver was due for the tube check out and the testing would eliminate a bad tube as the source of any other problems. After the tube testing was finished a closer check of performance provided the vital clue. By operating the R-390 with the antenna disconnected and using the Calibrator as the signal source I changed tuning ranges up and down the bands. I noticed that from 500kc up to 2.0mc the receiver had normal sensitivity and the bands from 4.0mc on up also had normal sensitivity. The vital clue was that only the 2mc to 4mc coverage was affected. This meant that the problem was in RF section and specifically in the slug rack/RF transformers that tuned the 2mc-4mc range. A quick look and the problem was apparent. The slug for the Antenna stage had come off of the slug adjusting screw and was all the way down the Antenna transformer coil barrel. It's an easy fix only requiring that the 2-4mc slug rack be lifted up to pull all of the slugs out of their RF transformers. I left the rack springs connected and only pulled the rack up as high as necessary to have the slugs clear the slug barrels. I then removed the loose slug out of the Antenna transformer and removed the adjusting screw out of the rack. Using a soldering iron, I heated the adjusting screw up and melted a small amount of solder into the hole and then inserted the slug's flexible carrier wire into the hole. After the solder had set up, I tested to make sure the flexible carrier wire was in the hole securely. I then rethreaded the adjusting screw back into the rack and then guided all of the slugs back into their respective barrels as I lowered the slug rack back into position. I performed a "quickie" alignment on the Antenna stage and that completed the repair. I'm relating this little problem because it shows that even in an excellent condition rebuilt receiver minor little problems will come up from time to time. Familiarity with the receiver's design made the problem easy to locate and repair.|
Restoring the IF Module
|Next in difficulty is the IF Module. There are several
components that need to be carefully checked and, if you are replacing
the paper caps, this module has more capacitors than any other. The area
under the chassis is very limited and the capacitors must be mounted in
the same position as the originals. This means that you will have to use
716P type 400wvdc SBE Orange Drop capacitors. These are somewhat flat in
shape rather than round and allow easier installation. Besides, the 716P
are better quality polypropylene types. The 715P 400wvdc caps can be
used in some places but the value .033uf is not available in 715P types.
The BFO PTO - In order to replace the three capacitors mounted on the side of the chassis under the bellows-coupler, you'll have to remove the bellows-coupler from the BFO PTO. Be sure to mark the position of the BFO PTO shaft so reinstallation will end up with the shaft in the same position. To remove the bellows-coupler you'll have to heat the spline socket set screws with a soldering iron tip to weaken the green Lok-tite. Loosen the front BFO shaft bearing to allow moving the front shaft out of the way and the bellows-coupler will just have enough clearance to remove it while leaving the BFO-PTO and the front shaft in place. After installing the three capacitors, reinstall the bellows-coupler.
|Note that the BFO PTO shaft threads in and out of
the housing as it's turned, however, the front shaft is fixed in
position. This is the reason for the bellows-coupler. It must be able to
compensate for the movement of the PTO shaft and the non-movement of the
front shaft. When installing the
bellows-coupler, tighten the PTO set screws first, then just slightly extend the coupler before tightening the
front shaft set screws. Make sure as you adjust the BFO PTO a half of a turn in each
direction from the index mark (that you put on the shaft, right?) that
the bellows-coupler compensates for the PTO shaft movement.
Plate Blocking Capacitor (for the Mechanical Filters) - When replacing the Plate Blocking capacitor C-553, remember you are replacing a 300wvdc Vitamin-Q type capacitor that, in the 1950s, was one of the best capacitors available. Don't replace this vintage quality capacitor with a Malaysian-made, less-than-one cent total-cost capacitor. If I was really worrying (like loosing sleep over it,) I'd use two .022uf caps in series for the .01uf C-553. This redundancy is pretty much fool proof for any future problems. Normally though, I use a .01uf 600wvdc 715P type SBE Orange Drop. The "double the working voltage of original" is usually enough precaution on this controversial component.
Component Corrosion - Depending on how the R-390A was stored you might find a lot of corrosion on the Vitamin-Q type capacitors. Shown in the photo to the right is C-531 mounted on TB-502 from a 1960 EAC IF module (EAC built spares for R-390A at that time.) This IF module is out of a "Blue Striper" type R-390A. These receivers were typically stored outside, stacked on pallets. This photo shows what can happen with high humidity, large temperature excursions with no protection for the receiver from the environmental conditions. Luckily, most R-390As were stored much better than the "Blue Stripers" from St. Jullian's Creek Annex and this severe of corrosion is rare. It's also interesting to note that this "Blue Striper" IF module has an open input transducer in the 4KC mechanical filter probably due to an unsuspecting technician (not me) powering the receiver up without first checking out the condition of C-553. Luckily, only one mechanical filter was destroyed.
|Testing the Mechanical Filters - Sometimes individual IF modules turn up for sale. Most of the time we can't test the module and have to rely on luck that everything will be usable in the module and just replacing the capacitors will let us end up with a good condition unit. If you have the opportunity to check out a prospective IF module before purchase, testing the condition of the mechanical filters is fairly easy with the IF module out of the receiver. Each of the mechanical filters will have an input transducer and an output transducer. These, essentially, are pick-up coils that are somewhat like a magnetic pick-up in that they respond to the movement of the discs and wire supports that make up the rest of the mechanical filter. You can measure the DC resistance (DCR) of each mechanical filter input and output transducer to see if it's likely to function correctly. An open transducer coil will mean the filter is not operational and can't be repaired. When the BANDWIDTH switch is set to a specific filter, say the 16KC filter, only that filter can be measured for DCR since the BANDWIDTH switch grounds the inputs and outputs of the non-selected filters. So, the procedure is to set the BANDWIDTH to 16KC and measure the DCR of the 16KC mechanical filter input and output. The DCR will be around 20 to 25 ohms for each coil. Select 8KC on the BANDWIDTH and measure the DCR of the 8KC mechanical filter input and output. The DCR will be around 25 to 30 ohms for each coil. 4KC DCR will be around 35 to 45 ohms and the 2KC DCR will be around 60 to 90 ohms. Notice that as the bandwidth of the filter decreases that the DCR of the transducer coil increases. The DCR mentioned are approximate and can vary by several ohms - depending also on your particular DMM among other things. Just be sure that each input and output transducer has continuity and the DCR is approximately as mentioned above. If you measure an open circuit that filter will not work and it can't be repaired - it must be replaced with a good filter. Though you probably wouldn't buy the IF module (unless it was really "price reduced" after testing) if you were considering repairing the IF module, then be suspect of C-553 if it happens to be the input coil that is bad. I've only run into this once but it can and apparently does happen, that is, a shorted C-553 allowing B+ to appear on a mechanical filter input transducer and causing the coil to go "open circuit." The only repair is to replace the defective mechanical filter and replace all of the capacitors in the IF module (especially C-553) before powering up the module.|
|Other Problems - Some problems won't be discovered
until you are performing the alignment. An example is the problem shown
in the photo to the left. This is the IF module from the 1956 Motorola
contract receiver. When adjusting the mechanical filter trimmers it was
found that the 16kc top trimmer wouldn't adjust. Closer examination
revealed that the trimmer connection was physically broken. These thin
brass connections are very fragile and will break if they are bent too
far. I don't think this break was a result of mishandling, I think it
just broke due to a flaw in the brass. Trying to find someone with a
"parts set" who would
remove the top trimmer assembly was going to be difficult. Also, a
set" IF module would be fairly expensive. I decided to repair the trimmer
instead. I first dismounted the entire top assembly and then dismounted
just the 16kc trimmer to allow good access to the broken trimmer. I used
very thin brass to make a duplicate connection tang. I "sweat soldered" it
to the bottom three finger spring/retainer. I tested the trimmer with a
capacitance meter to confirm that the repair did function correctly. I
then bent the tang to conform to the original broken tang and "sweat
soldered" that to the new tang. Once the trimmer was repaired and the
connections made, all that remained was to remount the parts. Now the
16kc mechanical filter could be adjusted for maximum gain. Of course,
the best repair would have been a new assembly but most of the time
small parts that are unique to the R-390A are difficult to find without
buying an entire module as a "parts unit."
During the alignment of the receiver, anytime you run into an adjustment that doesn't seem to do anything to the voltage reading, it usually indicates that something is defective in that part of the circuit. Inspect the circuit components and many times the problem is mechanical in nature and easy to spot. Of course, sometimes troubleshooting is required but an inspection should always be tried first.
photo left: The mechanical filters and the output trimmers showing how unexpected damage can occur even in protected assemblies.
|IF Module Alignment -
Accessing the Mechanical Filter Input Trimmers - These
trimmers were added with the 1956 contract R-390A receivers. The 1954
and 1955 contract receivers will have fixed value mica tuning capacitors.
The trimmers allow "tuning" the input and output transducers of each
mechanical filter to exactly 455kc where the early MFs that had fixed
capacitors didn't allow exact tuning at all.
It will be noted that the trimmers for the MF input transducers are on the side of the IF module and cannot be accessed when the module is installed in the receiver. Most of us don't have the extension cables that allow operating the module outside the receiver but you can still do the mechanical filter trimmer alignment by this following procedure.
Loosen the Bandwidth and BFO shaft couplers and pull the shafts forward. Then unscrew the three captive screws. The coaxial cables are all disconnected but connect the short IF Output coax to E513 on the IF module. This provides a BNC connector for signal injection. A VTVM is connected to the Diode Load as an output indicator. Leave the IF module power cable connected. Now, carefully lift the IF module up in the front and insert a piece of thin cardboard under it to act as an insulator (or you can use tube boxes as shown in the photo right) and rest the front of the IF module on the upper edge of the front panel. If you position it correctly, you'll now have access to the four filter input trimmers through one of the holes in the side panel (see photo right and note the four trimmers accessible in the side panel hole.) Install a knob on the SELECTIVITY switch because you are going to have to select each MF as you adjust the two associated trimmers (input and output.)
After peaking these adjustments, return the IF module back to its normal position and reinstall the all hardware. If you are proceeding on to the IF transformer adjustments then leave the IF output (E513) connected to the IF output BNC on the back panel.
|3TF7 Ballast Tube - The ballast tube is essentially a length of "heat/current versus resistance" wire (iron, usually) in a gas-filled tube (helium, most likely.) Since the resistance increases as the wire heats up, which is a function of an increase in the current flowing through the wire, this increase in resistance then decreases the current flow. With the current flow decrease, the wire cools and the resistance decreases which allows more current to flow thereby increasing the heat and resistance. This variation in the wire's resistance is what regulates the voltage drop. Of course, the current flow is normally very stable but the ballast tube provides regulation for slow changes like variations in the line voltage to the receiver. Ballast tubes are generally used as tube heater regulators and the 3TF7 in the R-390A is used to regulate the PTO tube and the BFO tube heaters. The 3TF7 is getting to be fairly expensive and, although they are pretty reliable, you probably will experience a ballast tube failure sooner or later. At present the 3TF7 is about $40+. If you don't want to spend the money for the correct component, you can substitute certain types of 12vac heater tubes, such as the 12BH7. This will require a couple of TC wire jumps on the tube pins for the correct connections. The current requirement for the 12vac heater acts similar to the 3TF7 and drops around 12 volts so the two oscillator tube heaters (6.3vac series wired) also drop 12 volts and that satisfies the 24vac provided by the R-390A power supply. There are many other schemes to replace the 3TF7, some that use active voltage regulators or even bypassing the ballast tube socket altogether and installing 12 volt tubes for the PTO and BFO. Several methods are detailed on the Internet.||An Interesting Problem in an Amelco IF Module - N7RCA wanted me to do an alignment and check out on his Amelco R-390A. I was told that one of the tubes in the IF deck was intermittent and had to be moved every now and again to get the receiver working. A quick inspection of the IF module didn't reveal anything unusual. However, when powered up the R-390A would often not receive the CAL signal and physically moving V-504 (the 4th IF amp) in its socket would eventually get the module working. I traced the problem to the ground return on the tube heater at the tube socket. If an external jumper was used, the tube worked fine but it wouldn't work without the external jump. Obviously, there was no ground return for the heater. I thought that I had inspected the wiring so I decided that the socket must have a cracked pin connection and needed replacing. However, when the socket was about half removed I discovered the actual cause of the problem. The TC wire that was routed from the tube socket to a ground lug was NEVER soldered at the ground lug. The ground lug was hidden somewhat by the wiring harness which probably explained why the unsoldered joint was never found at inspection. Apparently, the crimp joint functioned fine and the module passed test and worked for years. Only after many more years of aging with expansion and contraction plus maybe minor oxidation did the crimp joint begin to cause problems. This is the only time I've ever found an unsoldered joint in any R-390A module but it shows that almost anything is possible even with first class builders and thorough inspections. The Amelco did get a new tube socket in the process since removal does usually end up damaging the old sockets. Of course, with a proper heater ground return on V-504, the IF module worked fine. March 2014|
Restoring the Audio Module
|The Audio Module not only has the two audio output components
for LOCAL and LINE but also contains the Power Supply filter capacitors
and chokes along with the 800 cycle bandpass filter and Break-in relay. I usually check
continuity of the chokes and audio transformers while I have the AF
module out. If it worked before hand, then there's no need to check.
Sometimes, though, you've never operated the module (or receiver either)
and that case I usually check the iron just to confirm I won't have any
problems after installation back in the receiver.
Reforming the Electrolytic Capacitors (it might work) - If you don't rebuild the two filter capacitors, at least fully test and reform them. When I used to reform these capacitors I used an adjustable Lambda 25 power supply that had a variable 200vdc to 375vdc output. I connected a 10mA FS current meter in series to watched the current draw as the capacitor is reformed. Only did one section of the multi-section capacitors at a time. Due to the limitation of the Lambda 25 starting out at 200vdc, I checked the capacitor with another power supply that adjusted from 0vdc up to 50vdc. Then I switched over to the Lambda 25. When the electrolytic was first connected to the power supply, the current draw would be fairly high (probably around 100mA) for a very short time. Almost immediately the current would drop down below 10mA. I kept increasing the voltage every few seconds. The current increased and then almost immediately would drop down. When I reached +300vdc on the section, I watched the current. It should slowly drop to below 1mA. I let the voltage remain on the capacitor section for an hour or so. When checked after an hour, the current was usually around 500uA or less. This indicated a good condition capacitor that was reformed. I usually fully discharged the tested section and then measured its capacitance. If it was within spec, I considered the capacitor ready to use.
If your reforming acts differently than described above be suspicious of that section of the electrolytic capacitor. Sometimes you'll see the current "pulsing" or "bouncing" up and down. This indicates that the dielectric is breaking down and the capacitor will not be functional very long. Installed in the R-390A, these caps seem to work but you may notice a random "popping" or "thumping" sound in the audio. It's best to weed-out these caps before using them in the receiver. Of course, if you rebuild the electrolytics, you usually don't have to worry. The procedure for rebuilding using the original can but with new capacitors inside is below.
|Replacing the Paper Capacitors - There are some restorers that change the value of the coupling
capacitors to the grids of the 1st AF amp and the audio output grid.
Originally these were .01uf in value. The normal change is to replace
them with .02uf for better bottom end of the audio. This is normally
only done on the LOCAL AUDIO section. The LINE AUDIO section is left
stock because this was usually dedicated to running teletype converters
or other auxiliary devices. However, if you want the drive another 600Z
ohm speaker with the LINE AUDIO, then you might want to replace these
coupling caps too.
Be sure to use the 716P type 400wvdc SBE caps since they are flat and will not cause interference with the Main Frame bed plate when the AF Module is reinstalled. You will still have to watch the mounting of these components. The PC board that the components mount on is fairly high resulting in tight clearance between the components and the Main Frame bed plate. 715P and 600wvdc caps are too large for the clearance available.
You should also replace the small
electrolytic (tantalum) capacitor mounted on the PC board.
Rebuilding the Multi-Section Electrolytic Capacitors
When I wrote the section above (about five years ago) it seemed like most of the original electrolytic capacitors could be successfully reformed. This year (2016) when restoring a 1967 EAC, I found that the triple-30uf cap had one defective section (cap date-coded '67.) I pulled out my spare '390 electrolytics and found that ALL of them had at least one section defective - either entirely open or erratic current draw while reforming. Maybe it's time to realize that these electrolytic capacitors are at least fifty years old and most are many years older. I think as these units age more and more that we're going to find more and more failures. I would recommend to all restorers working on R-390A receivers nowadays to go ahead and rebuild the electrolytic cans using new capacitors for best reliability and performance. The following is how I rebuild the can electrolytics.
|First, obtain three 33uf 350wvdc axial lead electrolytic capacitors
and two 47uf 350wvdc axial lead electrolytic capacitors. When the caps
arrive, test them for value (just to be sure.) Next, you'll need to open
up the two cans. You can use a tubing cutter if you have one that will handle
1.5" diameter tubing. If not, use a hack saw. Scribe a line around the
cans about .25" above the crimp. This will be below the black tar fill
but above the crimp that holds the base to the can. The scribe line is
so you cut a straight cut. Be sure to scribe a vertical line across the
horizontal scribe as a reference for reassembly.
Once you have made your cut, go ahead and then cut the leads that connect the internal capacitors to the base terminals. This separates the top part of the can from the bottom. The top will have the black tar and old caps inside.
Get your heat gun and a heavy pair of leather gloves. Place the top can on a large metal tray and begin heating its exterior with the heat gun. Rotate the can so you heat all around it. This will take about 2 minutes to get really hot (why you need the heavy gloves.) Use a pair of needle nose pliers and grab the leads and then pull the old cap out of the can. If you've heated it up long enough, it will just pull out easily. If not, apply more heat until it does pull out easily. Discard the old cap. Using a long blade, scrape out all of the black tar while it's still warm. It will be pretty easy to get out at this time. >>>
|>>> Cut a heavy paper circle that will just fit
into the inside of the top can and push this down inside the top of the
can to act as an insulator. This really isn't necessary on the dual 47uf
capacitor but is extra safety for the 33uf assembly which is a "tight
fit" and pushes up against the top inside when assembled. The can is not
connected to anything in either of these two capacitor assemblies.
Next you'll need to clean the base. Notice that the terminals are aluminum that are crimped. Using a pin vise with a 1/16" diameter drill bit, start a hole centered in each terminal. Once you have a centered hole switch to a hand drill and drill the 1/16" hole down into the terminal about 1/4". See photo 1. The left base terminals are drilled. The right base terminals aren't drilled.
Build your capacitor assembly with two 33uf on top and one 33uf on the bottom. Use sleeving to insulate the leads. See photo 2.
Insert the capacitor assembly leads into the correct terminals. Pin 1 is common negative and Pins 3,5 & 7 are + 33uf. Check assembly to make sure that the can will easily fit over the assembly and mates with the base. Remove the can and crimp the leads in the terminals using fairly heavy needle nose pliers. These aluminum terminals are very soft and don't require a large tool for crimping. See photo 3.
Test the capacitor assembly for value before proceeding further. Make sure the terminal connections are correct and capacitor value is correct. Make sure the terminal crimp connections are tight and are holding the leads tightly. Test by pulling the leads with needle nose pliers - don't pull like you're trying to pull them out, just test that the crimp joint is tightly holding the lead. >>>
| >>> Cut a piece of heavy paper and form into a tube about 1.5"
tall. This is going to hold most of the epoxy so there is something for
the epoxy to bind to. Mix a batch of 5 minute Epoxy.
Coat the heavy paper tube and coat the inside of the base then fit the tube in place. Coat the inside of the top can and slide it into place over the tube. Make sure the scribe lines match. Wipe off any excess Epoxy from the outside of the can. Use masking tape to secure the two parts of the can together. Let the Epoxy set up for at least one hour before removing the tape.
I either paint the joint with silver paint or wrap the joint with aluminum tape to hide it. See photo 4. The cap on the left has paint over aluminum tape. Cap on right is just paint.
Test cap assembly again for value and then it is ready to install.
This particular description is for the triple 33uf cap which is the most difficult to rebuild but the dual 47uf is rebuilt in the same manner. The dual 47uf cap can have the two caps side-by-side and they will fit into the top can that way. Just be sure of the terminal connections - Pin 1 is common negative and Pins 3 & 5 are + 47uf. Pin 7 is not connected to anything.
Checking Out the Power Supply Module
|Since the filter capacitors and the 0A2 voltage
regulator circuit is located on the Audio module, there's very few parts
of the power supply located on the Power Supply module. Usually all
that is required is a general clean-up. Check to be sure that the
transformer is set for the proper AC input voltage. The primary voltage
setting is accomplished by how the wires are connected on the terminal
strip under the chassis. You should clean the
tube sockets and check the condition of the 26Z5 rectifier tubes. Check
the condition of the Amphenol connector and make sure that the pins
don't have any corrosion. See photo to the right.
restorers change the 26Z5 tubes to solid-state because of the expense of
a set of tubes. The military did this a lot and had no problems because
the receivers were basically turned on and left on - no cycling of full
B+ on tubes with the cathodes cold. Some restorers install an in-rush current
limiter device to "soft start" the power supply. It's a decision that
needs some consideration. Original 26Z5 tubes are somewhat expensive but
very reliable. Going solid-state requires a few small modifications.
Continuous Operation? - If you decide that you're going to leave the receiver on continuously, consider this,...if you use your R-390A for an hour a day that will equal seven hours per week. Left on continuously for one week, the receiver will have 168 hours on it, 161 hours that were essentially non-productive. In one year, you'll have put 8,736 hours on your R-390A's tubes and other components and only used it for 365 hours. Although tube heaters don't like to be cycled and cycling does affect their total hours of usability, even if cycling the power on and off reduces the tube's life by one-half, you'll still get more usable hours by turning the receiver off when it's not being used.
photo right: This is a Collins-built R-390A Power Supply showing that there are very few parts used in this module. It still needs to be cleaned and given a thorough inspection.
Setting up the PTO
photo above: A Collins version R-390A PTO. Note that to the left of Z-702 is hex head slotted plug that covers the end-point error adjustment L-701. Look just to the right of the hex stand-off that supports the front sheet metal mount and you'll see the slotted hex head of the plug. To access this plug for removal does require the PTO be dismounted. It's then reinstalled without the plug so the end-point adjustment can be made with the PTO installed in the receiver. This requires a long, fairly thin screwdriver to accomplish. (see text below.)
- There are two basic types of PTOs used in the R-390A. The Collins PTO, the older of the two types, has a large hex head slotted plug that
has to be removed to access the L-701, the end-point adjustment
inductor. The plug is located to the left of Z-702 as seen in the photo
of the Collins PTO to the left. The Collins-type PTO was replaced with an
version built by Cosmos Industries, Inc. However, the access
to L-701 is slightly different. Where the hex head plug was on the
Collins PTO, the Cosmos has a large blade screw plug that was for
filling the PTO with nitrogen during assembly (most PTOs have leaked out
the nitrogen over the years - either through reworking the PTO or
failure of the seals.) To the right of this large screw plug and almost
behind Z-702 is a smaller blade screw plug. This is the access to L-701
on a Cosmos PTO. Adjustment of L-701 requires a very long and thin blade
screw driver with a very small head (.062" diameter and 8" long.) L-701 is accessed with the R-390A
front panel lowered and through a holes in the KC tuning locking plate
and holes in the gearbox front and rear plates (the procedure for
adjusting end-point error is below.) Without the long and very thin blade
screwdriver it is impossible to perform the EPE adjustment on the Cosmos
PTO in the manner of the Collins PTO. The tedious Cosmos procedure is to make an adjustment with the PTO out of
the receiver, then reinstall to test. This has to be repeated until the EPE adjustment is within specification.
If you have to do any rework on the inside of the PTO, you'll find that after removing the outer shell there is a wrapping of fiberglass material. Under the fiberglass is another can shield that will have to be removed. Originally, this area was pressurized with nitrogen to keep air and contaminates out. If your PTO was never worked on and still has good seals, there might still be pressurized nitrogen inside. Slowly remove the screws and if there's any nitrogen still inside it will depressurize slowly and not be any problem. Nitrogen is almost inert so it won't react with anything and there's only a small amount inside anyway. When the rework is finished, just install all of the shields and don't worry about the fact that now air is inside the PTO. The nitrogen was to protect the PTO in the harshest environments.
photo above: Cosmos PTO. Note that the EPE adjustment is now almost behind Z-702 making access more difficult than on the Collins PTO. It's difficult to see but it's the black quarter-circle visible just behind Z-702 and behind the brown fiber front of the PTO case.
|PTO Removal - A little care in removing the PTO will save you a lot of work later. First, set the KILOCYCLE tuning to XX.000, it doesn't matter where the MEGACYCLE tuning is set but the KILOCYCLE setting must be at .000 (not +000, see note below.) Next, disconnect the PTO output cable from the RF module and unplug the power cable at the PTO. Loosen the captive screws in the front of the PTO. Now, pull back on the PTO body until the oldham coupler center piece comes off of the gear box-side coupler face. Now lift out the PTO. Be careful to not move the tuning shaft of the PTO now. It is set for .000 and you should pay attention to the orientation of the oldham coupler or you can mark the shaft so you know the proper setting. If no work is going to be done to the PTO, when reassembling the R-390A you merely have to assure that the PTO is still set to .000 and that the gear box and Veeder-Root counter are set for XX.000 when the oldham coupler is assembled. In this manner, the mechanical settings have been maintained and calibration should be very close. NOTE - XX+000 is the highest frequency tuned on a specific band and XX.000 is the lowest frequency tuned on a specific band. All of the settings here are at the lowest tuned KILOCYCLE frequency on any of the MEGACYCLE settings.|
Synchronizing the PTO to
the RF Module (if you didn't do the "pre-set" part first) - If you've
pulled the RF module without setting the Veeder-Root counter to XX.000,
or if you had to do some work on the PTO, or
if the PTO shaft has been moved and you don't know where it needs to be,
or if you just want to be sure that the PTO is correctly aligned with
the RF module then
you'll have to synchronize the PTO. This requires a digital frequency
counter (DFC.) The PTO should output a specific frequency range, 3.455
mc to 2.455 mc, in ten turns. All that is necessary
is to monitor the frequency output of the PTO and set it to 3.455 mc. This frequency will be equivalent to XX.000 on the Veeder-Root
counter. Here's the procedure,...
If the Veeder-Root counter wasn't set to XX.000 before removing the RF module, then the PTO was not pre-set to 3.445mc. This is a minor inconvenience that requires the PTO be set by powering up the R-390A and measuring the frequency out of the PTO with a digital frequency counter. It is assumed now that the PTO was out of the receiver so the oldham coupler is not together. Connect the PTO to the power plug in the PTO bay. Then route the PTO output coaxial cable to the back panel IF output connector. Disconnect the IF output cable and install the PTO output cable in its place. This provides you with a BNC connection for the PTO output that is then connected to the digital frequency counter. It isn't necessary to have the PTO mounted at this point but it will have to be in the PTO bay because of the cable lengths. With the R-390A powered up, the PTO will show the output frequency on the counter in about 30 seconds or so. Now rotate the PTO coupler and adjust the frequency to 3.455mc as measured on the DFC. Be sure the R-390A Veeder-Root counter is set to XX.000. Now the two faces of the oldham coupler should be correctly aligned (one projection oriented 90 degrees difference from the other face's projection.) Next, insert the coupler center piece. This may require moving the PTO body around a bit to get the two faces and the center piece to fit together. Once they fit, then go ahead and tighten the three captive screws to remount the PTO. Fit the backlash spring to the pins on the two coupler faces. Now, test that the PTO output frequency is 3.445mc at XX.000 and is 2.455mc at XX+000. Power off the R-390A and reconnect the PTO output cable to the RF module and the IF module's IF output cable to the IF output connector
|End-Point Error Adjustment
- Collins PTO
- It's rare to find an R-390A PTO that has excessive end-point error
that is beyond adjustment.
Most of the End-Point Error (EPE) horror stories come from the 70E-15
PTO that was used in the R-388 receiver. The R-390A PTO used high
material for the ferrite core and consequently stability is maintained
of a period of decades. This applies to both Collins-built PTOs and to
the Cosmos-built PTOs. Most of the time just a slight "touch-up" is all
that is necessary and luckily that can be accomplished without a test
jig or with having to operate the PTO outside of the receiver (as you do
with the R-388 receivers.) The
TM manuals direct you to remove the PTO and remove the access plug for
the adjustable compensation inductor, then reinstall the PTO and dismount the
front panel (lower it down) for access through the holes in the KC
tuning lock plate and the front and rear
gearbox plates back to the PTO compensation inductor adjustment. You'll
need a long, thin small blade screw driver for this procedure. The first step is to check your EPE
and see what it is. Usually, it will be pretty close. The greatest
excursion I've found on a Collins-type was around 4.0kc Most EPE encountered are around 1 or 2 kc. The TM manual will
give examples of which way to turn the EPE compensation inductor based
on whether the ten-turn coverage is greater than or less than 1.000MC.
Make a small adjustment to the EPE compensation L and then return the
R-390A to XX.000 on the Veeder-Root counter. With the CAL on, loosen the
oldham coupler on the gearbox side and readjust the PTO shaft for zero
beat. Retighten the gearbox side oldham coupler and recheck your EPE. If
you've adjusted the compensation L in
the correct direction your EPE should be less. Repeat the procedure
until you've gotten the EPE to less than 500 cycles. You can adjust it
even closer if you want to since the "tic-marks" on the Veeder-Root
counter are for 200 cycles. When you're satisfied with the EPE, remove
the PTO from the receiver and install the threaded plug that covers the
access to the compensation inductor adjustment. Remount the front
panel. Pretty easy when compared to the hassle of doing a 70E-15 PTO
from an R-388.
EPE Adjustment - Cosmos PTO - With the Cosmo PTO the location of the EPE access hole was moved slightly to the right and almost behind Z-702. This makes it difficult to access in the normal "thru the dial locking plate" method. The screwdriver used has to be minimum 6" long with a shaft diameter of around .060" (8" length is better.) Even then, it's mostly achieved "by feel" rather than sight. If you don't have the long very thin screwdriver the only option is to remove the PTO for EPE adjustment (with a shorter but still thin very small blade screwdriver) and then reinstall to test. This will take around five or six operations to get the EPE close to spec. It depends on how far off the EPE is. The greatest EPE I've encountered on a Cosmos PTO was 8kc. My long thin screwdriver that works on Collins PTOs would not adjust L-701 due to the very small access hole in the Cosmos PTO. I had to do the remove, adjust, reinstall and test operation about six times to get the EPE down to 0.5kc. With the PTO out of the receiver I could use a very small screwdriver to make the adjustments. The upshot is, expect more difficulty in doing EPE adjustment with a Cosmos PTO.
Front Panel Restoration
Silk-screened Panels - All Collins and Motorola R-390A receivers use front panels that have silk-screened nomenclature. This presents a problem if the panel is in rough condition. About the only solution is to look for a decent condition replacement panel. If originality isn't an issue, a silk-screened panel can be replaced with an engraved panel. Be aware though, that Collins and all of the other contractors used a short serial number tag with the exception of Motorola. The Motorola contracts used a 3" long tag with different locations for the mounting holes. If you're trying to maintain originality with a Motorola contract R-390A, then you're going to have to find another Motorola front panel. (Note: Early contract Collins R-390A receivers also use the long 3" data tag.)
Engraved Panels - Repainting an engraved front panel is very easy. Be sure to mask the back of the panel (if you're going to paint it) where the panel mounts to the Main Frame and also a small area by the upper left mounting screw for the Carrier Level meter. Some panels will have nomenclature on the back that identifies some of the components. These are usually the earlier silk-screened front panels as later engraved ones have nothing on the back but the paint. Be sure to use automotive grade paint that is purchased from an automotive paint dealer. This type of paint will have special hardeners that make the paint really durable. Also, professional paint will dry "ultra-thin, hard and flat" which will help make filling the engraving a lot easier. After painting, let the panel dry at least overnight before doing the engraving fill.
End-user Panel Repaints - The R-390A specifications state that the front panel is to painted medium gray. The manuals give a specific part number for the paint but it seems the shade of gray did change over the years from contractor to contractor. When choosing the color for repaint, try to get as close as you can to your receiver's original panel color by having the original paint matched at an professional automotive paint supplier. Nearly all R-390A panels are found painted gray, however, sometimes the end-users did repaint the front panels totally non-specification colors. The USAF had banks of R-390A receivers at Clark AFB in the Philippines that were painted flat black. Once and a while, olive drab panels turn up, supposedly painted that way by the USMC. At any rate, there is some evidence that R-390As were painted colors other than gray when the end-users had some reason to do so. Remember, all R-390A receivers left the contractor's facility with gray panels (and that's original) but it can be considered "acceptable" to paint the R-390A panels colors other than gray if there is believable evidence that the color was actually used on a receiver that was operating in a commercial or military capacity.
Engraving Fill - I use Artist's Acrylic paint to match the engraving fill paint. If you use pure white it will look way too bright. You should mix a color that is close to that found on manila folders - kind of beige color. Apply the fill paint to one part of the panel nomenclature at a time. Use a small paint brush and dab the paint into the engraving. Don't try to just paint into the engraving - you have to dab the paint into the engraving to have enough there and, of course, you'll have some paint around the engraving - that's normal. Let the fill paint set for about one minute. You'll now have to remove the excess paint around the area. Use a rubber squeegee to remove the excess paint. Be sure to wipe the paint off the squeegee each time you make a pass to remove the excess paint. Next, use a damp paper towel folded very flat to remove the small bit of remaining paint on the panel. You'll have to be careful not to "pull" the fill paint out of the engraved area, so keep the paper towel pieces small and only use them once. You'll have to have several damp paper towel pieces ready as you do each area on the front panel. Also, I've found that if you dampen the paper towel pieces using Glass Plus instead of water the Artist's Acrylic comes off much cleaner. These paper towel pieces should be just damp - not wet! When finished let the panel dry overnight. The next day you can apply carnauba wax to protect the panel and the engraving fill which will enhance the overall panel appearance.
IMPORTANT NOTE: Don't use Windex to dampen the paper towel pieces. Windex contains ammonia which might damage the new panel paint. Glass Plus doesn't contain ammonia but works very well to remove the excess paint without damaging the panel paint.
The Back of the Front Panel - The backside of the front panel has several clamps for holding the harness in place. Also, there is a printed circuit board mounted on the back of the front panel just above the frequency readout bezel. There is a lot of mechanical stress on the various wires when the front panel is lowered so check all of the wires to the pots and switches for any breaks or other problems. Since you have removed the front panel for repainting (or replacement,) then you'll be remounting all of the pots and switches along with the phone jack, dial lock and the zero adjuster. Note also that there is a grounding lug on the upper left (as seen from the front) stud of the CARRIER LEVEL meter. This provides a chassis connection for one of the AGC delay capacitors.
Use the Correct Lock Washers - Each nut that secures a potentiometer or switch should have a internal tooth lock washer installed. There are five 6-32 flat head phillips screws that mount into the Main Frame bed and into one of the vertical dividers (into pem-nuts) on the underside of the bed. These screws each have a #6 conical external tooth lock washer installed. If you're missing the conical lock washers, they are available from McMaster-Carr (boxes of 100 - they're cheap.) Notice that the three 6-32 flat head phillips head screws that mount the cable clamps have split ring washers mounted on the back side of the clamps (along with nuts.) The eight 10-32 flat-head screws that mount the panel to the main frame vertically will thread into Nylock pem-nuts, so no lock washers are required.
Mounting the Front Panel to the Main Frame - When mounting the front panel, note that the Dial Lock has to fit over the KC tuning shaft lock-plate. Leave the Dial Lock loosely mounted so it can be rotated to clear the lock-plate. Once the front panel is mounted, you can rotate the Dial Lock into position (the locking grips on each side of the locking plate with the locating tab in the hole one the backside of the front panel) and tighten the mounting nut.
If you have the two large shaft bushings (KC and MC Tuning) and the three small shaft bushings mounted to the front panel you'll find it difficult to guide the shafts thru the bushings because of the harness length. Although you could dismount the harness clamps, it's easier to just plan ahead and slide the rear panel shaft bushing onto the shafts and then mount the front panel to the Main Frame. You'll find with the large openings, it's really easy to guide the front panel over the shafts with the harness clamps tightly mounted. Once the front panel is mounted, then slide the rear bushing forward and slide on the washer and thread on the front bushing.
Once everything is mounted to the front panel and the front panel is fully mounted to the Main Frame (and tightened,) then you can go ahead and adjust the panel shaft bushings for the best feel when rotating the controls.
Front Panel Bearing Adjustment - If you want your R-390A to tune "light and easy" then you're going to have to adjust the front panel feed-thru bearings. These are on the KILOCYCLE and MEGACYCLE tuning shafts.
After a thorough cleaning of the RF module gear box, you probably noticed that the KILOCYCLE tuning was very light and easy to manipulate. As you reinstalled the slug racks, the tuning became slightly more difficult to manipulate but was still very light and easy. When the front panel was installed, all of a sudden the tuning seemed to drag and was noticeably more difficult to manipulate. This is caused by the two panel feed-thru bearings. When the RF module is removed and then reinstalled, it's very slightly, differently oriented and the same goes for the front panel. Only a slight misalignment of the panel bearings will cause a "heavy-feel" to the tuning.
Before the front panel is reinstalled, slide the rear bushing onto each shaft. Fit the front panel into position over the shafts and begin installing the mounting screws. With all of the screws tightened that secure the RF module to the Main Frame and all of the screws tightened that secure the front panel, then slide the rear bushings forward and mount the washer and front bushing nut. Note how the bearings can be moved within the feed-thru mounting hole. This is to allow proper placement of the bushing to act as a guide and bearing for each shaft.
Using a 5/8"open-end wrench, lightly tighten the KILOCYCLE bearing nut being careful to not move the position of the bearing itself. Then try the KILOCYCLE tuning. If the tuning is very light then try to just slightly tighten the bearing a bit more - not too much - the bearings don't have to be mounted "super-tight." If the tuning is still light then the adjustment is fine. Usually, no matter how the bearing is adjusted, there will be a slight increase in the "drag" because of the bearing itself. The adjustment is to achieve the lightest "feel" while still providing support for the shafts.
Do the same procedure for the MEGACYCLE tuning although this tuning is much more difficult anyway since you're moving so many of the slug-racks and there's also a detent about every turn of the shaft. Adjust this bearing for the best "feel." You can also apply a drop of machine oil on the shafts to help lubricate the oil-lite bronze bearing that is inside each of the feed-thru bearings.
The end result will be a KILOCYCLE tuning that is very easy to manipulate and feels great when fine tuning is required.
The three small (.25" shaft) bushings are adjusted in the same manner (ANT TRIM, BANDWIDTH, BFO.)
Meter and Line Level Meter - If an R-390A receiver has
been obtained from either a surplus source or other "official-type" dealer,
both meters may have been removed. There was (is) a concern that the
radium-coated scale and needle leaked too much radiation and that was a cause for meter removal and "proper" disposal.
Whether or not the radiation level is anything to be concerned about is
up to the individual user/owner but that's why many meters are missing from
the R-390 and R-390A receivers.
When proper replacement meters are found they will likely be in "rough" condition. It's easy to mask the glass and give the body a light coat of flat black paint. If there are heavy scratches or gouges, these will have to be removed with either a file or Al-Ox paper followed by a paint job. I've also "touched up" the meter cases and then used 0000 steel wool applied "lightly" to even out the finish. Be sure to use the gaskets (if you have them) between the meter body and the front panel.
If you want to get inside one of these meters it is a difficult operation that usually ends up ruining the meter. Proper tools are necessary and one should always where protective gloves. Avoid opening the meters if at all possible. If you are contemplating changing the scales to something non-radioactive, this would probably create more of a "radiation problem" than to just leave the meter "sealed." Besides, you still have the radium-coated meter needle to deal with. Most user/owners feel that the meters are safe when used properly. In other words, don't eat the needle, don't tape the meter to your chest and leave it there for a year or other things that normal users wouldn't do anyway. At a distance of three inches the meter's radiation leakage is not even measurable. That's why it was sealed in the first place.
- These are made out of stainless steel and can be easily cleaned up
with 0000 steel wool and a small brass brush for the washers and the
flanges. Wash with Glass Plus before installing.
Dial Cover - This cover, unfortunately, takes a lot of "hits" and as a result is sometimes found dented, scratched or both. Inside the cover is painted with zinc-chromate primer which is bright yellow-green color. Usually the inside is okay but if the cover has dents to be removed, it might need repainting after the body work is finished. Outside the cover is semi-gloss black. Automotive-quality paint should be used for painting the exterior of the cover.
Knobs - The knobs also take a beating and many times will need to be restored. First strip the old paint off with a methyl-chloride type stripper. Go over the knobs with a wire brush afterwards. Use a high quality automotive paint in semi-gloss black. Let the paint set overnight. Mix "manila" Artist's Acrylic as described above for front panel nomenclature fill and use the same procedure to add the index line for the knobs. Let this set up for a day and then give the knobs a coat of carnuba wax and install.
|A Note on All-Matching Modules - This is generally an indicator that the receiver has not been used extensively and has not gone through any sort of echelon rebuild. These types of receivers are desirable in one sense since they usually haven't been brutalized by careless technicians. Most enthusiasts consider the "non-matching modules" equipped R-390A to be inferior since it has obviously been worked on in the past. If you intend to use an R-390A "as delivered" then the all-matching modules type gives you a chance that this "out-of -the-box" operation might be possible. However, if you intend to rebuild the R-390A before putting it into operation, then the "non-matching modules" type will be a more reasonably priced option. >>>||>>> All of the modules are basically the same regardless of which contactor built them. There are minor differences but all are interchangeable. You will find that some of the early Collins and Motorola IF modules don't have adjustments on the inputs and outputs of of the mechanical filters. That was corrected with an ECO in mid-1956. You might find the the early Motorola RF transformers are of a higher quality than the later EAC units (that used American Transformer units.) That was just an evolution of production methods. The contactors had to meet a detailed specification when building each module and all modules will perform to spec after a rebuild. Certainly, if you enjoy the rebuilding process, then a "non-matching modules" R-390A will be your most economical route and you won't have to be concerned about disturbing the unit's originality.|
The Receiver Alignment
after Rebuild You should have the R-390A receiving
stations on all bands with power on. You can turn on the CAL and the BFO. Then tune to
a 100kc calibration signal. Now rotate the MC knob through each of the
bands listening for the calibration signal. Normally, you'll hear the
CAL signal on every band although it will be at various tone-frequencies depending
on each band's particular alignment at this time. You should hear the
signal on every band though. This will assure you that everything is
basically working and the receiver is responding to an input signal on
each tuning range.
Things to do Before Proceeding to the Fixed-IF, Variable-IF and RF Alignments - you should first do your PTO end-point error correction, if necessary (it will be.) Be sure that the Calibration Oscillator is set correctly and the Veeder-Root counter is in sync with the PTO (you should have checked both before doing the EPE adjustment.) Also, check the Crystal Oscillator output at E-210 and be sure the voltage there is between -3.5vdc and -8.0vdc on the 8mc to 31mc tuning ranges. The voltage should appear when the MEGACYCLE knob is on its detent for each band. You don't have to check the .5mc to 7mc bands because these crystals and associated oscillator circuits are both fundamentally and harmonically operated and also work in either a double or triple conversion scheme. They were actually checked in the higher ranges. Be sure your DIAL ZERO is mechanically centered within its approximately one-sixth turn of the KILOCYCLE tuning when calibrated to XX.000 on the Veeder-Root counter.
Error in Army TM 11-5820-358-35 - Field Depot and Maintenance Manual for the R-390A from December 1961. Alignment instructions, page 116, paragraph 76b (2) indicates that URM-25 Signal Generator should be tuned to 18.75mc. Actually, the correct frequency is 18.25mc. This error was very obvious to technicians doing the alignment and is very well known. Interestingly, the earlier TM 11-856A Technical Manual for the R-390A has the correct 18.25mc information, so this later error was probably a typo that wasn't caught in proof-reading.
How to do the Balanced Input Alignment - Make up a test resistance that consists of two 68 ohm 1/2 watt carbon resistors in series. Each separate leg of the resistors will push into to each terminal of the Balanced Input Twin-ax connector. The junction of the resistors will be connected to the signal generator. When performing the alignment of the RF stages first adjust the Balance trimmer on the the RF transformer for the minimum voltage on the DIODE LOAD as read on an (analog) VTVM. The Balance adjustment will not reduce the DIODE LOAD voltage to zero - you're setting the Balance trimmer for the minimum voltage. Be sure the ANT TRIM is set to 0. With the minimum voltage set, now proceed with the RF adjustments for that section of the receiver. Recheck the Balance trimmer adjustment after the particular RF section has been aligned. The Balance should still be close but will probably need just a slight adjustment for minimum voltage at the DIODE LOAD. Recheck the RF alignments - but there should be no significant change and just a slight "tweak" should be all that's required.
Does it Really Matter - Balanced or Unbalanced Input? - If you do the Balanced Input alignment, then the answer is yes. Correctly aligned you might see an improvement if you are using an adapter that grounds one side of the two terminals and connects an unbalanced antenna (that is matched for the received frequency) to the other terminal. This method runs the signal through a set of tuned coils before going to the RF amplifier stage. If you haven't performed the balanced alignment then it's very possible that a reduced signal level may be experienced with this method of connecting the antenna. In this case, connect your antenna to the Unbalanced input (but, eventually, you'll want to do the Balanced Input alignment.) Originally, the Balanced Input was for dipole antennas that used a balanced feed line in the 100 Z ohm range utilizing the "twin-ax" type coaxial cable. Nowadays, hardly anyone runs a balanced antenna directly since most transmitters operate into unbalanced loads. Additionally, the "twin-ax" type cable has a significant db loss per foot. If you're using a tuned antenna with antenna coupler, you might find that the Unbalanced input works better. This may also be the case if you're using a vertical antenna directly fed with coax. Go ahead and do the balanced alignment and then test both Balanced and Unbalanced inputs to see which nets the best results with your particular antenna.
Stagger Tune the IF or Peak Adjustment of the IF? - Stagger tuning will give the IF bandwidth the maximum flat top possible with the mechanical filter selected. Early receivers were "peak" tuned for maximum response and may give a frequency bandwidth somewhat less than the mechanical filter bandwidth. Stagger tuning was used on the later IF modules and does give a better response in the receiver that is generally flat out to the limits of the mechanical filter's bandwidth. You can align early IF modules using the Stagger Tune method. Be sure to check the IF transformers to verify that the Q-spoiler resistors are present. Some IF transformers were modified by clipping out the Q-spoilers to have more IF gain but this also narrows the bandwidth. The Q-spoilers should be installed and connected for the best bandwidth. Updates to TM11-856A have the later procedure for stagger-tuned IF alignment, as does TM11-5820-358-35, or it can also be accessed from many sources on the Internet.
IF Cans without the Alignment Hole - Early production IF modules will have shields over the IF transformers that don't have a hole for alignment. This was to assure that the receiver's IF alignment wasn't tampered with in the field. When the receiver went back for repair or alignment the technicians had a set of covers with holes that were installed for the alignment and when the alignment was finished then the "non-hole" originals were re-installed. If you can't locate an extra set of IF cans then it will be necessary to drill an access hole for alignment. All later IF shields had the hole anyway and many early ones are found nowadays with the hole already drilled since the TM directs the technician to drill a hole for alignment purposes.
What to do about early IF modules without the Mechanical
Filter trimmer caps? -
The trimmers were added with the 1956 contract R-390A receivers. The
earlier IF modules will have fixed-value 110pf mica capacitors to tune the input
and output of each mechanical filter. When checking these earlier IF
modules, it will be necessary to measure the output
and see whether or not each of the Bandwidth positions (16kc, 8kc, 4kc
and 2kc) are more or less equal for a constant, known-value input
signal. The easiest way is to use the CAL and tune to zero beat and
watch the CARRIER LEVEL meter. For instance, if 16kc and 8kc measure
50db on the meter but 4kc measures 40db and 2kc measures 50db, then
something is wrong with the 4kc mechanical filter. When doing this test,
you'll notice that the fixed tuned mechanical filters are not equal but
are usually fairly close - within 5db of each other. If the four filters are not close in their equal response it
will be necessary to reselect the tuning capacitors on the filter that
is different (hopefully, it's only one filter that has been affected.) Try to select capacitors so
that each Bandwidth position results in a fairly equal output in all
four Bandwidths (mechanical filter derived bandwidths, that is.)
Make up an adjustable trimmer that has a range of about 80pf up to around 130pf. Remove the cover from the mechanical filters on top of the IF module and then remove the 110pf fixed capacitor on the particular mechanical filter. >>>
|>>> Now "tack solder" the trimmer in its place. Power-up the
receiver and select the bandwidth for the particular mechanical filter
and adjust the trimmer for maximum reading on the CARRIER LEVEL meter.
Remove the trimmer and measure the capacitance with a digital
capacitance meter. Install a silver mica capacitor of that value to the
mechanical filter. This may be enough to get the mechanical filter tuned
enough for equal response but it usually isn't.
To do the capacitance selection for the mechanical filter output will require accessing the underside of the IF module. You can loosen the BANDWIDTH and BFO knob-shafts and pull them forward. Then undo the IF OUTPUT coax cable and loosen the three captive screws. The IF module can now be lifted in the front and placed in a vertical position. Use rubber spacers to "prop up" the IF module. Remove the fixed silver mica on the particular mechanical filter and "tack solder" the adjustable trimmer. Power-up the receiver and switch the BANDWIDTH control for the particular mechanical filter and adjust the trimmer for maximum reading on the CARRIER LEVEL meter. Remove the trimmer and measure its capacitance and install that value silver mica on the mechanical filter. Replace the IF module and the knob-shafts and give the receiver a final test to see how the new tuning compares. Hopefully you'll be able to retune the mechanical filter to be within 5db of the other mechanical filters. Needless to say, the 1956 upgrade that added adjustable trimmers to all of the mechanical filters made everything a lot easier.
|I've used many R-390A receivers in my various ham radio
station set-ups over a period of many years. My first R-390A was a 1959 Stewart-Warner version
that worked pretty well "as-is" when I got it from a ham swap meet in 1991. I
used it with an Eldico SSB-100F transmitter I had and performance was
great. A few years later I obtained an excellent EAC version from 1967 but
I sold it to
buy a 1951 contract Collins R-390 installed in a CY-979 cabinet. I still
own and use the R-390 on a regular basis. I sold the
1959 Stewart-Warner after obtaining a 1955 Collins R-390A. This Collins
R-390A was given to me as payment for repairing and rebuilding a Motorola R-390
for a fellow ham. Eventually, I "wheeled and dealed" my way into a 1956
Motorola R-390A, a 1961 Capehart R-390A and a "Blue Striper" survivor
from St. Jullian's Creek Annex. My latest (2016) R-390A find was a
bargain-priced 1967 EAC that needed a little TLC but turned into a great
receiver. I've used all of these receivers except
the "Blue Striper" at one time or another, both as an SWL receiver or as
a Station receiver.
Here's what I like about the R-390A,...
1. If you absolutely must know exactly where in the electromagnetic spectrum you are listening, the R-390A and its family are the most frequency-accurate readout available in vacuum tube receivers. It's easy to achieve 1/2 kc accuracy or better. The mechanical-digital readout eliminates the vague interpretations of reading analog dials.
2. If you're bothered by QRM, remember the R-390A was designed to intercept radio signals from the USSR, China, East Germany and other Communist countries and be able to successfully copy those signals through any kind of interference whether natural or man-made. The mechanical filters allow the best in steep slope bandwidths. When operating CW, you can also switch in an 800 cycle audio filter. You can literally copy one CW signal with another CW signal almost on top (yes, I've done it,... many times.)
3. When the R-390A is rebuilt and correctly aligned it is very competitive as far as sensitivity is concerned. Are there more sensitive receivers? Of course, but sensitivity isn't all that's required to successfully copy weak signals. When all the available controls are taken into account and the user is very familiar with the operation and capabilities of the receiver, the R-390A is almost unbeatable as a station receiver.
4. Stability is the best in vacuum tube designs. Drift is non-existent.
5. You have two individual audio outputs on an R-390A. The LOCAL AUDIO is normally used to drive a 600Z ohm speaker set-up but you can also use the LINE AUDIO for the same thing - simultaneously! The LINE AUDIO was normally used to drive data devices like RTTY TUs, etc., but there's no reason it can't drive any 600Z load - like another speaker. I've set up a speaker in one room run by the LINE AUDIO and a second speaker in another room run by the LOCAL AUDIO. Independent audio levels in separate rooms. Really neat.
6. Everything about the R-390A's construction is "heavy-duty" and its use metal knobs imparts a massive "feel" to the receiver's operation. The R-390A has a certain impressive presence that attracts the attention of ham shack visitors. This seems to be true whether the visitor is familiar with the R-390A receivers or not.
The following might be concerns for some users,...
1. On SSB and the Meters - Although there were a couple of military SSB adaptors , the CV-591A (aka MSR-1 or MSR-4) and the CV-157, available and several modifications have been published and other add-on devices for demodulating SSB produced, none of these are necessary for demodulating SSB signals. Unfortunately, many new R-390A owners have only used modern equipment (with SSB Product Detectors) before going to the R-390A which only has a simple Envelope Detector. They expect the R-390A to be adjusted for SSB reception just like their modern receiver - RF GAIN at maximum with the AVC (AGC on the R-390A) on and volume level set by the AF GAIN (LOCAL GAIN on the R-390A.) The R-390A can't be operated like that when receiving CW or SSB. Before product detectors came along it was standard procedure when receiving CW or SSB to reduce the RF GAIN and advance the LOCAL GAIN (AF GAIN) so that the proper ratio of incoming signal to BFO injection could properly demodulate either CW or SSB. AGC was usually turned off but it depended on the receiver. With the R-390A, AGC can be left ON to limit the maximum response, if desired. If the R-390A's BFO is properly set-up, its position allows selecting either upper or lower sideband. Now, you do lose the function of the CARRIER LEVEL meter in this method of reception but who cares? The CARRIER LEVEL meter measures DB over 1uV and its accuracy depends on the RF GAIN setting. If you were planning to use the CARRIER LEVEL meter for CW or SSB signal reports, most stations wouldn't even know what you're talking about when your report was so many "db over 1uV." It's all a relative measurement anyway, dependent on the frequency and conditions. It's better to operate the R-390A as a standard pre-product detector receiver for CW and SSB and just give your contacts an estimated R-S-T report.
2. More on the subject of SSB reception and Modifications - The CV-591A family of SSB adapters were built by The Technical Materiel Corporation. These adapters work from the IF output, therefore you lose the Noise Limiter function, the 800 cycle filter function and the dual audio section of the R-390A receiver if you utilize only the audio output section of the CV-591A. Now, if you happen to have an extra speaker, you can connect one to the CV-591A output (8.0Z or 600Z) and the other one to the R-390A's LOCAL AUDIO (600Z only.) If you want to do SSB or CW you can use the CV-591A and its speaker. If you want to do AM, then use the R-390A's LOCAL AUDIO and speaker. The CV-591 will provide excellent, distortion-free SSB reproduction and they are well-worth using. The only disadvantage is the price that the CV-591A is fetching today, sometimes selling for as much as the R-390A .
On modifications to enhance SSB reception,...most of the mods that have been published do not improve the receiver's overall performance. Most modifications on any piece of vintage radio equipment will enhance performance for one area at the expense of overall performance. Besides, modifying a vintage receiver to make it operate like a "modern" piece of equipment seems to go against the whole idea of collecting, restoring, operating and preserving these classics in the first place. You're better off to learn how to use the R-390A properly and when you do, you'll find that modifications are not necessary for great performance in all conditions and in all modes.
3. On Audio Quality - Audio reproduction is not as bad as a lot of "AMers" complain it is. The mechanical filters provide a specific, very steep-sided bandwidth but some AM op-listeners are used to the "bell curve" that many early vacuum tube receivers had with only two fairly broad-tuned IF amplifiers. "Ringing" or a "hollow sound" were the usual complaints about the mechanical filter bandwidth. If you change the LOCAL AUDIO coupling capacitors to .022uf and then use a high quality 600Z transformer with a large speaker in a good enclosure, the audio sounds very nice, especially in the 8KC bandwidth (which is really close to 11KC) on AM with marginal signals or 16KC with a really great signal level (like from a retired AM-BC transmitter.) You'll have to do the same thing to the LINE AUDIO if you want to run dual audio lines to two separate speakers. If you're really into high-fidelity, then you can take the signal from the DIODE LOAD and run it through a shielded cable to a high-fidelity audio amplifier that's connected to a large hi-fi type speaker system. At 16kc bandwidth, AM signals will sound incredible. As with the SSB adapters though, you'll lose the NOISE LIMITER and 800 cycle audio filter functions with this "hook-up" unless you provide a separate speaker on the LOCAL AUDIO line (just in case you want to do CW.) For most users though, the stock audio sounds pretty good and with a good speaker the original .01uf coupling caps are fine.
|Here are some disadvantages to using the R-390A - minor stuff,
Break In Operation - The R-390A "Break In" function requires using a T-R relay with Normally Open (NO) contacts that change to Normally Closed (NC) when in transmit. This function operates the R-390A Break In relay and the R-390A Antenna Relays. The Break In function of the R-390A is opposite that of a typical receiver Stand By function that will require the T-R relay to provide a NC state for receive and NO for transmit. Most T-R relays, like the Dow-Key type, will usually provide a set of DPDT auxiliary contacts that allow connecting the R-390A Break In to one of the NO set of contacts.
The Weight Issue - No doubt, the R-390A is a heavy receiver weighing in at close to 80 lbs. Here's a hint for when you have to move the receiver. Remove the Power Supply module and the AF module. These two modules will reduce the weight over 15 lbs or more. With the covers off and the two modules out the receiver weighs about 60 lbs - much easier to move. Of course, you do have to get the receiver to the work bench to remove the covers and modules. This hint is for moving the receiver longer distances, like to another room (when you don't have a roll cart) or up and down stairs.
Cabinets - If you want the R-390A to be mounted in a cabinet you have two choices. First, is to find the proper CY-979 aluminum cabinet. This is a high-quality, military cabinet that is designed for the R-390A receivers. They are expensive. Also, the military did have a very late production run of these cabinets in the 1990s. Sometimes these are mistakenly advertised as "reproductions" but they are actually a military contract and identified as such and dated. They don't have the data tag on the top, however, and the ID is ink-stamped inside the cabinet. Once in a while these later CY-979s can be found at a lower price. If you are going for the CY-979 cabinet and willing to pay a high price be sure that the one you decide on has the shock mounts and the skids. Otherwise it's incomplete and should be priced much lower. Other than the CY-979, any other cabinet that is for 10.5" by 19" panels with a depth of 15" will also work.
Miscellaneous Information on the R-390A and its Variants and Accessories
Dual Space Diversity Operation with the R-390A Receivers
|If you're lucky enough to own two R-390A
receivers and have room for widely separated antennas, you can easily set
up the pair to operate in Dual Space Diversity. Good separation of the
antennas would be at least one wavelength at the frequency of operation but
usable diversity effect can usually be obtained with closer spacing if
necessary. Space Diversity assumes you will be using two similarly
polarized antenna and are relying only on the phase differences of the
radio wave based on the spacing of the antennas. You can also try "Polar
Diversity" which relies on a vertical antenna for one receiver and a
horizontal antenna for the second receiver. Polar Diversity doesn't
require that the two antennas be separated by great distances and
assumes that there will be a benefit from the reception of two different
polarizations of the incoming radio wave. This assumes that some
splitting and rotation of the
radio wave will occur as it propagates through the ionosphere and is
returned to earth. Generally, space diversity helps with fading signals
and polar diversity helps with phase distortion due to wave rotation.
With either method of Dual Diversity reception, the receiver set-up is the same. You will be connecting the DIODE LOAD from each receiver together. The receiver that you plan on operating as the "master" will have to have the DIODE LOAD terminals jumped while the "slave" receiver doesn't have the terminals jumped. The "slave" receiver is only operating to the detector stage and its audio output is not used. You can connect 500 ohm resistors across the LINE AUDIO and the LOCAL AUDIO on the "slave" receiver. You will also have to install the jumps to connect AGC DIV terminals together on each receiver. You will also have a wire connecting the AGC DIV from each receiver together. A speaker on the LOCAL AUDIO is only required on the "master" receiver. To listen to just the "slave" receiver, turn the RF GAIN on the "master" receiver to 0 and what you hear thru its speaker is the "slave" receiver. Also, if you want to listen to just the "master" receiver, turn the "slave" receiver's RF GAIN to 0 and what you hear thru the speaker is the "master" receiver only. With both receivers operating and connected to their respective antennas, tune in a strong shortwave broadcast signal. Have both receivers' RF GAIN set to about 8. Don't set the RF GAIN on either receiver to "full on" (10) or each receiver will "fight" the other one for control of the AGC line. By alternately reducing the RF GAIN of each receiver to 0 you should be able to end up with both receivers tuned exactly to the signal. Once the SW BC signal is tuned in on each receiver you will need to "balance" the RF GAINs. Slowly increase the RF GAIN on the each receiver alternately to the point where you see the CARRIER LEVEL meter showing some response. Adjust the RF GAIN on each receiver until you have the highest CARRIER LEVEL readings on each receiver without one receiver or the other "overloading" the AGC line. When "overloading" occurs the CARRIER LEVEL meter on one receiver will drop much lower in its reading and with a reduction in the RF GAIN of the other receiver you'll see the meter reading jump back up. By "balancing the receivers" you get the best diversity response and the best sensitivity. You will note that the two receiver's CARRIER LEVEL meters will react differently since each receiver is responding to a phase difference in the radio wave based on the separation of the antennas. You should see deep fades that cause one CL meter dip while the other receiver's meter remains steady. You should also see a reduction in phase distortion if you are using the polar diversity set up.
Remember, you can only use AM reception on this type of Dual Diversity. That's because CW or SSB reception requires the BFO to be in operation and the BFO dominates the detectors and spoils the diversity effect. For RTTY reception special TUs were used, like the CV-116 that was designed for diversity RTTY. Diversity CW reception required Tone Keyers.
So, give Dual Diversity reception a try if you can. It's interesting and sometimes beneficial to copy.
photo above: A Dual Diversity set-up with the 1956 Motorola on the bottom and the 1955 Collins on top. Both receivers are fully rebuilt (including recapping) and both are fully aligned. Though the speaker looks like a Hallicrafters PM-23, it isn't. It's a homemade wooden cabinet that houses an 8" Jensen speaker and the louvers are made of aluminum. The cabinet is painted gray wrinkle finish.
1967 Electronic Assistance Corporation R-390A SN: 974 - Restoration Log (2016)
This is a little different from my usual restoration write-ups. I've written this one in the form of a journal or log that has daily input as the project progresses. I hope this approach gives the reader an idea of the order in which the restoration-rework takes place and the problems encountered along the way as the work progresses. It will also show the reader about how long it takes to complete an R-390A that isn't in terrible condition but certainly was non-operational and incomplete. - H. Rogers, Aug 2016.
|July 16, 2016
- Ham & Hi-Fi, a vintage audio, vacuum tube and ham radio business in Sparks, Nevada,
had its semi-annual Open House Sale today. Lots of bargains and
"freebies." I was interested in a decent-looking R-390A that was priced
at $70. Not complete by any stretch but I was pretty sure I had all of
the missing parts. I asked owner Ethan, "Is this R-390A seventy dollars?,"
just to verify. "Yep!" Hmmm. I paid Ethan and went looking for a hand
truck to move the receiver out to the car. When I returned, my old
friend Mike W7MS, was giving the R-390A a real "going over." "Well, I see the
3TF7 is still there. That's a surprise. It's missing a slug rack and RF coil though. I
bet it's missing all of the crystals, too." Mike had flipped the R-390A over
when I broke-in, "I already bought it, Mike." To which we both
had a good laugh at the fact that Mike was critiquing my receiver purchase before I
could even get it off the table.
July 17, 2016 - Once I got the R-390 home I was able to inspect it more thoroughly. Mike was right,...all of the crystals had been taken out of their sockets. There should have been 17 crystals and all were missing. The cover had been screwed down so tightly, I thought that nobody had been in there,...ever. Also missing was the 4-8mc RF-Ant transformer, the 4-8mc slug rack and slugs, all of the tubes in the RF deck, top and bottom covers, the Utah plate, the front panel bushing for the KC tuning. Both meters were non-original types that were similar types but not correct. On the good side,...the dial had a security flip-down dial display cover, the 3TF7 was good, both 26Z5 tubes were present (and tested good,) the receiver was a 1967 EAC contact with the correct data plate and all of the modules were correct EAC with the correct contract number on each module. None of the sheet metal was "bashed" and, mechanically, everything looked okay. >>>
|>>> Luckily, I had a couple of "parts sets" and
extra modules so the missing parts weren't going to be too much of a
problem. The receiver was extremely dirty with loads of greasy, oily
contamination that had very fine black powder mixed in. The receiver had
obviously been stored in a garage or machine shop or some other location
where oil and fine black powder would be everywhere.
July 20, 2016 - Started complete tear-down. All modules out, front panel off, all parts plastic bagged and tagged.
July 21-23, 2016 - Cleaned front panel. I was amazed. I thought the front panel was kind of rough but it was all just the oily dirt and black powder getting into the engraved nomenclature. Careful cleaning first with WD-40 and a soft brush followed by Glass Plus to remove the WD-40 residue resulted in the front panel looking first-rate.
Complete disassembly of the Main Frame was necessary because the oily dirt had worked into the side panel joints due to loose screws. Again, under the dirt everything was in excellent condition.
|July 24 - 28, 2016 - Took RF deck outside for a WD-40 flush of the gear box. The Veeder-Root counter was very dirty and I thought the digits were damaged but, again, it was just the black dirt causing the problem. Luckily, it comes off easy with WD-40. Stripped down the RF deck by removing all 24 RF and VIF transformers, slug racks, return springs. With no load, the KC tuning was checked for "feel" which was normal. Same for MC tuning. Checked cam synchronization by setting tuning to 07+000 and found that the 4-8mc cam was way off. Probably why the 4-8mc RF coil was missing - a former owner was chasing a problem in the wrong area of the receiver (also noted that the mica capacitors had been replaced in remaining two 4-8mc RF transformers.) Mechanically reset the 4-8mc cam to correct position. Replaced the missing 4-8mc transformer and also replaced the remaining two that had been "worked on." All other cams were in alignment. Cleaned and inspected ALL 24 RF & VIF transformers checking for proper rotation of trimmer on each. The replacement 4-8mc RF coil needed to have the trimmers "unstuck" and repaired before it could be used. Cleaned all slugs of dust and any other dirt. Cleaned all slug racks and lubed cam rollers. Reassembled the RF deck, adjusted the fit of all slugs into their respective coil barrels and then checked operation of all of the cams, slug racks, slugs and cam rollers.|
|July 29 - 31, 2016
- Installed a set of crystals in the RF module-Crystal Oscillator (17
crystals required.) Cleaned rotary switches. Reinstalled the RF module
back into the Main Frame. Cleaned and lubed all controls and switches
for front panel. Remounted harness and all controls to the front panel.
Installed two #328 bulbs in the dial cover (originals gone.) Mounted front panel to the
Main Frame temporarily - checking for fit. I only installed four screws
since this panel will have to be "dropped down" for PTO end-point
adjustment and probably for other things, it's best to just mount it in
this manner for now. Original knobs were rough.
Since I had a full set of restored knobs, these restored knobs were mounted to the
August 1, 2016 - Tested the two can electrolytic capacitors and found the triple-30uf to have one defective section. The dual-45uf seemed to reform okay but the values are not very close. Same with the two good sections on the triple-30uf. I will have to rebuild these two units using new electrolytic capacitors for best reliability and performance. I had several spare R-390A can electrolytics and when testing them I found that ALL were defective in some way. Probably time to admit that you can't use and reuse the original, fifty-year-old caps anymore. New electrolytics ordered and on the way.
August 2 - 6, 2016 - While waiting for the replacement electrolytic capacitors, I tested all of the tubes and cleaned the tube sockets on the IF module and re-installed it into the Main Frame. I also had to locate tubes for the RF module since all were missing. The other modules all had W.P.M. heat-reducing tube shields so I also needed to find tube shields for the RF module tubes. All I had was IERC-type, which are very good heat-reducing tube shields, but they don't look like the W.P.M.-types. Since most of the tubes in the RF module are covered by the Utah plate, I went ahead and used the IERC-types.
August 7, 2016 - I can't find a 6DC6 (RF Amplifier) anywhere in the R390A spare parts or in any of the tube boxes. I will have to order a couple. They aren't expensive tubes but I never seem to have any NOS ones around for some reason.
August 8 - 11, 2016 - Rebuilt the two multi-section electrolytic capacitors. Photographed for the added write-up on this procedure that is now in this web-article in the Audio Module section. Picked up four NOS 6DC6 tubes from Ham & Hi Fi. Installed the remaining modules into the main frame.
Since I didn't set the Veeder-Root counter to XX.000 before removing the RF module, the PTO was not pre-set to 3.445mc. This is a minor inconvenience that requires the PTO be set by powering up the R-390A and measuring the frequency out of the PTO with a digital frequency counter. The procedure I use is above in the PTO section of this web-article.
Since I had already powered up the R-390A, I went ahead and hooked up a 600Z ohm speaker. I had lots of noise but the calibrator seemed pretty weak on 40M. I connected an antenna and tuned around 40M and heard a few SSB stations. Now this R-390A has been completely apart and is certainly quite a bit out of alignment but still it picked up a few signals. This should be a very good sign of things to come.
|August 12-14, 2016 - I've been checking out performance of the R-390A before alignment by listening to various signals on different bands. This gives everything a chance to operate at voltage for awhile to make sure everything is going to function. Since the only "repair/changes" occurred in the 4-8mc RF section with the installation of different RF transformers along with different slugs and slug rack, it was kind of a surprise that the 4mc, 5mc, 6mc and 7mc bands actually would tune in the Calibration oscillator. I was really pleased with the performance on the bands 8mc and up. I had manipulated all of the trimmers on all of the RF and VIF transformers to verify that the trimmers weren't stuck, so I was surprised that 20M and 19M SW performed quite well. I checked for the Calibration oscillator signal on all bands and it was present. I tested the end-point error on the PTO and found it to be 8.0kc. That's the greatest EPE that I've ever encountered, so we'll have to see if the compensation adjustment can correct that much error. I dropped the front panel since that was going to be necessary for the EPE adjustment. I installed all of the correct hardware to mount the correct type meters and soldered the connecting wires up for both meters. After the EPE adjustment I will be able to fully mount the front panel with all screws and washers and proceed with the full IF, VIF and RF alignment.||August 15-17, 2016
- See PTO section on Cosmos PTO. I added my experience with this Cosmos
to that section of this article. The EPE adjustment is virtually
inaccessible from the front through the locking plate and the front and
rear gearbox panels. I had to remove the PTO each time to make the
adjustment and then reinstall to test. Very time consuming. I was able
to adjust the EPE from 8kc down to 0.5kc. Remounted the front panel with
all screws, locking washers, shaft bushings, etc. Checked output on the
Crystal Oscillator section and adjusted all trimmers for peak output.
August 18-20, 2016 - Peaked the mechanical filter inputs and outputs. This requires having the IF module dismounted but still connected to power. Photo and method described in "IF Module" section further above in this article. All mechanical filters were pretty close so, just minor tweaking.
August 21, 2016 - Completed the full alignment. Most adjustments were pretty close but, as expected, the 4-8mc section was quite a bit out of alignment. Installed the Utah plate, top and bottom covers. Connected receiver to the full-size ham antenna. 40M reception is normal now. All other bands are functioning correctly. Adjusted Carrier Level Meter and IF Gain for best performance.
|1967 EAC Performance and
Observations - Here's what I've noticed on this receiver that
is somewhat different than the earlier versions, such as the Collins or
1. Components - many ceramic disk capacitors in RF and IF modules. These modules also have several capacitors that appear to be metalized film capacitors. The AF module appears to have similar capacitors to the old Vitamin-Q types but I think the construction is different with better seals. Certainly the multi-section capacitors are of the same construction and questionable reliability. They have the same problems that are found in any electrolytic capacitors that are half-a-century old. Overall, the capacitors seem to be better types than those used in the R-390As built in the 1950s. Cosmos PTO is difficult to adjust the EPE due to the new location of the L701 adjustment behind Z702.
2. Performance - is definitely equal to a rebuilt and recapped earlier version receiver. IF Gain is set at about 60% which is pretty close to where it's adjusted on the recapped earlier versions. With IF Gain set at 60%, most SSB signals demodulate nicely with the RF Gain at about 5 to 7 and the AF Gain at 7. This is using a 135 ft center-fed tuned inverted-vee antenna. SW BC stations usually run about 50 to 60db on the Carrier Level meter depending on the station and the time of day. The coupling capacitors in the AF module were NOT changed to .02uf but the audio sounds very good with noticeable bass response on AM BC and SW BC. Also, strong SSB stations and AM ham signals seem to have good bass response with the original .01uf coupling caps. Overall, a nice performing R-390A that is going to be set up with one of my ART-13A transmitters for awhile.
Time to Complete Project - It took just about one month to complete the rework on the 1967 EAC. This is from a non-operational, incomplete receiver to an entirely functional and totally complete receiver. I didn't work on the receiver everyday so total time actually spent on the project was probably around 20 hours.
Update - Sept. 4, 2016 - I guess I should have cleaned the Antenna Input relay contacts. The procedure is described in the Main Frame section further up this page. It's not difficult to do, even if the receiver is already back together. The symptoms were no (or very little) carrier level indication, a change in the normal position of the ANT TRIM for resonance and relatively weak signals. If STAND BY or BREAK IN were actuated then the signals would return to normal levels. This was the typical indication that the antenna relay contacts were introducing some resistance due to poor contact. In this particular case I don't believe the cause was oxidation because inside the arm, NC and NO were very clean looking. I used just a slight bit of DeOxit and paper to clean the contacts to have them measure zero ohms. Problem might have been some kind of rosin-like coating or something that dissolved with DeOxit. Other than this minor and easy to correct problem, the '67 EAC R-390A has been performing very well.
Other R-390, R-390A Variants and Accessories
R-392/URR - Component Receiver for the AN/GRC-19
AN/GRC-19 - The AN/GRC-19 was a portable transmitter-receiver that utilized the Collins-designed T-195 transmitter, a 100 watt output, completely autotuned antenna matching marvel that was combined with the Collins designed R-392/URR receiver, also a marvel of packaging most of an R-390 receiver into a cabinet about half the size of the standard R-390. Although separate units, the T-195 and the R-392 were interconnected via a Power Input-Trans Cont cable that allowed the T-195 to power the R-392 receiver. Both units mounted to a large shock mount that was typically bolted to the back of a Jeep or other type of vehicle. The GRC-19 had to be almost weather-proof and the R-392 is virtually a sealed unit. Most R-392 receivers are in great condition inside because of the weather-proof housing. The entire receiver operates only on +28vdc.
R-392 Circuit - A stout, small and fairly lightweight receiver, the R-392 still has a lot of the features found on it's big brother, the R-390. Frequency coverage is .5mc to 32mc in 32 tuning ranges each with 1mc of coverage. Permeability tuning using slug racks driven by a complex gear train with a PTO, variable tuned IF and fixed Crystal Oscillator providing double and triple conversion is very similar to the R-390 receiver's front end as is the frequency read out provided by a Veeder-Root digital counter. 25 tubes are used in the double and triple conversion circuit that also provides 2 RF amplifiers and 6 IF amplifiers. Also, the IF stages are similar to the R-390 in that mechanical filters are not used for the selectable 8kc, 4kc and 2kc bandwidths. Data modes, e.g., portable RTTY, could be received via the IF output connector (the T-195 was capable of FSK transmission.) The Audio Output is 600 Z ohms and accessed from either of two twist-lock type connectors marked AUDIO or it can also be accessed from the POWER INPUT-TRANS CONT (PI-TC) connector. There is no phone jack on the R-392 because in the GRC-19 configuration the audio was routed to the T-195 (via the PI-TC connector) where typically a telephone handset, the H-33, was used for both transmit (microphone) and receive (earpiece.) The typical field speaker, if used, was the weather-proof (and terrible sounding) LS-166. A Noise Limiter circuit is activated with the Function switch and a Squelch function is also available. When operated as the GRC-19 there is a short interconnecting cable between the T-195 transmitter and the R-392 receiver using the PI-TC connector that allows the two units to function together with the T-195 providing Break-in or Stand-by functions along with receiver to transmitter Signal Relay capabilities.
|photo left: This is the upper deck of the R-392. The modules
are the RF Module that has all of the RF transformers, the tuning slug
racks and slugs along with the variable IF transformers and its slug
racks and slugs. The module the has all of the trimmers is the Crystal
Oscillator module. Although reduced in size, these modules are very
similar to the R-390/URR receiver.
photo right: This is the lower deck of the R-392. The tuning gear box is visible behind the front panel. Center left is the Crystal Calibrator module, then the PTO and center right is the Audio module. At the rear top is the IF module. Like the R-390/URR, the R-392 modules interconnect using cables and plugs. All modules are secured using captive screws that have their heads painted green. The blue dots on the tube tips are my indication that the tubes have passed a tube test.
Operation of the R-392 as a Stand Alone Receiver - To operate the R-392 as a "stand alone" receiver, a separate +25vdc to +28vdc power supply will be required and it should be capable of at least 3 or 4 amps. The typical +24vdc computer-type power supply with the voltage adjusted up to +26.5vdc will work fine. BE SURE TO USE AT LEAST +25VDC - - - +26.5VDC IS BETTER! Receiver performance will begin to drop off as the supply voltage is reduced below +25vdc and the R-392 will barely function below +24vdc. The GRC-19 system was designed to operate with the vehicle running or with some sort of charging system used with a 24vdc battery set-up. The typical battery-charging system voltage would have been around +28vdc (although this depends on the engine RPM and the condition of the batteries.) The GRC-19 will only operate marginally at +24vdc since the T-195 is spec'd at +28.5vdc input. When operating the R-392 as a "stand alone receiver" the operating voltage is applied directly to the receiver power input rather than through the GRC-19 system (which usually had some IR drop in the cabling to the receiver power input, thus the +28.5vdc was somewhat less at the receiver power input.)
More "Stand Alone" Info - When the receiver was in active use with the military in a GRC-19, it wasn't really a problem that the supply voltage to the R-392 was a bit high. After all, better gain was then available at the receiver and, at the time, the tubes were easily available. Today, the tubes are still fairly cheap but why abuse them unnecessarily? Although the apparent gain of the receiver can be increased by running the supply voltage at +28vdc, most of the tubes utilized in the R-392 are 26.5 volt filaments and a properly operating R-392 will function great at +26.5vdc supply voltage. This assumes that only one power supply is going to be used and LINE and PLATE are connected together. Increased performance is possible by operating the LINE at +26.5vdc and operating the PLATE at a slightly higher voltage, up to maybe +30vdc. This requires two power supplies and separate voltage wires in the power cable going to the PI-TC connector.
IMPORTANT NOTE ON TESTING 26.5 VOLT TUBES: When testing the 26.5 volt tubes, be suspicious of readings using a typical mutual-conductance tube tester, e.g., the TV-7, etc. Some tubes will show very little gain, perhaps as much as 60% lower than minimum specified useable test level, and yet these tubes will function fine in the R-392. This is probably due to the tube tester's method of powering the tube versus the R-392 circuit's application of DC voltage on the heaters with +28vdc plate voltage. The best indicator of the tube's usability is by substitution in an operative receiver. Naturally, tubes that read high on a tube tester are going to work best but don't necessarily discard the 26.5 volt tubes just because they show "bad" in a tube tester. Try them in the receiver, you might be surprised.
|Servicing the R-392
- Since the R-392 circuitry is essentially sealed from the outside
world, most of the examples found are in excellent physical condition.
Since the environment has been kept out, the typical corrosion or
oxidation is not normally encountered. This results in a receiver that
is usually very easy to service and get operational. Here's a list
1. Test all tubes on a quality tube tester. Be aware of the caveat on the 26.5 volt tubes and how they will test on mutual conductance tube testers. Clean all tube sockets with De-Oxit. This is probably not necessary on most R-392 receivers but is a routine problem preventative measure.
2. Disconnect all cables and clean sockets and plugs with De-Oxit. I usually pull all of the modules and check everything over. I clean the bed plate although usually it's not all that dirty. If the interior of your R-392 is dirty or the receiver has been out of the cabinet for a very long time, you may want to pull some of the RF transformers to inspect their contacts. If cleaning is necessary, the procedure is very close to that used on the R-390A RF module and is covered extensively in this web-article in the "RF module" section further above.
3. Check operation of the gear box. The feel of tuning the receiver should be fairly light but probably not quite as light as a recently lubed R-390A gear box. It's pretty much the same box and the Veed-Root counter is exactly the same as the one used in the R-390A receivers. If the R-392 gear box seems tight then it might benefit from a cleaning and light lube. They are usually pretty well preserved since the receiver is in a sealed cabinet. Again, the gear box cleaning and lube procedure is above in the section "RF Module" further up this web-article.
4. Check operation of all switches and pots. Lube switches with De-Oxit. Pots are usually sealed (Allen-Bradley types) but if they aren't then give them a shot of De-Oxit.
5. Reinstall the modules. Connect all of the cables. Reinstall the tubes. Power up the receiver.
|Alignment of the R-392
- If you've aligned the R-390 or R-390A receivers then aligning the
R-392 will seem very familiar. It's essentially the same receiver but
packed into a much smaller case. Use the alignment procedure in the
military manual. Essentially, these are the steps for alignment.
1. Check mechanical alignment. Make sure the PTO and the gear box are close +/-15kc between maximum and minimum span.
2. Adjust Carrier Level Meter first, then proceed to IF alignment. This is like the R-390 (non-A) in that the IF is peaked rather than stagger-tuned. This is because there are no mechanical filters in the R-392.
3. Variable IF alignment, then the RF alignment.
4. Synchronize the PTO. Check that the Crystal Oscillator is adjusted to give very little error when going from band to band.
5. Adjust Calibration Oscillator.
Variations in the R-392 Receivers - The initial contract in 1951 was from Collins Radio Co. but soon, just like the R-390 and R390A, many other contractors built the R-392 receivers. There are some variations from early production to the later receivers. Early receivers will use 26A6 tubes for the RF amplifiers while later production used an improved version of that tube, the 26FZ6. The change to the 26ZF6 was to help with cross-modulation problems when using the receiver near operating transmitters. Most of the later manuals specify that either the 26A6 or the 26ZF6 can be used as RF amplifiers. Early panels have silk-screened nomenclature while later panels are engraved. The 2kc-4kc-8kc BANDWIDTH nomenclature layout is closer together on early panels but spaced at 90º on later panels. Cabinets on early models have large flutes that run front to back while later cabinets have five "ribs" that entirely encircle the cabinet running parallel with the front panel. These "ribs" strengthened the cabinet significantly. Like many contractor-built items, the color tint of the olive drab paint used varies from contract to contract with some receivers appearing very light brownish-OD while others appear dark greenish-OD. R-392 production ended in the mid-1960s. Ample sensitivity, super-accurate frequency readout and decent audio (from a good speaker - not the LS-166) not to mention the "extreme" military looks along with a small and lightweight package (well,...52 lbs) have made the R-392 a popular receiver with many military collectors and even some BA collectors.
|The R-648 is the so-called "Airborne R-390A." Certainly the nickname
comes from the intended use of this Collins-design/build along with its
many similarities of construction and operation to this receiver's "bigger brother." Since it was destined to be used in aircraft, the R-648
had to be light-weight. Rather than the 80 pound, hefty-weight of the
R-390A, the R-648 weighs-in at about 30 pounds. To achieve this lighter
weight package, the receiver is much smaller, with reduced-size
mechanics and components. Of course, many of the R-390A features and
circuits are not even present. Still, with 17
tubes that provide two RF amplifiers, three 500kc fixed-frequency IF amplifiers, frequency coverage from
190kc to 550kc and from 2mc to 25mc, two mechanical filters (9.4kc and
1.4kc - the 9.4kc MF was later changed to 6.0kc) and a
mechanical digital frequency dial, the R-648 does have quite a few
"R-390A" features. Double conversion is used on all frequency ranges
except for one range, the 2mc to 3.999mc tuning range where single
conversion is used. Audio output has three stages and was designed for a headset of 300Z ohms
or greater. Typical antenna input Z is 50 ohms.
operated on +28vdc battery-charger buss available on the aircraft. The +28vdc buss supplied
the series-parallel connected tube heaters, the dial lamps and also operated the
"on-board" dynamotor. The dynamotor supplied +250vdc at 100mA. A 0A2
was used to also provide regulated +150vdc from the +250vdc. A divider network also
provided +31vdc for the AVC bias. Unfortunately, like a lot of the dynamotor-operated military
gear, many R-648 receivers have been converted to operate on an AC power
supply. Some modifications were well-designed and their incorporation
into the receiver was accomplished with minimal modification to the
receiver while still providing the necessary voltages. Most AC power
supplies utilize the original dynamotor module chassis since this then has all
of the circuitry for the 0A2 regulator and the divider network for the
30vdc. Usually two power supplies and two transformers are necessary, one for the +250vdc
supply and one for +24vdc for the tube heaters. Some better conversions utilize a DC-DC converter to provide the
+250vdc and then the receiver is operated on +24vdc input. In essence, providing
a solid-state substitute for the dynamotor only. If you don't have the
original dynamotor, this is probably the best solution to keeping the
R-648 as original as possible.
Shown to the left is the top chassis of the R-648. Directly behind the front panel is the gearbox and the RF module containing all of the RF transformers, VIF transformers, slugs and slug racks. To the right of the RF module is the PTO. To the right of the PTO is where the dynamotor module should be but this receiver has an AC operated power supply that provides +24vdc and +250vdc to operate the receiver. This PS is built on an original dynamotor chassis. Behind the PS is the Audio and Calibrator modules. To their left is the Crystal Oscillator module and to its left is the IF module with the two mechanical filters. The front panel also "plugs in" and has enough wiring to be considered another module. The front panel hinges down for access.
|As to the R-648 operation, the GAIN control operates RF gain when in CW
and AF gain when in AM. The Sensitivity is a screwdriver adjusted pot
accessible on the front panel behind a "toilet seat" cover. The overall
Audio Gain is a screwdriver adjusted pot accessible on the rear panel.
The audio output impedance works well with 600Z ohm loads and a
loudspeaker with a 600Z ohm matching transformer will have plenty of
volume. The manual doesn't specifically give an output impedance only
stating that the load should not be less than 300Z ohms.
Selectivity is controlled by two mechanical filters. On early versions, the AM filter is 9.4kc. This was later changed to 6.0kc to improve selectivity in the Voice mode. CW selectivity used a 1.4kc mechanical filter. The selectivity is automatically switched between the two mechanical filters when either VOICE or CW is selected by the EMISSION switch. The "SHP" positions are "sharp" selectivity and add an audio filter to reduce bandwidth to improve copy in noisy conditions.
Although the R-648 is called "The Airborne R-390A," don't expect the receiver to have the robust construction or all of the capabilities of its namesake. The R-648 has ample sensitivity, adequate selectivity in most circumstances, a minimal amount of knobs to adjust and lots of volume available if the proper load impedance is supplied. Its light-weight and small size make it easy to find a place for on the bench.
NARF NORVA - NARF stands for "Naval Air Rework Facility" and NORVA stands for "Norfolk, Virginia." These stickers showed the receiver rework dates. The first quarter of 1975 is marked on this tag. Also note "IRAN." At that time, 1975, the Shah of Iran was our ally and Norfolk was contracted to repair and rework electronics not only for Iran but also for the UK (and several other allied countries.)
Other R-390 and R390A Versions
|R-389 - VLF, LF & MF
version of the R-390. Two tuning ranges. Motor drive tuning, if
selected. 15kc to 500kc and 500kc to 1500kc.
R-391 - Motor tuned, seven channel presets, if selected. Manually tuning also. R-390 based. 500kc to 32mc
R-391A - same as "non-A" except based on R-390A.
Other Miscellaneous Versions - There was also a version, possible an end-user conversion, that replaced the IF module of the R-390A with an IF module from the R-390. This provided the user six IF stages and eliminated the use of mechanical filters. Stewart-Warner may have been involved in this version.
Additionally, some R-390A receivers will be found with a ten-turn vernier dial installed on the BFO. Sometimes this is referred to as the "CIA" or "NSA" conversion but there's no confirmation that the source of the turns-counter dial on the BFO is from either agency. Sometimes this is also referred to as the "RTTY" conversion. Again, no confirmation.
There was also a US Navy modification that added a front panel pin jack that will be ink-stamp ID'd as "DIODE LOAD." This was supposedly added by the USN to make a routine check on the receiver easier. The diode load could be measured at the front panel without removing the receiver from the rack mounting (or the necessity of somehow gaining access to the rear panel.)
Security Dial Cover
|The Security Dial Cover is found installed on R-390A receivers once in a
while. The "cover" mounts using the two upper screws of the dial cover
and is hinged with spring-loading so it can be placed in almost any
position and remain there. All are painted black and all have a
cream-color felt pad on the inside to prevent damage to the receiver dial cover
What were these used for? The most-often-heard purpose is for preventing unauthorized (or unintentional) observation of a listening frequency by radio surveillance room visitors that didn't have the necessary security clearance. This didn't necessarily mean "spies" or "moles," it was just that in certain radio surveillance rooms, the frequencies monitored were classified and not everyone that might have a valid reason to come into that radio room had the necessary security clearance for knowing the frequencies monitored.
The usual procedure for the radio operator was to raise the cover, tune to the desired frequency and then lower the cover back down. That way the actual tuned frequency was only visible for a short time. Sometimes the covers were left up but immediately lowered if anyone entered the room.
It was also possible to use these covers in a "lights out" situation where the receiver had to be kept in operation. The lowered cover would block the light from the illuminated dial and kept the area darkened.
LS-206A/U Rack Mount Dual Speaker
|So-called "Diversity Speaker" - These dual speakers were
built by a couple of different contractors, Crosley Radio Corp. was one and Oneida
was the other. The Crosley versions are the earlier versions and have circular cut-outs with a maroon
grille cloth. The later Oneida versions have a pattern of holes that
create a square opening for the speaker with a beige grille cloth behind
the hole pattern. Both types of
speakers have matching transformers that provide a 600Z ohm load for the
input. The channel switches will silence that speaker selected by switching from
the speaker line to a 600 ohm load resistor. The front panel to the
housing mounting is entirely sealed with gaskets and the nuts used are
all nylock types. One would think that the LS-206 cabinet is sealed but
it isn't. The cables exit out grommet holes to attach to the terminal
strips. Also, the speakers aren't water-proof.
If you have an LS-206, one use for it is to connect one speaker to the LOCAL AUDIO output and then connect the other speaker to the LINE AUDIO output. That way you can run both audio outputs from your R-390A and also operate both speakers in the LS-206 simultaneously.
The LS-206 was a component part of the AN/GRC-26 Mobile RTTY Communications Hut. The receiving portion of the GRC-26 had two R-390 receivers that were rack mounted with a CV-116 Diversity RTTY Converter and the LS-206. The dual speakers allowed the operator to monitor the signal while tuning in each receiver. Once the RTTY signal was tuned in on each receiver the speakers could be turned off.
photo above: The Oneida Electronics version of the LS-206A/U. The contract number on this LS-206 is 15488-PP-63 implying that the contact date is 1963. Serial number on this unit is 19.
CY-979/URR Cabinet Electrical Equipment
The CY-979/URR cabinet was specifically designed for the R-390 and the R-390A receivers. Either receiver fits into the enclosure perfectly with a "wrap around" fit to the back of the receiver which provides a good seal along with screened ventilation louvers to keep out most live insect invaders. The cabinet is made out of aluminum painted smooth gray. The original CY-979 included "skids" that were part of the mounting system. Each skid had a slight up-bend on each side that would slide into the receiving part of the mounting system and thus secure the cabinet in position. The skids were made out of heavy gauge steel painted gray. The shock mounts were made by Barry Industries and bolt to the skids with four flat-head screws and nylock nuts for each mount. The cabinet mounts to the top of the shock mount with a single bolt. Some versions of the CY-979 have a metal data plate that is mounted top-center of the cabinet. Other versions will have a decal data tag in the same location. The decal data tag is shown in the photo below.
Most of the CY-979 cabinets were built in the 1950s and early-1960s. During the 1990s, there was a contract issued for a new run of CY-979 cabinets. These newer CY-979 cabinets are identical to the originals except for the data plate or data decal. The new versions are ink rubber stamped on the inside to identify cabinet and also the contact date. Sometimes these newer CY-979 cabinets are called "reproductions" which, in a way, they are, but these new CY-979 cabinets are official military contracts and are identical in appearance and functionality to the originals.
Many times you'll run across CY-979 cabinets that are missing the shock mounts and the skids. This seems to have been commonly done to make the cabinet look less "military." If the CY-979 is incomplete, that is, missing the shocks and skids, it should be priced substantially less than what the complete CY-979s sell for (which is a lot!)
|Conclusion - The R-390A was "the ultimate" vacuum tube receiver that could be built in the 1950s and 1960s. It used absolutely the best parts in its construction and its design provided the best performance available. The R-390A was able to cope with any reception conditions from terrible atmospheric noise to deliberate interference and was able to successfully achieve good copy under the worst imaginable conditions. Operating an R-390A today is a pleasure that is derived from knowing that your receiver was built to exacting specifications by some of the foremost American companies and that its performance is the best that could be achieved using vacuum tube technology. Also, in knowing that it would be literally impossible to produce such a receiver today. The cost would be staggering and complexity something most companies would either be incapable of building or something they would want to avoid building due to the excessive cost factor. Admire your R-390A in knowing that it could only have been built during one specific era - when vacuum tube technology and vacuum tube receiver designs were at their zenith. At a time when this country produced the most advanced electronics apparatus in the world and our government was willing to provide the military with the very best equipment, regardless of cost.||References
Hard Copy Material:
1. Military TMs, Navy Manuals, etc. - the originals provide detailed information on all aspects of receiver design, operation and repair. There are several manuals available and the proper one for your receiver depends on when the receiver was manufactured. Check the publication date of the manual versus your receiver build-date.
On Line Material:
1. www.r-390a.com - the most complete instructions for gear box teardown and reassembly, lots of other information, too.
2. Literally, there is so much data on the R-390A on the Internet it's impossible to list it all. Just do a search on "R-390A" and you'll have pages and pages to look through not to mention photographs galore.
3. Thanks to all of the R-390A enthusiasts that have provided information over the years either by e-mail, eye-ball or over the air conversations.
Henry Rogers/ Western Historic Radio Museum © March 2012, major additions July 2012, trimmer cap info May 2013, R-390 Slug Problem Aug 2014, add 1967 EAC restoration Aug 2016, add sections on R-392, R-648 and LS-206 Aug 2016
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