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Radio Boulevard
The Main Frame - The RF Module - The IF Module The AF Module - The Power Supply Module - The PTO Front Panel Restoration - Alignment - Performance Today
photo:1955 Collins contract R-390A |
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| 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 the being able to operate one of the best performing vacuum tube receivers ever designed. - Henry Rogers - Feb. 8, 2012 |
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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. |
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| R-390A/URR
- 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. Most of the maintenance "quirks" of the R-390 were
corrected in the R-390A. 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 easier, it was still a complex receiver. 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 designs 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. >>> |
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Although the R-390A will probably operate pretty well on all original
parts, it certainly has some of the many paper capacitors exhibiting
some 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. 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.
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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. |
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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 they are 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
1960s and later will probably not have the molded capacitors. These
modules have capacitors that appear to be film-type caps are installed
instead. Aerovox is one of the manufacturers of these later capacitors.
Since these capacitors are going on half-a-century old, even if they are
film caps, they probably should be replaced. The more diligent
rebuilders also point out that 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 are "self-healing" in that
they 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 the R-390A receivers 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 has changed my mind. I rebuilt the 1961 Capehart and didn't replace the paper capacitors. It works and seems 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. It's performance is "light-years" ahead of the Capehart (which is going to also be subjected to a thorough rebuild, including capacitor replacement, this summer.) 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. |
| 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'
sockets) will
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 lower green head screw that secures the harness clamp. 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. 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. |
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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. |
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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. |
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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
module performs.
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 contact receiver. Note the Motorola ID in the upper right corner of the front gear plate. |
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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. 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. |
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 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
through
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
uncommon to
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
change.
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. |
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2 |
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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. |
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| 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 clip to disengage the rotor pin. Looking at photo 2 the
clip would be moved to the right to disengage the rotor pin. 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. |
1 Photo 2 - The rotor clip secures the assembly Photo 3 - Gasket and the stator contact pins Photo 4 - Stator placement over gasket |
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2 |
3 |
4 |
| 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 you want to replace the three paper capacitors, 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 but these should probably also be replaced, if encountered. 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. 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. |
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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 that 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. |
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| 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, 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 powering the receiver up without first checking out the condition of C-553. Luckily, only one mechanical filter was destroyed. |
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| 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 DDM 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. |
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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
complete "parts
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 are a lot easier to deal with. However, it will be noted that the trimmers for the input 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. Leave all of the coaxial cables connected except those removed as described in the manual's procedure for IF alignment. Leave the 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 and rest the front of the IF module on the upper edge of the front panel. If you position it right, you'll now have access to the four filter input trimmers through one of the holes in the side panel. After peaking these adjustments, return the IF module back to its normal position and reinstall the shafts and tighten the captive screws. |
| 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 wires 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. |
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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 - If you don't rebuild the two filter capacitors, at least fully test and reform them. I use an adjustable Lambda 25 power supply that has a variable 200vdc to 375vdc output. I connect a 10mA FS current meter in series to watch the current draw as the capacitor is reformed. Only do one section of the multi-section capacitors at a time. Due to the limitation of the Lambda 25 starting out at 200vdc, I check the capacitor with another power supply that adjusts from 0vdc up to 50vdc. Then I switch over to the Lambda 25. When the electrolytic is first connected to the power supply, the current draw will be fairly high (probably around 100mA) for a very short time. Almost immediately the current will drop down below 10mA. Keep increasing the voltage every few seconds. The current will increase and then almost immediately drop down. When you've reached +300vdc on the section, watch the current. It should slowly drop to below 1mA. Let the voltage remain on the capacitor section for an hour or so. When checked after an hour, the current should be around 500uA or less. This will indicate a good condition capacitor that is reformed. If you have more advanced capacitor checkers that can test the power factor of the capacitor, do so. I usually fully discharge the tested section and then measure its capacitance. If it is within spec, I consider 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'll notice a random "popping" 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. |
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. 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.
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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.
Many
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 giving a thorough inspection. |
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Setting up the PTO |
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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.) Now,
loosen the clamp on the gear box side of the oldham coupler. Next,
disconnect the PTO cable from the RF module and unplug the power cable
at the PTO. Loosen the captive screws in the front of the PTO. Remove
the two screws that mount the rear bracket to the bedplate. Now, pull
back on the PTO body until the oldham coupler comes off of the gear box
tuning shaft and 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 coupler clamp is tightened. In this manner, the
mechanical setting 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 frequency on any of the MEGACYCLE settings. Synchronizing the PTO to the RF Module - If you have to do some work on the PTO or if the shaft has been moved and you don't know where it needs to be, 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. After the PTO is set, then tighten the clamp on the oldham coupler. You can monitor the PTO output by disconnecting the PTO output cable from the RF module and attaching a DFC to the BNC connector on the end of the cable. Doing this procedure without a digital frequency counter is
much more difficult. In fact, the manual says to use another R-390A
receiver to tune in the output of the PTO. If you don't have the DFC,
try to borrow one as it
will make the procedure easy and accurate. |
| End-Point Error Adjustment - 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 quality 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 compensation capacitor, then reinstall the PTO and dismount the front panel (lower it down) for access through the hole in the front gear plate back to the PTO compensation capacitor adjustment. You'll need a long, thin 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 was 4.0 kc. Most EPE encountered are around 1 or 2 kc. The TM manual will give examples of which way to turn the EPE compensation capacitor based on whether the ten-turn coverage is greater than or less than 1.000MC. Make a small adjustment to the EPE compensation cap 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. Now, recheck your EPE. If you've adjusted the compensation cap in the correct direction your EPE should be less. Repeat the procedure until you've gotten the EPE to better 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 capacitor adjustment. Remount the front panel. Pretty easy when compared to the hassle of doing a 70E-15 PTO from an R-388. |
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Front Panel Restoration |
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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. 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. |
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| Carrier Level
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 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. Grab Handles - 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.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. 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, loosen these bearings to make sure that the threads are not locked or that there is some other problem that would prevent moving the position of the bearing after the panel is mounted. 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, now note how the bearings can be moved within the feed-thru mounting hole. Also note, where the bearing is in relation to the 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. 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. |
|
Other Details |
| 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 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 |
| Initial Power-up
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. 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 should 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 loss per foot. 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. |
|
Expected Performance |
| I've used many R-390A receivers in my various ham radio station
set-ups over a period of several 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. 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 and other Communist countries (they were our enemies,...then) 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 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 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 it's 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 - Though at one time I had three of the CV-591A SSB adapters, I sold them all because of the disadvantage of using them. 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 receiver. You have to use the audio section in the CV-591A. Now, these units do provide excellent SSB reproduction but that's about all. You're better off just using the receiver as it was designed, without mods or additional equipment, for SSB copy. 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 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 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 loose the NOISE LIMITER and 800 cycle audio filter functions with this "hook-up." |
| Here are some disadvantages to using the R-390A - minor stuff,
really,... 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 and 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. |
| 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. 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 converations. |
Henry Rogers/ Western Historic Radio Museum © March 2012, major additions July 2012, trimmer cap info May 2013,
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