Collins Radio Company
Series of Receivers - Part 2
70E-15 "M" PTO vs "CR" PTO, BFO Inversions, Break-in Set ups,
by: Henry Rogers WA7YBS
51J Series - Oddities
No receiver is perfect and the 51J Series receivers do have a few "oddities" that can cause user frustration during set-up and operation. The sloping top on all 51J receivers almost necessitates using some type of cabinet if you like to utilize the area on top of the receiver for various accessories. The oldham coupler can cause some minor backlash issues in the tuning. The five "oddities" that cause the most problems are covered below. The PTO end-point error is probably the most serious issue but, luckily, it almost always only affects the R-388. The PTO serial numbers involve both the R-388 and the 51J-4. The Break-in is a headache but only affects the R-388, 51J-3 and 51J-4. The BFO "inversion" will complicate selecting the proper upper or lower sideband and is a function of the front-end design so it affects all versions of the 51J receivers. The audio issues are integral to the 51J design and affect all versions on the 51J Series. The R-388, 51J-3 and J-4 might cross-modulate in the presence of a strong RF field when used with a large, non-resonant antenna.
Break-in Operation for the R-388, 51J-3 and 51J-4 - What a Headache! - Perhaps the most inconvenient requirement for the R-388, 51J-3 or the 51J-4 is how to provide the Break-in function. Break-in requires an external +12vdc 200mA source that is switched "on" when in the "transmit" mode to isolate the receiver input and to mute the audio output. Inside the receiver, relay K101 is operated by the Break-in voltage and the relay contacts disconnect the antenna input line, ground the receiver input circuitry and also remove the voltage from the IF amplifier plates to mute the receiver. There is also a neon bulb as a protection device on the antenna line but this is mainly a static voltage drain and not intended to handle large amounts of RF energy. Break-in should be used when the receiver is paired with a transmitter and both are using the same antenna switched by an external T-R relay. Usually, the T-R relay does provide positive isolation and many Dow-Key T-R relays had an additional spring-loaded contact switch inside the receiver-side coaxial fitting barrel that further increased receiver isolation. These types of T-R relays afford protection when used with the Standby position for muting in the short term and it's fine for testing and preliminary set-ups but, ultimately, the permanent set-up should be using the Break-in with the T-R relay to provide a positive "double" disconnect of the antenna line and providing the grounding that protects the receiver input,...just in case there's a T-R relay failure and arcing happens inside the relay.
IMPORTANT NOTE: The front panel STAND BY position does not actuate the Break-in relay. The Break-in relay can ONLY be operated by applying +12vdc Break-in voltage to pins 2 and 3 of the REMOTE terminal strip (pin 1 provides a ground for a cable shield.)
NOTE 1 - If you can't use an external +12vdc for Break-in: There are a couple of methods that provide the +12vdc "Break-in" within the receiver. Either a small power supply mounted under the chassis of the receiver or a voltage doubler circuit that runs off of the 6vac tube heater supply. Since < 200mA is required to operate the relay K101, very small components can be used for either power supply circuit. See "R-388 Expected Performance and Operation" further down in the "Rebuilding the R-388/URR Receiver" section.
NOTE 2 - Sensitvity down on 80M and 40M?: If your R-388 or 51J-4 receiver seems to have somewhat less sensitivity on Bands 4 through 7, which happens to be the bands that include the 80M and 40M ham bands, check L-104 for burn marks and for its DC resistance. Refusal by some hams to use the "Break-in" function and instead relying on a T-R relay for isolation (along with using the STAND BY for muting) can lead to accidental high levels of RF making it to L-104, the Antenna coil for Bands 4-7, causing damage. Typically, the receiver still works but sensitivity will be down on Bands 4-7. A check will involve testing reception levels on Bands 8 and higher and noticing an increase in sensitivity on those bands. Also, check the response using the Calibration function. If received signals seem down on 4mc but switching in the Calibration oscillator shows a good response on the CL meter, then suspect damage to L-104 from excessive RF levels (and not using Break-in.) The Calibration oscillator is injected at the grid of the RF amplifier tube which is after L-104 in the signal flow. Be sure in your testing that the lack of sensitivity isn't due to an antenna mismatch. The final test and confirmation requires using a RF signal generator to input known signal levels and to actually confirm that the sensitivity is down on Bands 4-7. When inspecting L-104, often excessive RF damage can be seen as melting, discoloration or burn marks on the coil. Sometimes the excessive heat will only change the DC resistance of the wire of the coil and the damage shows up as an increase in the DCR of the coil. Another observation is that even though the damage may have happened during 80M operation, the damage also affects 40M sensitivity and vice-versa.
NOTE 3 - The 51J-1 and 51J-2 have conventional stand-by provisions providing external relay connections that parallel the front panel STAND BY switch. The auxiliary contracts on the T-R relay can provide the remote Stand By function. There isn't any antenna input protection circuitry provided in these early version receivers.
BFO/IF "Inversion" - Selecting Upper or Lower Sideband with the 51J - The double conversion process in the 51J circuit involves mixing the incoming RF with a Crystal Oscillator (using fundamental and harmonic frequencies) to create Odd and Even Bands for the dual Variable IF that are in turn mixed with the PTO output. The PTO tunes from 3.0mc to 2.0mc but the Even Bands use the Variable IF that tunes from 1.5mc to 2.5mc. This requires using the formula PTO f - Vari IF = +500kc fixed IF (mid-range example 2500kc-2000kc=+500kc.) However, an Odd Band uses the Variable IF tuning 2.5mc to 3.5mc and this requires using the formula PTO f - Vari IF = -500kc fixed IF (mid-range example 2500kc-3000kc= -500kc.) The 500kc fixed IF signal is actually changing phase (180º) between Odd Bands and Even Bands. Since the BFO is a 500kc oscillator heterodyning with that 500kc IF, when the 500kc fixed IF phase changes, the BFO phase relationship inverts. This shows up, for example, as tuning in an USB 14.200mc SSB signal will require the BFO to be to the right of the center index line for proper demodulation. But, if an USB 15.035mc SSB signal is tuned in then the BFO must be to the left of the center index line for proper demodulation. Since there aren't any calibration index lines to comprise a scale for the BFO position, the 51J design never intended using the BFO for selecting a specific sideband or in providing a "calibrated position" for the BFO (as with the later R-390 and R-390A.) It's really not that much of a problem but it's something that's noticed quickly when first operating a 51J receiver nowadays and tuning in SSB signals.
Even Collins had a hard time with the Odd-Even Band changes and the relationship with the BFO operation. When the 51J was designed at the end of WWII, a BFO was only used for CW reception and, since a CW signal has no sidebands, the position of the BFO didn't matter as long as it provided a heterodyne. By the time the 51J-4 was introduced, SSB had been developed and was being promoted by Collins. The 51J-4 manual has a specific set of instructions for determining what Collins thought was the proper setting of the BFO for SSB signals when using the J-4. The procedure requires determining center frequency of a marker signal and then tuning below the f center for a carrier level drop of 18db and then adjusting the BFO CCW for zero beat and noting the knob's index position. The same procedure is again performed with by tuning above f center for an 18db drop and tuning the BFO CW for zero beat and noting the knob's index position. NOTE: It's strange that the Collins instructions actually states the signal levels and the signal drop as "3 S-units" and, of course, all of the 51J-4 receivers had Carrier Level meters that read in "db over" but Collins did provide the 18db level in parentheses. The instructions finish up by stating that if the SSB signal can't be demodulated with the BFO in the proper CCW position then rotate the BFO control to the proper CW position. In other words, there's no specification of either "upper" or "lower" sideband mentioned since obviously that changes depending on whether the band being used is Odd or Even, which isn't mentioned either. It must have become too convoluted to explain at a time when SSB transmissions were just becoming popular and just beginning to be heard on the air, so Collins avoided it altogether. It also seems strange that Collins (at the time) didn't mention the BFO inversions since this changing BFO relationship would have affected reception of RTTY FSK signals. The BFO position determines whether the RTTY FSK signal is going to be received "Mark high f" or "Mark low f." Of course, it depended on the type of RTTY TU. Those TUs that ran off of the IF and supplied their own BFO might not have been affected. However, some of the TUs were audio input types and relied on the receiver BFO. These would have definitely been affected by the Odd-Even BFO inversion but most RTTY audio TUs had the ability to invert the relationship, if necessary, for this very reason. Since the BFO inversions are a product of the Vari-IF and PTO relationship, they will be experienced with all versions of the 51J series. I guess it's just one of the 51J "oddities."
Communications-grade Audio, Loudspeakers and External Audio Amplifiers for the 51J Series - If you're expecting high fidelity audio from any of the 51J receivers, you're in for a disappointment. The initial intent of the receiver was communications and especially for receiving data-type transmissions. Communications-grade audio response was generally considered 300hz to 3000hz for voice transmissions. For CW or data, the audio bandwidth could be much narrower. Most military transmitters (and most amateur transmitters at the time) that were used for voice communications kept the audio response approximately from 300hz to 3000hz. This was to assure that the best intelligibility was available at the receiving-end and took into consideration that many transmitters were fairly low power and that poor receiving conditions would also affect intelligibility. All 51J-1 and 51J-2 receivers have audio specification of 200hz to 2500hz at -6db down. Additionally, the 51J-1 and 51J-2 have a very narrow IF bandwidth of about 4kc at -6db. The R-388, 51J-3 and 51J-4 have audio specifications of 200hz at -3db down on the low end and 2500hz at -7db down on the high end and 1.5 watts available audio power at less than 15% distortion. The IF bandwidth on the R-388 and 51J-3 was widened to about 6kc at -6db. The 51J-4 added mechanical filters to the 500kc fixed-IF that created a steep-sided, flat topped bandwidth that, while great for selectivity, created serious audio issues. Certainly not impressive specs from any of the 51J Series,...especially if you're an audiophile. But, the 51J receivers are "communications receivers" and their audio specifications are appropriate for that type of end-use.
All of the advertising or the manuals never specified a particular matching speaker for any of the early 51J series of receivers. The 51J-1 and J-2 manuals provide dimensions of the intended loudspeaker and these dimensions describe the Collins 270G-1 (used an 8" loudspeaker.) The 51J-4 manual actually specifies the 270G-3 loudspeaker. The 270G-3 used a 10" loudspeaker. During early production there was a 270G-2 10" loudspeaker available. There is conflicting information out there (even from Collins) but it seems that the 270G-2 was identical to the 270G-3 with the exception of the three small vertically-spaced holes towards the front on each side of the cabinet for installing rack-mount brackets. Some sources say it's the other way around, that is, the 270G-3 had the rack bracket holes but the 1959 Collins production catalog shows the 270G-3 loudspeaker without the mounting holes for the rack brackets. Also, in the 1959 Collins catalog the 270G-3 is listed as 6-8Z ohms however actual impedance measurements of original loudspeakers will normally show they are 4Z voice coils. Note that this is a nominal impedance shown and that would vary depending upon how the measurement is made, specifically the frequency employed. Figure that the impedance specified is "NOMINAL" and whether the testing shows 4Z, 6Z or 8Z, all 270G-3 loudspeakers sound great when connected to the 4Z output terminals of any 51J receiver. R-388 receivers were generally used for specific military set-ups such as RTTY or other data transmissions where a loudspeaker would be part of a larger, rack-mounted collection of equipment. In addition to the rack-mounting option of the 270G-2, there also was a rack mounted dual speaker system available for diversity RTTY set-ups. There was a USN version of the R-388 referred to as the AN/URR-23-A that came in a standard Collins cabinet, CY-1235/U and included the 270G-3 speaker (mil ID: LS-199/U.) The 51J-2 was also supplied in a similar fashion and designated as the AN/URR-23 (no -A.)
For a vintage mil-rad station, perhaps the easiest speakers to find (that are period and manufacturer correct) would be either the Collins 270G-1, an 8" speaker, or the Collins 270G-3, a 10" speaker. The 270G-1 was typically supplied with the 75A-1 receivers and the speaker cabinet had the Collins "winged" emblem on the front with chrome trim strips with a felt-flocked perf-metal grille (usually a sort of brownish-maroon color.) The 270G-1 also has a serial number tag mounted inside the cabinet showing the model designation and the serial number. The 270G-3 was typically supplied with the 75A-2, 75A-3 and 75A-4 receivers and these cabinets don't have the Collins WE and have black trim strips with an white-cream color felt-flocked grille. The 270G-3 doesn't have a serial number tag and wasn't assigned a serial number. Either of these Collins speakers are fine sounding, communications-grade reproducers and, though usually outrageously expensive, they are easy to find. However, any 4.0Z speaker will function fine with the 51J receivers. There's not too much of an advantage in using a high-quality, large diameter speaker installed in a bass reflex cabinet because the audio output circuitry of any of the 51J Series receivers was always "communications grade" in quality. The audio range available with all later 51J receivers lacks any significant bass since the audio output frequency response was rolled-off starting at 200hz on the low end and the upper limit roll-off was at 2500hz while the audio power can only produce about 1.5 watts maximum. Even the latest version, the 51J-4, still has audio that was specified as 200hz at -3db down to 2500hz at -7 db down in the manuals with the 1.5 watts output power maximum (at <15% distortion.) These less-than-impressive specifications will become more and more apparent with better quality, high fidelity speaker systems. Using the 600Z output to drive a loudspeaker through an appropriate matching transformer is also an option. Using a good quality transformer and a typical 8Z or 4Z loudspeaker will result in typical communications audio reproduction. I do use the 600Z output from the front panel SPEAKER jack to operate a rack mounted loudspeaker with dual R-388 rack-mounted receivers. The resulting audio is a little thin due to the open rack nature of the loudspeaker but the communications quality is excellent.
With the very late R-388 receivers and most of the 51J-4 receivers, the Diode Load was brought out to the rear chassis apron as a test point jack. It would be possible to use a shielded cable connected to the Diode Load and run that to the input of a High Fidelity audio amplifier (using the aux. input or some other fairly Hi-Z input) to drive a quality loudspeaker. You would lose the Noise Limiter function but it was designed for repetitive pulse noise that isn't heard very much nowadays. Also, the 51J-4's 6kc mechanical filter will still be limiting any high quality AM reproduction so very few enthusiasts have bothered with the external Hi-Fi amp set-up.
Since the 51J receivers have always been considered "communications receivers," it's not unexpected that the standard Collins communications-grade loudspeakers, the 270G-1 or the 270G-3, will give the best results when used with any of the 51J receivers.
Cross-Modulation with R-388, 51J-3 and 51J-4 - The first receivers, the 51J-1 and 51J-2, used RF transformers in the Ant/RF input section of the receiver but this resulted in a fixed antenna input impedance of 300 ohms. The Army found this terribly inconvenient since most of their antennae were whip verticals or dipoles with impedances a lot less than 100 ohms, let alone 300 ohms! Collins modified the RF/Ant input section when implementing the changes needed for the R-388 receiver. The change removed the primary winding on the coils and just used a parallel LC for tuning the Ant/RF along with adding an Antenna Trim to allow matching the antenna low impedances better. It all works fine except if there's a really strong RF field present. This could be a nearby AM-BC station or a neighboring ham running a high power transmitter. To have a strong enough RF level that could cause cross-modulation generally requires using a very large random length wire antenna that's directly connected to the receiver antenna input. The receiver's parallel LC Ant/RF input section doesn't have much selectivity and that's why it's possible for cross-modulation to happen if the RF field is sufficiently strong. The Army used mostly whip antennae and so it wasn't much of a problem for them. I've heard about this cross-modulation problem with the R-388 but I've never experienced it. Certainly, in large urban areas where powerful AM-BC stations might be nearby and numerous, along with the possibility of neighboring hams operating powerful transmitters, the cross-modulation issue might be more likely to occur. Using an end-fed wire antenna without any type of matching network (tuner) will contribute to the susceptibility to cross-modulation. Here in Western Nevada, I've always used large wire antennae but I've always used dipoles fed with open wire feedline in combination with a selective type of antenna tuner. I believe that the "tuned" antenna, which is essentially a selective LC combination, adds another "tuned stage" ahead of the receiver to compensate for the reduced selectivity in the Ant/RF input section. Using large broad-band or untuned (non-resonant) antennae might tend to result in the R-388 being more susceptible to cross-modulation. A selective "tuned" large wire antenna seems to work for me. Also, even if I'm just listening to station outside of the ham bands, I'll always "tune" the antenna for that frequency of reception. Again, the tuning and resulting high-Q of the antenna might be why I haven't experienced any cross-modulation issues.
How Collins Radio Company and the 51J were Responsible for the RACAL RA-17
British Homage to the 51J
- RACAL Engineering, Ltd. began as a small electronics company in England.
RACAL started operations in
1950. The founders were two friends, Raymond F. Brown and George Calder Cunningham, who had been working
together at Plessey Company (one of Britain's largest electronics
companies at the time.) The RACAL name was derived from RAy and
CALder, the founder's names. The small RACAL company built almost anything at
first from golf clubs to shelving units along with some electronic
Things changed for RACAL in 1953 when the British Royal Navy became interested in having RACAL build a couple hundred Collins 51J receivers for Royal Navy use. The 51J was being successfully used for RTTY communications in the USA military. It provided frequency-solid stability along with a dial readout that was "to the kilocycle" accurate. The 51J was the first receiver used by the US military that provided reliable RTTY reception without modifications. The US military R-388/URR was the most successful 51J version and was produced in the greatest quantity. Certainly the Royal Navy was aware of the success the 51J/R-388 afforded US military RTTY users and they needed the same type of reliability for their RATT comms (British for RTTY.)
If RACAL was going to become involved in British-built Collins 51J receivers, they would have to be licensed by Collins to do so. Naturally, RACAL wanted to use mostly British parts (probably so did the Royal Navy) but Collins Radio Company insisted that only parts from the USA could be used and only assembly of Collins 51J receivers could be performed by RACAL. Ultimately, the proposal ended up with a group from Collins Radio doing an inspection of the (then) small RACAL manufacturing facility. Apparently, Collins wasn't impressed and simply refused to license the manufacture of the 51J by RACAL. The only option left was for RACAL to design their own receiver that would meet the requirements for the Royal Navy contract.
RACAL contacted Dr. Trevor Wadley, a well-known South African electronics engineer, to help with the project and the result was the famous RA-17 receiver. Though entirely different in electronic design concept, using the famous "Wadley Loop" to eliminate drift, the RA-17 met or exceeded the 51J specifications and went on to be produced in many variations over the next 20 years. In looking at the RA-17 receiver to the right, it's immediately apparent that the physical appearance of the RA-17, especially the dual-dial arrangement and the slanted top (can't be seen in the photo,) paid homage to the Collins 51J receivers.
General Information on Reworking 51J Receivers
- The 51J Series are fairly difficult receivers to work on because their
construction isn't modular in mechanical design and the receivers will
need to be substantially disassembled to work on the commonly
encountered problems, including those problems involving the PTO that
was used on the R-388. The 70E-15 "M" PTO that was used in all R-388
receivers will almost always require a slight modification of L002
inductor-trimmer to be able to reduce the end-point error to <1kc.
If work is required in the receiver's front end, it is extremely difficult to access any of the parts in the crystal oscillator and most of the other front-end stages aren't much easier to get into. Additionally, the wires from the various coils are very fine and easy to accidentally break when doing rework in the front-end of the receiver. Most of the time it's easier to dismount the particular coil assembly for the repair and then reinstall. All R-388 receivers will be MFP'd which adds to the difficulty of rework. The 51J gearbox is complex and rework is difficult.
To take on a poor condition 51J receiver, you should be experienced in complete disassembly and reassembly of advanced vintage communications equipment. You should have professional soldering equipment, be experienced and possess a good soldering technique and only use real SnPb solder. If you're going to work on the R-388s you should be experienced in reworking military equipment that has been MFP coated. Necessary for working on almost any Collins equipment, you'll absolutely need the Xcelite 99PS-60 Bristol Multiple Spline Screwdriver 11 piece set (this set includes the extension piece.) This set is expensive at around $85 but absolutely necessary to access many of the Bristol set screws deep inside the receiver can't be reached unless you have the extension piece to use with the already "long" spline drivers. Your test equipment should be laboratory-type although your skill at RF/IF alignment will determine the ultimate quality of the receiver's performance. The 51J Series is within the capabilities of restorers who have good mechanical ability and a solid electronics background with considerable experience working on vintage sophisticated communications receivers. Don't be in a hurry and always be thorough.
|Some Thoughts on Rebuilding
- Any of the Collins 51J-3, R-388 and 51J-4 receivers, if they have been
well-cared-for, will usually operate as found.
However, these receivers are probably not operating to their full capabilities
and generally give the new owner a feeling that the 51Js are over-rated. Although
you won't find handfuls of bad capacitors (as in the early Hammarlund
SP-600s) there are several common problems with the 51J receivers now
that they have aged for the past six or seven decades. Probably the most
important step is to install good condition tubes and to perform a full IF/RF
alignment. These two steps are always required for top performance and the new tubes should be
installed and the alignment performed before any critical assessment is
made about the receiver's capabilities or any circuit modifications are
However, most of the 51J receivers encountered these days haven't been well-cared-for and most suffer from poor storage and many are compromised by owner-induced abuse (unwarranted modification.) Since most of the receivers were used extensively when new and then were probably stored poorly, you will usually find some mechanical problems that will need to be repaired. Once all of the circuits have been gone-through and rebuilt if necessary and the mechanical issues addressed, a full IF/RF alignment is always going to be necessary. Though not essential, the original alignment tools will make many of the adjustments easier (repro tools were available and they worked great. Can't find anyone selling them now, in 2023.)
Early 51J receivers that use the 70E-7A type PTO usually align easily. There are exceptions and one example I found had a 12kc EPE. The ease of adjusting the 70E-7A makes finding excessive an EPE unusual but it can happen. However, all R-388 receivers will have the 70E-15 PTO with the "M" prefix serial number and these PTOs will almost always require a slight modification to the PTO L trimmer coil L002 to function at the design level of performance. The 51J-4 70E-15 PTOs have the "CR" suffix serial number and these types seem to have faired much better and rarely, if ever, require going inside the PTO (the problem is more with how the receivers were operated than with the PTO itself.)
When completed, the 51J Series receivers are fully competitive with any other mid-fifties communications receiver. Highly sensitive, very accurate dial resolution and great selectivity. Note, that high fidelity audio was not included. The 51J receivers have highly restricted audio since they were primarily designed for communications.
Typical "As Found" Condition Issues
51J-1 and 51J-2 - A thought for consideration is that some of the 51J-1 and 51J-2 receivers, have had thousands of hours put on them by former commercial/military users. Many receivers were sometimes roughly treated, carelessly modified and are usually well-worn, needing much more than the usual re-cap and alignment to function at their design level of performance. The 51J-1 and 51J-2 gear boxes had the small drive gears made out of brass where the later R-388/51J-3 and J-4 versions have these small gears made out of steel. The relatively soft brass used on the small gears (under a lot of pressure and working against similar metals) wore quickly and the result is that sometimes the wear on early gearboxes is so severe that binding and jamming are experienced when changing the Megacycle position.
The early IF transformers often seem to sustain internal damage with rough handling of the receiver requiring disassembly and re-gluing of the coils and ferrite shields back into their proper position for correct operation.
The kilocycle dial on the 51J-1 and 51J-2 was photosensitive and would darken (tan to brown) over time if left in the same position and exposed to a lot of sunlight. The J-1 and J-2 kilocycle dial was a three-part laminate with the outer clear plastic pieces not being photosensitive. The white inner piece is what was photosensitive and it might have been a cellulose-acetate material which would explain the tan to brown discoloration that happens. Starting with the 51J-3 and R-388 the kilocycle dial inner material was changed and it isn't photosensitive. Luckily, darkened 51J-1 or 51J-2 kilocycle dials can be replaced with the kilocycle dial from a R-388 receiver. There is a very slight difference in the thickness of the material and the number font is very slightly different but mechanically the two types of dials are interchangeable.
Add to those 51J-1 and 51J-2 problems, the fact that for the past half-century, most of these receivers have been severely modified both by hams and earlier by commercial technicians attempting to "modernize" the "company receiver." Also, some of the J-1s or J-2s were military R-381 receivers that the Army depots "modernized" without too much concern for appearance, ergonomics, workmanship or functionability. It's extremely rare to find either the 51J-1 or the 51J-2 that's in completely original condition. The J-1 receivers seem to have taken the brunt of this modification mayhem with most examples being totally "hacked" beyond restoration. J-2 receivers sometimes surface that are mostly original and are in pretty good, restorable condition, though I haven't seen one in that condition in a long time. The J-2 that I rebuilt fifteen years ago needed a second J-2 to provide a "parts set" to allow selecting the best parts and components between the two receivers to use for the rebuild. Also, the "parts set" was able to supply any missing or destroyed parts. Figure that taking on a J-1 or J-2 restoration will probably require two receivers to make one operational and relatively original receiver. NOTE: In April 2023, I found a 51J-1 receiver that was a "survivor." In years of looking at many 51J-1 receivers, I'd never seen one that wasn't totally hacked beyond restoration. This 51J-1 is 95% original and only has a couple of minor modifications. The front panel is all original and so is the top of the chassis. So, I can't say that it's impossible to find a nearly all-original 51J-1, but it will take a lot of looking.R-388, 51J-3 and 51J-4 - The R-388 and 51J-4 receivers seem to be in much better overall condition and usually don't require extensive rebuilding, although there certainly are many exceptions lurking out there. Most component problems and certainly all mechanical problems will be aggravated by poor storage conditions. Fifty years of storage near any coast, in an unheated shed that's also inhabited by the local rodent population (along with copious insect and arachnoid ingression) is going to be devastating to all parts of these receivers. Challenging storage conditions can cause irreparable damage to all sheet metal surfaces and finishes. Sometimes corrosion will set up underneath the front panel paint causing "white blisters" to appear through the paint. These "severely storage compromised" receivers have, in reality, slid into oblivion and are really only usable as "parts sets." However, because of the Collins-name, very few of these corroded hulks are ever priced as "parts sets."
For those R-388 and 51J-4 receivers that have faired better, there are still a few things that should be checked, but, most of the problems will be minor, such as defective tubes, filter capacitor needing reforming, poor alignment or other minor issues. Generally, the better the physical condition of the receiver is, the less likely that any serious problems will be encountered.
Capacitors & Resistors
- Nearly all of the capacitors used in the 51J front-end and IF sections
are tubular ceramic caps which are extremely reliable and never seem to
fail. The bypass capacitors are usually paper dielectric types that
should be replaced in early versions (some a tub mounts.)
The filter capacitor assembly plugs into an octal socket. These are dual electrolytic capacitors with 25uf per section used in the 51J-1 and 51J-2 receivers. The R-388 and 51J-4 use either 35uf per section or 40uf per section depending on if the capacitor was ever replaced. Working voltage is usually 450vdc on all types. These are very reliable, well-sealed capacitors that seldom fail. Check for leakage current before powering up the receiver. Capacitors that haven't had voltage applied for decades should be reformed. Leakage current on a reformed electrolytic should be less than 100 uA at full working voltage.
Resistors are generally Allen Bradley JAN-types that are extremely stable and never seem to drift in value.
Dealing with Stuck Trimmers - A stuck trimmer capacitor can be a fairly common problem with almost any of the 51J receivers and is mostly due to decades of poor storage conditions and dirt. The variable trimmer capacitors are integral to their fiber mounting boards in the front-end. "Gunk" gets into the rotor to stator space and sets up over years resulting in the "stuck" trimmer. Do not force any stuck trimmer. Try a little bit of heat using a hand-held heat gun. Only apply the heat for about 5 seconds because not much is needed. Gently try to move the trimmer and it will normally easily break loose. If the trimmer still refuses to budge, apply another 5 seconds of heat and try again to move the trimmer. Usually no more than three times will be required as the heat will melt or loosen whatever is sticking the rotor. Don't apply the heat all at once, gentle persuasion is best. This approach works most of the time and is the safest method for loosening stuck trimmers. You don't want to break the trimmer since they can't be directly replaced without considerable difficulty.
- If you are working on a 51J-2 receiver
be aware that the 1950 version of the 51J-2 manual is fraught with
errors in almost every section - almost on every page - almost in every
paragraph. The alignment section, circuit description section, the
component designations and component identification layouts seem to
contain the greatest number of errors. Misidentified components are the
most common errors but some alignment procedure errors also exist. How this manual
ever got through the proof-readers is a mystery. The 51J-1 manual is probably
just as bad. The later Signal Corps R-388 and the Collins 51J-4 manuals are excellent
with very few, if any, errors found. However, the R-388 schematic found in some
Army TM manuals has several component identification numbers that are
transposed, e.g., R102 might be identified as R201. This
makes correct ID'ing of components referenced in the text or parts list
a little difficult but, once you know about the transposition,
identification becomes a bit easier. The schematic that is glued inside the
top cover of the R-388 is correct. The Navy manual for the AN/URR-23A (R-388) is
an excellent, comprehensive book but it does have a couple of
photo plates that have erroneous "figure" identifications.
I've also found some components in the parts list that have incorrect
values shown. Well,...nobody's perfect.
Parts Availability - At one time it seemed to be impossible to find anyone that was "parting-out" a 51J receiver. No matter how bad the receiver's condition, the seller always believed that it could be restored,...by someone. Things have changed and nowadays (2023) it's fairly common to see many 51J parts being offered for sale, primarily on eBay. Of course, eBay prices tend to be high but, not always, especially when just dealing in parts. Power transformers, chokes, 70E-15 PTOs, dial drums, KC dials, front panels, grab handles, dial escutcheons, meters, crystals, knobs are all often seen for sale. Even 51J-4 mechanical filters show up more often than one would think. Also, repro drum dial overlays are easily found. More difficult items like any of the front-end parts or IF transformers, a complete Crystal Filter assembly or gearbox parts probably would require obtaining a parts set.
- There are plenty of modifications that have been published for the 51J
Series. A search on the web will result in several to choose from. Nearly all of them are concerned with two areas of the receiver
First is the AVC circuit which many users feel has too short of a
release time. However, Collins was very specific as to why the AVC
time-constant is relatively short and that was for quick receiver
recovery when going from transmit back to receive. In a
commercial-military Voice communications set-up with break-in being used, quick exchanges
would have been the norm and Collins anticipated that with the AVC
time-constant they selected. Additionally, for RTTY, quick AVC recovery
is a necessity for accurate copy after a static burst or other
interruption of the signal. Yes, you can leave the AVC on for both RTTY
and even for CW but you do have to reduce the RF gain as required for
the signal level.
The second is the standard diode detector used in the stock receiver which many users want to replace with a Product Detector. The one 51J-4 that I used with the in-circuit Product Detector mod (and AVC mod) still distorted SSB signals if the RF Gain was advanced too far. The object of the mod was to allow the RF Gain to be fully advanced and not distort SSB signals. I thought the receiver functioned much better after I removed these two mods. >>>
The usual "ham mod" caveats should be seriously considered before
actually corrupting the original receiver circuitry. When the receiver
is functioning correctly and in good
alignment, the AVC does work fine and so does the diode detector.
However, Collins designed the receiver in the late forties, long before SSB
transmissions became the standard voice communications mode. When operated as a typical late-forties communications
receiver, no serious problems will be encountered with the 51J Series receiver.
This means that you will have to reduce the RF Gain when receiving SSB or CW signals. If
signal quality is not an issue, the AVC can be left on but, in either
case (AVC on or off,) the RF Gain
will have to be reduced to about 8 or less for minimum distortion of a
typical SSB signal. Of course, signal strength will affect the RF Gain
setting for best SSB demodulation. The AVC, the BFO injection and the diode
detector circuits are the primary reason that input signal level,
determined by the RF Gain, must be reduced so the proper ratio of BFO
injection to signal level will result in good SSB demodulation.
Additionally, when receiving CW signals with an older receiver,
excessive BFO injection into the detector can "mask" very weak CW signals.
Maximum sensitivity in CW was always achieved by "riding" the RF Gain control
with the receiver out of AVC and the Audio Gain near fully advanced. Additionally, the Carrier Level meter will
no longer be useable for relative signal strength measurements when the AVC is in the off position.
The primary reason for the AVC/Product Detector mods is to allow reception of SSB net operations without having to "ride the RF Gain" for every participant in the net roundtable. How much of the receiver operation is dedicated to this type of activity will determine the level of interest in these mods.
More on Mods -
The most common in-circuit modification was to replace the 6BA6 BFO tube with a
6BE6 mixer tube and install a few additional components to create a Product Detector for operation with the BFO
on - that is SSB and CW signal reception. This was a fairly involved
modification that could be accomplished with "no holes drilled" if done carefully.
It functioned fine for SSB or CW and it could be relatively easy to
reverse and put the receiver back to stock, if that's desired. The mod's
origin was from a Bill Orr article "Modifying the 51J Receiver for
SSB" published in the February 1978 issue of Ham Radio
I had a 51J-4 receiver that had the older style receiver circuitry modification (it was something like Orr's mods but not as extensive) and its performance wasn't very good. The receiver still distorted the SSB signal with the RF Gain at maximum (the object of the mod is to be able to run the RF Gain at maximum level with AVC on and have no SSB distortion.) I ended up removing the mod and returning the receiver to stock configuration with much better performance operating the receiver as a typical "late-forties" communications receiver. Additionally, the 51J-4 had the AVC mod that adds capacitance to increase the release time. This was also returned to stock configuration. The person who installed the mods did an excellent job with no damage to the circuit components that remained. I'm fairly sure the mod was working as intended because it did change how SSB and CW could be demodulated but "improvement" would be a subjective judgment.
Nowadays there are "plug-in" circuit boards that allow an easy, non-destructive method to incorporate a Product Detector to many types of older receivers including the 51J Series. Although these PCBs do plug-in, there are a few wires that need to be connected under the chassis but that's much better than total circuit modification. Treetop makes the best of the plug-in Product Detector, AVC units. It plugs into the detector tube socket and has a few wires that need to be wiring into the circuitry. This unit also has a voltage doubler that is used for its requirements and has enough current capability to operate K101, the Break-In relay. The Treetop units are made in Canada and there is more detailed information on the Internet.
Certainly how you intend to use your 51J Series receiver will determine your interest in any of the published mods or plug-in circuit boards. Certainly the plug-in circuit board approach is best and allows easy installation or removal. Remember that most ham modifications are "amateur-level engineering" and will enhance one area of performance at the expense of another. The circuit altering product detector mods may work fine for SSB or CW and the AVC mods might allow for better SSB response but learning how to operate the stock receiver in the manner in which it was designed will also give you great performance in all modes of reception.
But, if you want the best in SSB reproduction, without modifications, take a look at TMC's MSR-8 500kc IF SSB Adapter shown to the right. >>>
|Another Mod - I've run across this mod once in a while on early 51J receivers, like the J-1 and J-2. These first versions of the 51J receiver had a very narrow IF bandwidth with the spec being around 4 to 5kc at -6db. This selectivity was a deliberate design feature for a receiver that was intended for communication and primarily for RTTY, CW or other data-types of transmissions. The 51J-1 and 51J-2 weren't intended for aural pleasure or for listening to wide audio frequency AM Broadcast transmissions (even though all 51Js do tune the AM-BC band.) Almost all Voice "communications audio" had the frequency response limited to 300hz to 3000hz because that favored typical voice characteristics with the best intelligibility given that most propagation conditions severely hampered Voice comms anyway. However, I've found a few 51J receivers that have used 3pf capacitors connected across the primary to secondary on the IF transformers (pin 1 to pin 4.) This increases the coupling and that broadens the bandwidth. The later R-388 receivers and all 51J-4 receivers had 2pf to 3pf capacitors installed to increase the bandwidth to 6kc at -6db. The J-4 has mechanical filters for determining selectivity and the increase in the R-388 bandwidth was probably at the request of the Signal Corps (since the caps first appears in the R-388.) At any rate, the 51J-1 and the 51J-2 properly shouldn't have coupling capacitors installed. Yes, the IF bandwidth is pretty narrow but that produces the characteristic "sound" of these early 51J receivers. This is about nostalgia and wanting to experience what the original operators heard when using these receivers,...isn't it?||One More Note on the Fallibility of Mods - Bill Orr's Feb 1978 Ham Radio article does have one suggestion that seems like a very easy check and repair if necessary. It involves the RF stage grid bias voltage affecting sensitivity above 15mc. According to Orr, he had checked several R-388/51J receivers and found that many had very high negative bias voltage on the RF stage grid. Orr thought the problem was caused by resistor value of the 820 ohm R149 resistor drifting to a higher value thus increasing the negative bias. Orr suggested that R149 should be changed to 680 ohms for the desired bias voltage -1.4vdc. All very logical,...but should the bias voltage be -1.4vdc? It's the spec out of the tube manual - it must be correct. Much later, Dallas Lankford tried this mod and found the RF stage gain was increased but the dynamic range of the receiver was greatly reduced with the bias at -1.4vdc. His further research indicated that the -1.4vdc bias was probably a misprint in the Collins manual that was never corrected and that Collins had actually been using -1.8vdc as the target value. As far as sensitivity,...good tubes, a good alignment and, most importantly, a good antenna will do wonders for the receiver when tuning above 15mc. Also, hearing fabulous DX on 15M or 10M during the summer months is not likely no matter what you do to the receiver or antenna.|
|Lubrication - Unlike the R-390A receivers, with their roller bearings on all of the slug lifter racks, the 51J receivers just use a "rolled-end" on the slug lifter racks to act as a bearing against the brass cams. Where the roller bearings in the R-390A require lubrication with light-weight machine oil, the 51J receivers slug lifter rack ends require a light coating of grease. Any type of "sticky" grease will provide a good reduction in friction and reduce wear to the cams and slug lifter rack ends. Wheel bearing grease is probably a good choice since it's designed for high temperature and for staying inside the bearing. It doesn't take a lot of grease, just a light coat is all that's necessary. DO NOT use Lubriplate. It hardens quickly to become like cement. Carefully apply wheel bearing grease with a very small paint brush so the grease is only placed where needed. Other bearings are Oilite-type shaft bearings that don't really require any lubrication but if you feel it's necessary then only use one drop of machine oil for lubrication. Don't "over-lubricate."|
- Most of the time these two Garolite tools are missing from the
R-388 and the 51J-4 receivers, which is unfortunate. They work really
well, especially the tool for adjusting the ceramic trimmer capacitors.
The tool for aligning the IF transformers also is very helpful. At one
time, these tools were being reproduced very accurately and were easily
available. However, a recent search (2023) on the Internet didn't find
anyone selling these reproductions anymore. They may show up on eBay
sometimes but I've haven't really seen that happen either. If you find a
set, buy them.
Also, in advance of any alignments, build the series RC shunt for the IF transformers. Use 5" flexible wire leads with small alligator clips on the ends. Since one side is always connected to chassis, only one lead has to be moved. It really eases the IF load set-up. Also, have a series RC load for the antenna built consisting of a 47 ohm carbon resistor and a 100pf capacitor. It connects in series with the RF signal generator output to the receiver antenna input.
|Rubber Feet for Collins A-line Cabinets
(like those shown in the photo)
- If your Collins cabinet has really worn or missing feet that
are the type shown in the photo to the right don't
replace them with the typical "hole-mounted" rubber feet. The
correct rubber feet are actually called rubber bumpers. The originals
were 1" in diameter and 0.5" tall with about a 1" long 1/4" x 20
threaded stud secured with an external tooth washer and 1/4" x
20 nut. McMaster-Carr carries rubber bumpers listed as "Threaded
Stud Bumpers" but their 1/4" x 20 studs are too short. However,
the mounting hole in the Collins cabinets is 5/16" diameter and
McMaster-Carr does carry a 1" x .5" rubber bumper with a 5/16" x
18 stud 1" long that will work fine.
The rubber bumper type of feet are found mostly on the 32V transmitter cabinets mainly because of the weight of the transmitter. Some of the receiver cabinets might hole-mounted rubber feet with aluminum spacers. These cabinets don't have the welded riser mount for the rubber bumpers that the 32V cabinets had, so the separate aluminum spacer had to be used. Since the receiver weight was normally about 35 pounds, the hole-mounted feet worked fine.
|Reducing "High Line" AC using a Line Bucking Transformer - The 51J Series of receivers were designed to run on 115vac input voltage. Tube heater voltages and the non-regulated B+ voltage is dependent on 115vac being the line voltage used. Today, line voltages are nearly always minimum 120vac and often run up to nearly 125vac. Short term operation, such as testing or alignment, isn't usually affected by "high line" operation. Long term operation however will affect tube life and the higher non-regulated B+ can cause an increase in overall heat generated. It's very easy to add an external Line Bucking Transformer to lower the "high line" down closer to the specified input AC voltage. Using a 6.3vac filament transformer (that was designed for 115vac primary voltage) will lower the AC line by about 7vac to 8vac, e.g., lower a 124vac line down to 116-117vac. Our AC line here in Dayton is close to 124vac (123.8vac measured 7/29/22) and Line Bucking with a 6.3vac filament transformer lowers the AC line to 116vac. For receivers I use 6.3vac rated at about 3A minimum transformers and for medium power transmitters I use 6.3vac at about 8A minimum. It's not critical since the VA is only for the small voltage drop, not the entire VA the equipment requires. You can always use a larger current-rated filament transformer, especially if you want to use it to supply 115vac to a power strip, just be sure to have a primary switch on the bucking transformer. There are lots of easy hook-ups shown on the Internet (search on "line bucking transformer.") Of course, an autotransformer (Variac or Powerstat) could be used just as easily but filament transformers are plentiful and cheap. Variacs are also plentiful, just usually not cheap. Besides, the Variac belongs on the test bench.|
Easy Access to the Crystal Filter - How many times have you wanted to get into the Crystal Filter assembly on the 51J receiver but were intimidated by its lack of obvious and easily removable covers? Well, Jan Wrangel SM5MRQ, decided to get into his 51J-4 crystal filter and took photos to document just how easy it is to gain access to the Crystal Filter components. If the receiver is in a cabinet it has to be extracted out of the cabinet. Then remove the top cover and the bottom cover. From the top of the Crystal Filter assembly a single screw has to be removed. From the bottom, the choke nearest the front of the chassis has to be dismounted (just unbolt the choke - no need to unsolder the wires) to allow access to a 6-32 nut that is underneath the choke (between silk-screened IDs T101 and T102.) This nut mounts the bottom of the Crystal Filter top plate. Once the nut is removed now the top cover of the Crystal Filter can be removed from the top. This allows access to most of the circuitry inside. Reassemble in reverse order when checking, repairs or cleaning have been completed. If further access is required such as removal of the IF transformers inside, then more disassembly will be required. Complete extraction of the Crystal Filter assembly will require some unsoldering work. This procedure is only for simple checking, cleaning and minor repair work to the Crystal Filter.
|R-388 and 51J-4 Repro Megacycle Dial Drum Overlay - Some of the megacycle dial drums that still have their original band scale overlay appear quite different nowadays than they did when they were new. Many of the original overlays have darkened considerably with their present color ranging from medium yellow to dark amber. The color change is certainly expected and acceptable, if the dial overlay is in otherwise excellent condition. But, many drums will have nicks, gouges, stains, scratches, bubbling and flaking that can range from very minor and perhaps acceptable wear up to a completely destroyed original overlay. Luckily, the new reproduction overlays are very nicely made with real printing, in other words,...not a copy machine creation. The original color was kind of a cream color, that when illuminated with real #47 incandescent lamps, looked a light yellowish-cream color. The new overlay comes with a self-adhesive back to ease installation. The surface of the old drum must be sanded with 400 grit paper to provide a clean and level surface for the the adhesive to adhere to. Prep is all-important. Also, important is to not sand the original drum overlay completely away. Enough has to remain to see where to install the new overlay. Also, a pencil centerline should be lightly marked on the edges of the new overlay from a mid-scale point and also on the drum at the same point to provide accurate alignment for installation. The pencil lines can be erased after installation is complete. Start with clean dry hands. Start mid-center and work evenly in each direction being careful to avoid air bubbles. Go slowly and carefully because once the self-adhesive glue is in contact with the drum, the overlay can't be removed. As the installation proceeds, a soft paper towel can be used to lightly smooth out the overlay as it's installed. Once the overlay is completely applied and there aren't any air bubbles, the soft paper towel can be used to rub with slightly more pressure for good adhesion. Then the new overlay can be given a few sprayed-on coats of Krylon Clear Lacquer to protect it and, more importantly, to impart the slight sheen that the original overlays had. These repro overlays are necessary to restore a "rough-condition" original dial drum. They shouldn't be used on good condition drums just because of the age-related color change.||Original vs Restoration - Data Plates - Some restorations are so good, it's difficult to tell what's original and what has been replaced with good condition parts for another receiver. In most cases, especially when considering components that were always replaceable like tubes, circuit components or some assemblies, it's not important. But, then there are the items that indicate manufacturing date or contract or original end-user. These are the data plates that are so important but many times are missing and other times not seeming to fit the time-line of the particular receiver. Some restorers will install a proper data plate obtained from another receiver and this seems to be no problem in the short-term. But, when the data plate information is later used to try to date some of the manufacturing details, nothing seems to fit with the data plate information. My suggestion (and this is what I do) is to write on the back of the data plate that it's a replacement from a "parts set" receiver and not the original data plate for the receiver that it's installed on. Of course, it does depend on the curiosity of the researcher that they might be interested enough to remove the data plate for examination. Of course, on most R-388 receivers, this happens fairly often to check on what's stamped on the panel under the data plate. Nowadays, with decades of date plate swapping, starting with the Army depots and more recently with careless restorations, many questions arise when the data plate appears original but doesn't match some of the key manufacturing points. Just write "Replacement - Not Original" on the back of a replacement data plate. It might help someone in the future.|
70E-15 "M" PTO - Fixing the End-Point Error Problems
Excessive End-Point Error
in the "M" 70E-15 PTO -
The 70E-15 PTO tunes from 3.0mc to 2.0mc in ten turns. There is a
trimmer inductance (L002) provided to adjust the end-point error. The
end-points on nearly every R-388 PTO are excessive and beyond the range
of the trimmer inductor. Usually, if the EPE is greater than 6.0kc
(typical is 8kc to 12kc) it will be out of the
range of the adjustment. One important thing to observe is the PTO
serial number. If the PTO has "M" as a prefix to the serial number, that
PTO was for a R-388 and very likely that PTO will have excessive EPE
problems - probably due to excessive heat build-up from 24/7 operations. However, if the PTO has "CR" as the suffix to the serial
number, that PTO was for the civilian 51J-4 and more than likely the EPE can be
adjusted normally. Even though the "CR" PTOs seem to be better, still
most are going to require adjustment of the L002 trimmer for perfect
tracking. While the 51J-4 PTO may have a 4kc EPE which is correctable
adjusting L002, a R-388 might have a 10kc EPE which isn't correctable
with L002. All 70E-15 PTO end-point errors (EPE) are
similar in that the tuned range, which should be exactly 1.000mc change
in exactly ten turns of the PTO, has decreased. I've never
encountered a 70E-15 PTO where the EPE actually has an increased range.
The EPE issues are probably related to the type of the ferrite core
material used in the 70E-15 in combination with 24/7 operations that
most R-388s were subjected to. Since the ferrite has had about 70 years
to age (since nearly all 70E-15 "M" PTOs have been "unsealed" for
decades) it has become stable and since most of us aren't going into the 24/7
signal monitoring mode, by modifying the trimmer inductor for more
range, it then becomes possible to correct the EPE for the
ferrite core condition as it is now.
Use a Digital Frequency Counter - When the R-388 was being produced, used and maintained there wasn't an easy way to directly measure the output frequency of the PTO. Much of the calibration and alignment procedures for any 51J receiver are burdened with cumbersome details on how to use the 100kc crystal calibrator and its harmonics or how to use heterodynes to assure proper frequency output, sometimes even another calibrated receiver was used. As far as that being "the good old days," just read through any 51J procedure and see how much of the procedure is dedicated to just setting up these implied reference signals for calibration. Nowadays we have Digital Frequency Counters that can measure the PTO output directly, and that measurement is accurate and instantaneous. I started using a DFC when doing R-390A PTOs and found the process is so direct and so easy that I've started using the same method to do the 70E-15 and the 70E-7A PTOs for the 51J Series. It makes setting up the KC dial synchronization quick, calibration of the PTO EPE is quick and easy (unless the PTO came out of a R-388) and the accuracy is the best. You'll have to read through the procedure to determine what needs to be accomplished and almost anything requiring frequency determination is about ten times easier if a DFC is used to measure the frequency directly. I use a 10X oscilloscope probe to isolate the measurement load from the circuit.
L002 Details - It's common to find the 51J-4 receivers with virtually no EPE in their "CR" PTO. It's also fairly common that 51J-4s will be found with an EPE of about 4kc but that can usually be adjusted with the trimmer L002. If your R-388 "M" PTO has excessive EPE, that is >6kc EPE, correction will require removing one coil turn from the internal PTO trimmer coil L002. This requires disassembly of the PTO. When modifying the trimmer coil, be sure to remove only one turn. This coil only has four turns so one turn is quite a bit (also the coil wire is quite small at 29 gauge.) If more than one turn removal seems necessary in order to get the EPE in spec, you can perhaps remove another quarter to perhaps a half of a turn - but no more. Excessive turns removal will significantly reduce the inductance which in turn reduces the range of adjustability. Taking off 1.5 turns reduces L002 to just 2.5 turns which will narrow the range to the point where the adjustment of the slug doesn't affect the EPE at all. Bill Orr wrote extensively about the R-388 and correcting the 70E-15 "M" PTO EPE problems in the form of an article in Ham Radio magazine in the December 1969 issue. This detailed article should be read before attempting to rework your first 70E-15 "M" PTO. Orr's article can be found online in PDF form on the Collins Collector Association website - www.collinsradio.org
Linearity of the PTO Range - The linearity for the 1000kc range, or the 3.00mc to 2.00mc range, is determined by the adjustment of the "Corrector Mechanism" which is essentially a compressed "stack" of washers upon which an extension arm from the ferrite core rides. The slight "ups and downs" of the individual washers in the "stack" will slightly change the position of the ferrite core and thus create a "correction" for slight changes in the linearity as the ferrite core travels its 1000kc range. Normally, the original factory setting will be maintained if the EPE is corrected. Changing the "stack" can be tedious work that might end in disaster. Be sure to mark the stack for the beginning and the end of travel for the 1000kc range. Test the linearity and mark down where the correction needs to happen. Then set the PTO at that point and remove the cover to see where on the corrector arm is on the stack and where the adjustment is needed. If other washers in other sections move while doing the correction,...that's a problem. As mentioned, the stack is under a slight compression to hold adjustment and making changes requires loosening the compression enough to adjust where needed but not moving any of the other washers in other sections. Unless the linearity is totally "out" (which is not likely) it's best to leave the stack alone. Always adjust the EPE first because most of the time that will correct (or reduce) any apparent linearity problems. NOTE: Dallas Lankford wrote extensively in the Hollow State Newletter about the need to do the corrector stack linearity adjustment if the EPE was more than 4kc. Certainly, the farther out the EPE is the more the linearity will be affected. But, correct the EPE first. Then see where the linearity has gotten to. Lankford was trying to achieve the original specification of <750hz linearity error which is pretty tight.
|The Special Tool Required
for the 70E-15 EPE Adjustments - The end-point adjustment is behind a hex-head plug that has to
be removed and then there is a slotted locking nut that also has to be loosened
before the trimmer inductor adjustment can be moved. The tool required
to loosen the locking nut wasn't supplied with the receivers, but a
drawing with dimensions was generally included in the manuals
showing how to make the tool (the drawing to the right is from the 51J-4
manual.) The unlocking part of the tool is a small
round cylinder with two small projecting tangs that will mate with slots
in the locking nut. The blade screwdriver part of the tool fits down the
barrel of the cylinder part of the unlocking tool to allow adjusting the
L002 trimmer. The cylinder tool kept the locking nut from moving while
the L002 trimmer was adjusted.
After the EPE was "in spec" then the locking nut could be "snugged-up"
but not over-tightened. I made my own tool out of steel thin wall tubing
about 2.5" long and .20" in diameter with one end filed to create the
two tangs necessary. Since it's made out of tubing, a small thin blade
screwdriver fits down the barrel and can make the adjustments. Easy to
make. The drawing to the right gives you an idea of how the tool works.
There are some restorers that have built special right-angle tools for accessing the locking nut and the trimmer inductor with the PTO still mounted in the receiver but these do require a lot patience to use. Unfortunately, over the past several decades, who knows if someone has tried to adjust the EPE without "unlocking" it and has gnarled the trimmer slot. If you have the PTO out of the receiver you can visually examine the trimmer slot and make sure it's not damaged. Generally, if the PTO is the "M" version, it has to be removed from the receiver anyway for modification of the trimmer inductor for end-point correction. I find it easier to just remove the PTO, do the rework and do the entire EPE adjustment on the bench with a simple test jig.
An Easy PTO Test Jig - The test jig should be simple and easy to make. Orr's Ham Radio article shows his fixture using the KC dial but you don't really need to be that elaborate. Also, using the KC dial covers the access to L002, so the KC dial has to be dismounted from the PTO for each adjustment and then remounted for testing (it's not that difficult to dismount the KC dial for each L002 adjustment, really.) But, you can get by with a fixed reference index line on a clear plastic scale and a very narrow pointer. It's really all that's necessary. A very narrow pointer is attached to the PTO shaft. It can be made out of 22 gauge solid TC wire. The small transparent plastic index scale has a scribed "zero" line and is mounted to the PTO case using stand-offs and screws. Be sure mounting your small plastic index doesn't cover access to L002 EPE adjustment. Scribe several other lines on each side of "zero" that are about the width of the KC lines on the actual dial. This will allow you to accurately see if the end-point error adjustment is proceeding in the right direction. You'll have to count the ten turns the first time to be sure that the range is close to 2.000mc to 3.000mc but, after the first time, then just watching the output frequency is all that's necessary. How accurately you observe your pointer and the index lines on the plastic scale during the adjustments will determine your overall accuracy when the PTO is installed back in the receiver. You just need to determine a 1000kc change in frequency for exactly ten revolutions of the PTO shaft and a simple pointer and index does that. The PTO should tune exactly 3.000mc to 2.000mc in exactly ten turns of the PTO shaft and that should correlate to 0.00 to +00.0 (one complete turn) x 10 on the KC dial (when installed back in the receiver.)
I use a bench power supply connected to the PTO wires with clip leads. A digital frequency counter monitors the PTO output. In this way, the PTO can be completely removed from the receiver. It makes any rework (like modifying the trimmer inductor) much easier since you don't have to have the harness connected to the receiver. When measuring the PTO output frequency for EPE adjustment be sure to have the cover in place as its proximity to the PTO circuitry greatly affects the output frequency. >>>
|>>> Don't worry about the seals for the
PTO cover. At the factory, when a PTO was completed, that is EPE
adjusted and it was "ready to go," the PTO was heated, then, while still hot,
the L002 hex-plug was installed. As the PTO cooled, it formed a slight vacuum
inside which protected the components. But, the first time someone
decided to adjust the EPE, air leaked in and the vacuum was lost. That's why Collins didn't want anyone working on
their PTOs since, at Collins, after any adjustment or rework, the PTO
again went through the process to create the slight vacuum inside.
Collins claimed that correcting the EPE in the field was only for
temporary repairs and the PTO needed to be sent back to Collins as
soon as possible because the loss of vacuum would cause rapid
deterioration of the internal components. Hmmm, hype? Maybe. Nowadays, nobody worries about the vacuum loss in the PTO because
that certainly happened a long time ago anyway. The components
inside have already "aged" and are certainly stable. Besides, we all
take excellent care of our receivers and the environment they are kept
in is usually very clean and temperature-controlled,...isn't it?
If all you need to do is to adjust the EPE (like on the 70E-15 "CR" types) it's fairly easy to dismount the PTO, although you do have to lower the front panel and the KC dial has to be removed to access the three PTO mounting screws, but leave the PTO wiring connected to the receiver to supply the voltages. Attach (using a clip-lead) a shielded test cable to the PTO output coax and connect that to a digital frequency counter. The simple test jig fixed index should be mounted to the PTO and the pointer to the PTO shaft. Not too difficult and relatively fast if all you need to do is an EPE adjustment.
Rebuilding the Collins 51J Series Receivers
Rebuilding a 51J-2 Receiver
Origin of the Primary 51J-2 - I was initially given a cosmetically very nice 51J-2 by my old ham friend, W7ZCA, Paul Eisenbarth (now SK,) in exchange for re-capping his Collins 75A-4 receiver (this was around 2007.) Paul had been given the 51J-2 at sometime in the past and had done some rework on it. At one point, Paul had the receiver setting on the floor of his workshop with the bottom RF shield off, exposing the ten crystals of the Crystal Oscillator circuit. Paul's young grandson, who was probably about 4 years old then, found the temptation of the small shiny and removable crystals irresistible and pulled all of them out of the holder and hid them around Paul's workshop. This removal and hiding was all accomplished while Paul was off in another room. In fact, it was several days before Paul even noticed that the crystals were missing. Of course that was enough time for his grandson to forget where he had hidden all ten crystals. Paul was able to find a few of the crystals but seven were still missing. A telephone call to International Crystal Manufacturing Company had surprising results. After Paul told the sales person the story of how the crystals came to be missing, they found the story so charming that they sent an entire set of ten crystals for the 51J-2 to Paul - free of charge. Note: Unfortunately, International Crystal Mfg is no longer in business, 2022.
The crystals got the 51J-2 working again but there were other problems that seemed unfixable. The gearbox had severe wear that allowed the Megacycle control to be advanced maybe three or four bands before the gearbox would bind and jam up. You then had to rotate the Megacycle control back a band or two to undo the binding and then proceed forward three or four more bands. It was a cumbersome method but it did allow eventually getting to the desired band. Additionally, the Antenna Coil primary on Band 2 was open due to an excessive amount of RF accidentally injected into the receiver while operating on 160 meters. Paul had obtained another Antenna Coil but it was from a R-388 receiver (which doesn't have the primary winding on the coil.)
So, that was the condition of the 51J-2 receiver when it was given to me. I tried to use the receiver with the worn gearbox but it seemed like sooner or later (probably sooner) it was going to break something in the gear train. I decided to look for a parts set to rob the gearbox from to correct the problem.
photo above: The 1950 Collins 51J-2 showing the metal dial bezel, the square illuminated Carrier Level meter, the green "ham band" highlighting on the megacycle dial scale and the Collins' winged emblem - all characteristics of the early 51J receivers. (a 2007 photo)
Suitable Parts Set - After a few months, a suitable 51J-2
showed up on eBay. Nobody seemed interested in it since it was in fairly
rough cosmetic condition. The operational condition was described as a
"fun radio" - whatever that meant. So, two hundred dollars later I was
the owner of another 51J-2 receiver. When the second 51J-2 arrived, I
found out what a "fun radio" was - totally and incompetently reworked
besides being non-functional with a repainted front panel in black paint
with white rub-on lettering. I swear that the eBay photos were of a
different front panel (probably a different receiver) but by the time I
had the receiver, the seller had already pulled the auction
photos (at the time there really wasn't too much a buyer could do
except complain or give a negative comment.) It really didn't matter because all I wanted was the gearbox and
that was in good shape.
The plan was to use two 51J-2 receivers to build up one nice condition, fully functional receiver. In checking over the two receivers, it seemed easier to use the chassis of the second receiver as the starting point and rebuild the entire receiver using the best parts from both units. This allowed me to skip the tedious removal of the gearbox which certainly would have resulted in several extra steps to synchronize the switching.
The second receiver had been incompetently reworked and several of the front end coils had broken wires that were their connections to the trimmer capacitors. Also, all of the high quality tubular ceramic capacitors had been replaced with cheap disk caps. I decided to strip out the receiver from the front of the IF section back to the audio output and start over. I reinstalled the high-quality tubular ceramic capacitors harvested from the first 51J-2. I rebuilt the entire front-end of the receiver to repair the many broken coil leads. I had to replace the third conversion input coil because it looked like it had been burned. All parts used were either new parts or good ones harvested from the first receiver.
Once the electronic rework was complete, the front panel, the knobs, the megacycle drum dial, the meter and many other parts were transferred from the first 51J-2 to the new rebuilt receiver. I ended up with a great looking, complete 51J-2 - but how did it work?
- Upon power-up the new rebuilt 51J-2 seemed to be working fine. I
performed an alignment and the receiver worked fine - or did it? After
about 30 minutes of operation the sensitivity dropped down to where
signals barely moved the carrier level meter. From the start, the audio
would distort if the RF Gain control was higher than 8 when in AVC.
Clearly, there were a few more bugs to work out before the receiver
could be called "complete and working." However, other projects came
into the shop and the 51J-2 was put on the shelf and temporarily
In fact, "temporary" ended up being about three years before I got back to the receiver. I had recently been told that Paul W7ZCA had become an SK and that had me reflecting about all of the "deals" we had exchanged over the years. I remembered the 51J-2 and thought that I should probably finish that receiver since it was such a nice looking example of a fairly rare Collins. This was in September 2010.
The strangest problem was the erratic variable gain of the receiver. When on the bench in the normal position the gain was down but when the receiver was placed up on its side to troubleshoot the gain would then be somewhat normal. I could even tilt the receiver about 20 degrees and get the gain to go up and down with just a slight change of the angle - weird.
To narrow the problem down I used a clip lead type scope probe and measured at various points in the receiver while tilting the receiver back and forth on the bench. I had to inject a fixed-level signal into the receiver's antenna input so I knew the variations in the gain were occurring because of a fault and not the signal level changing. I could see the gain change problem was occurring just past the first IF amplifier tube. All components and the tubes had already either been checked or replaced except the IF transformer itself. I removed the IF transformer cover and discovered that the coil and the ferrite shields were at the bottom of the mount and loose. Apparently, moving the receiver around changed the position of the IF coil which changed the coupling and resultant gain. Upon removing all of the IF transformer covers, it was discovered that every IF transformer had loose coils, loose ferrite shields or both. Complete removal and rebuilding of all of the IF transformers was necessary. I used epoxy to secure the ferrite shields in position. I actually used the IF transformers from the first 51J-2 because they were in better overall condition but still required securing of the ferrite shields to prevent future problems. Upon power-up, I had more gain through the 51J-2 than ever before and an IF alignment only improved the response.
||Although I thought that the IF transformer problem solved everything
that was wrong with the 51J-2, I soon discovered that the AVC would
cause distortion after several minutes of operation. I had already
replaced all of the tubes in the receiver with a set that checked fine
on the TV-7 tube tester. However, you can't find all tube problems with
a tube tester. Once the AVC started to cause the distortion, I replaced
the AVC amplifier tube with no change in the problem but changing the
Dectector/AVC Rectifier tube cleared up the problem. I had already
rebuilt that entire section of the receiver so I was sure the problem
had to be tube related. NOTE: Most of us don't leave the
"tube being tested" installed in the tube tester long enough for these
types of problems to develop. With enough time in the tube tester, I'm
sure the tube problem would have become apparent. Fortunately, these
types of tube problems are uncommon.
An additional problem was only on the AM BC band or Band 1. Low gain was the result of a defective mixer coil that appeared to have gotten very hot. Fortunately, the first 51J-2 receiver provided a good condition coil. These coils are easy to replace as they are only held in place by two tangs that have to be pushed inward to dismount a coil and "snap" into place when installing the coil into the mounting hole. The wire leads are the same diameter as the coil wire itself and the connections to the trimmer capacitors are fairly delicate so care must be taken in the removal and installation.
A full IF/RF alignment had already been performed early on the receiver and a quick touch-up was all that was necessary for top performance that is at or better than the original specs. Audio is fairly nice as I did replace the .01uf across the audio output transformer with a .0047uf to increase the high frequency audio response. I usually run the Collins 270G-1 loudspeaker with this 51J-2 and that gives just a little bass discernable on strong SW BC stations or AM BC stations. Reception of ham AM stations benefit with the increased audio highs with better intelligibility. But, the 51J-2's audio is still communications-grade, not high fidelity.
Photographic Details on the 51J-2 - Five Photos
1. The top of the chassis of a 51J-2 receiver showing the earlier 70E-7A PTO. Under the metal shield on the PTO is a metal octal 6SJ7 tube. In front of the PTO tube is the glass holder for the desiccant that protects the PTO from moisture ingression. Otherwise the receiver is very similar to the later 51J-3 and R-388 receivers. 2. Close-up of the 70E-7A PTO
The back of the 51J-2 showing the unique slanted top cover that is used
on all of the 51J Series receivers. The top cover must be installed even
if the receiver is installed into a cabinet. The top cover shielding
spurious oscillations that will be tuned at various frequencies
throughout the tuning range ("birdies.") Note the ID plate - this is NOT
standard. The ID information is typically silk-screened to the rear of
the chassis. This particular 51J-2 had holes drilled thru the
information so a suitable "data plate" was made from the derelict "parts
set" 51J-2 chassis. The original AC line cord was a two conductor
black rubber "zip cord" but nearly all receivers will be found with a
later heavy-duty power cable installed (as shown.)
Rebuilding the R-388/URR Receiver - A Basket Case
The following "basket case" is not typical of that status since most of the parts were present. It was just that the receiver was mostly disassembled and the former owner had no intention of future reassembly.
|The Basket Case - 2013
- A friend of mine had the luck to find three R-388 receivers locally.
They had been listed on Craig's List. His telephone call to me was
mainly for advice on how to "power-up" the three receivers. I told him
that typically R-388s will pretty much work "as found" but it would be a
good idea to thoroughly check the filter capacitor and to test all of
the tubes before applying AC. This was accomplished but he found that
none of the receivers seemed to work. I was asked if I would "take a
look" at them. We made a deal that I would service and align the two
best receivers in trade for the worst of the bunch which, by this time,
had been disassembled for parts needed in the other two receivers.
I delivered the two serviced and aligned R-388s and picked up the "parts set" R-388 while at a ham swap meet in Reno. What I got was the R-388 chassis, the front panel and a box of parts. The gearbox and the main band switch had become de-synchronized during some of the disassembly. Also, about half of the tubes were missing as were nearly all of the screws. My friend sent the missing screws to me through the mail. I had plenty of tubes so that wasn't a problem. Really, all that was necessary was to put the R-388 back together and go from there.
The front panel was in exceptional condition and had been cleaned by the former owner (a machine oil rub was applied to the wrinkle finish after cleaning.) The kilocycle glass was missing but I had a glass piece from a "parts set" 51J-2 that fit correctly. When I purchased a "parts set" 51J-2 to complete the rebuild of another 51J-2 receiver I was lucky enough that the "parts set" 51J-2 that had an almost perfect R-388 megacycle drum installed. Fortunately, the original 51J-2 megacycle drum was excellent so I really didn't need the R-388 one that came with the "part set." It was so nice though I saved it and wrapped it up in plastic to protect it. So, here it was a few years later and I was rebuilding an R-388 that had a very well-worn megacycle drum. Obviously, that "saved" megacycle drum had "saved the day."
||Synchronizing the Band
Switch to the Gear Box - This is really much easier than
it sounds. Since the gear box has mechanical stops at each end all that
is necessary is to know which way the Megacycle knob is rotated to
change bands in the proper direction. Since counterclockwise rotation
increases the frequency, rotation clockwise to the end stop would be the
.5 to 1.5 mc band or Band 1. Once Band 1 is in position then a check of
S-106 can be visually accomplished to see where the arm of the switch
is. By checking the schematic versus looking at S-106 it was determined
that the switch was actually on Band 2 and all that was necessary was to
loosen the set screws on the coupler from the gear box to the band
switch and then rotate the switch shaft until Band 1 was in position and
then tighten the coupler set screws. Rotating through all thirty band
positions did verify that the switch rotated from position 1 to position
16 and then from Band 17 to Band 30 the switch shaft did not change
position. Then when returning from Band 30 down to Band 17 no change in
the switch shaft position but with Band 16 on down to Band 1 the switch
shaft did rotate correctly and stop in position one. NOTE: S-106
doesn't change position from Band 17 to Band 30 because it selects the
ANT/RF/MIXER coils and the same coils are used for the entire 16.5mc to
30.5mc tuning range. The position of the slug rack changes for each
17-30 Band selected. While doing these
tests all of the other movements of the slugs and slug racks were
verified and no problems were noted.
Dial Drum Drive Cable and MC Dial
Pointer Cable - These are special metal cables that have
a plastic covering over them. Luckily, both cables were present but were
loose in the box of parts. I think the cable material is available from
various Collins suppliers and the dimensions and installation
instructions are in the manual if your R-388 is missing these cables.
|Knobs and Tubes - Since this R-388 was a "basket case" it naturally didn't include all of the knobs or many of the tubes. Luckily, I had a complete set of Daka-Ware knobs that had come with the "parts set" 51J-2 receiver. I did have to search through the junk box to find a set of Daka-Ware kilocycle and megacycle skirted knobs since those weren't on the 51J-2 "parts set." Most of the tubes missing were the 6BA6 tubes. In fact, the R-388 uses seven 6BA6 tubes. Also, missing were the two 6AK5 tubes, the three 6BE6 tubes, the 6AQ5 and the 5V4 rectifier. All tubes were found in the tube "junk" boxes. All tubes used in the R-388 should test well above "minimum acceptable" for best performance.||Repro ID Tag - As can be seen, this receiver originally had two data plates installed that had been removed in the past. The center tag was the serial number identification plate and the tag to the right was a "Caution" tag that informed the user to refer to the TM manual. Luckily, there are a few repro tags available from Mike Chanter (Collins Radio Association.) He supplies a 1950, 1953-54 and 1957 type tags. Since this R-388 was probably built on a 1951 contract, I should have gotten the 1954 tag but instead I bought the 1950 version. The thinking was that the MFP date of "JAN 52" would have to be from an earlier build, thus the 1950 tag. However, knowing that Collins built R-388 receivers an put them into inventory for later contract sales, maybe the 1953-54 tag would have been a better choice. NOTE: THE original data plate found.|
|Interesting Find -
What's a USN acceptance stamp doing on an Army receiver?
- I didn't pay any attention to this when I was doing this
rebuild in 2013. I only came across it again ten years later! Just
recently, in 2023, I wanted to examine the "original" data plate closely
and when I dismounted it what should I find under it, stamped on the
front panel, but a "Navy Anchor" acceptance stamp! I thoroughly checked
over the receiver, looking for any of the usual Army SC acceptance
stamps but NONE were found. The CL meter has an orange triangle, the
typical SC-type stamp. But, this meter is one that I bought off of eBay
and restored so it's not original to the receiver. But, the Marion
Electric meter that was installed in this R-388 also had an orange
triangle SC stamp but it's likely that meter was a field
replacement. So, the Navy Anchor is the only "original" mil-stamp on the
receiver and it was covered-up with an Army data plate. The
implication is that this receiver started out as a Navy R-388 but ended
its military career as an Army receiver. Note in the photo below, very
faint in the upper right corner of the panel is the Army SC acceptance
Maybe this isn't unusual because navy-radio.com has a photo of R-388 with an white Navy Anchor stamp that has an orange triangle Signal Corp stamp directly over it and also has several listings of receivers that have both Navy and Army stamps!
|Restoring the Dial Bezel
- The dial bezel had been broken at one time and glued together to
repair. The break was at the bottom of the bezel - an area hidden by the
skirt of the main tuning dial. There was also a chip in the opening for
the kilocycle dial glass. I decided to use an epoxy fill to repair the
break because there were some missing sections. Tape dams were used and
epoxy was layered to complete the fills. After the epoxy had set-up the
areas were leveled by filing and then polished. Usually I would have
used black power coloring for the epoxy but I didn't have any around so
I carefully applied jet black nitrocellulose lacquer and polished this
to match the bezel bakelite. To finish the bezel required filling the
engraving "MEGACYCLES" and "KILOCYCLES" along with the "tick marks" for
the kilocycle index. I used Artist's Acrylic mixed to a manila color and
applied with a brush. The fill paint is left to set up for one minute
and then a paper towel piece dampened with Glass Plus is used to remove
the excess fill paint. The end result was a bezel that looked great and
had repairs that were very difficult to see. (see the Update for July
17, 2016 at the end of this section regarding an original replacement
bezel for this receiver.)
New Power Cord
- The original power cord had been cut on this R-388/URR. The
plastic strain relief was broken when trying to remove it.
Fortunately, most larger hardware stores carry the correct type of
power cable and also the same style of plastic strain relief.
photo left: The R-388 "basket case" immediately after restoration (Oct 2013.) Those aren't the correct grab handles. The meter is a Marion Electric field replacement. See below for updated photo of this receiver
|IF and RF
Alignment - The alignment procedure in the TM has a few
steps that are outdated. This is because today we have easy access to
digital frequency counters that are extremely accurate. When the TM
directs you to use the R-388's Crystal Calibrator to "beat" against the
signal generator's 500kc input to be sure that it is accurate, it's
because back in 1950 that was the most accurate way to assure that 500kc
was the input signal. Today's digital frequency counter (or synthesized
signal generator) is just as accurate and much easier to use. I like to
check the Crystal Filter's crystal frequency just to be sure. Usually
that crystal is within 0.01kc or so of 500kc but I use that frequency of
the crystal as the IF. This assures that the Crystal Filter works great
(and they will - if you take care in the IF alignment.) The TM
directs you to "de-tune" the IF transformers with a series RC load
consisting of a .01uf capacitor and a 4.7K resistor (a simple shunt-LP
filter.) The instructions are
very specific as to where to connect the RC load for each IF transformer
adjustment.* Since the load is to chassis-ground, only one end has to be moved
and using short clip-lead connections works fine. Though the special alignment tools (that are
always missing) will help with the alignment they are not strictly
necessary. Reproduction tools used to be available and if you can find a set
they are worth having. I think the original type alignment tools make the trimmer caps
easier to adjust but not the slugs. The IF transformers are also easier adjust with the
proper alignment tool.
* I believe the RC load is to isolate the 500kc IF and prevent it from mixing or interacting with the crystal oscillator or the PTO during adjustment since the bottom cover, thus the normal shielding, has to be removed to adjust the bottom slug in the each of the IF transformers. Many IF alignment procedures of single conversion superheterodynes instructed the technician to remove the Local Oscillator tube when performing the IF alignment for the same reason.
|Variable IF and RF
Alignment - Signal is injected into the Antenna Input
SO-239 connector. Align the Variable IF sections first. There are two
sections, odd and even, requiring different frequency inputs. Alignment
is standard in that the trimmer caps adjust the high end of the range
and the slugs (inductance) adjusts the lower end.
RF tracking is straight forward but you will only have to adjust six ranges as the remainder of the bands are tracked by oscillator heterodyne action.
Mechanical set-up of the kilocycle dial and the PTO should have been performed before the tracking alignments. All 70E-15 PTOs will have some end-point error unless they are removed and calibrated. If you are satisfied with the EPE and it happens to be a few kilocycles then be sure to "rock" the signal generator frequency at each calibration dial frequency rather than changing the actual dial readout. For example, if you have to align to 7.40kc but the actual peak output is on 7.402kc, then use 7.402kc on the signal generator and 7.40kc input on the receiver and then adjust the proper trimmers or slugs for peak output. Always set the receiver frequency and then "rock" the signal generator for peak diode load, then adjust the slug or trimmer. The more end-point error there is in the PTO the further out this procedure becomes but for just a couple of kilocycles of error it works fine. The correct procedure would be to rework the PTO for zero error and then all of the tracking will be accurate. Remember that the tracking accuracy is in the PTO and the crystals in the crystal oscillator. The slugs merely align the tracking of the Ant, RF, Mixers and the Variable IF and affect the overall gain of the receiver.
R-388/URR - Expected Performance and Operation
I've worked on many R-388s and this of course includes many full alignments. Probably the most important improvement that can be performed on the R-388 receiver is to do a full alignment. It's surprising how far out the alignment can be and the receiver still seems to be performing fine. So, this evaluation of R-388 performance is for receivers that are in good condition and have had a recent full IF and RF alignment.
The R-388 is probably the overall best performer of the entire 51J Series for vintage AM ham stations. This is because it doesn't have the mechanical filters that were installed in the 51J-4 receiver. Though the MFs are great for SSB and CW, the 6.0kc filter is not particularly ideal for AM reception. This is because Collins' engineers were thinking "communications" (and primarily data communications) and not wide-bandwidth audio reproduction. The R-388 relies on the designed-in bandwidth of the fixed IF section of the receiver which is probably around 6.0kc at 6db down or perhaps a little bit wider (it's rated at 12kc at 60db down.) However, the IF passband curve is a little more bell-shaped rather than the steep slopes and flat tops of the Collins' mechanical filters. So, the first "plus" is the IF bandwidth of the R-388 for AM reception. The audio output 6AQ5 does have a capacitor from plate to B+ for impedance matching which on the earlier J-1 and J-2 receivers seriously limited the upper audio response but the R-388 changed the value of this capacitor to 6800pf which doesn't affect the audio response. So, you should expect your R-388 to sound just a little limited in the upper end of the audio range but most listeners find this audio to be "pleasingly mellow."
Dial accuracy is legendary and is plus or minus one kilocycle if the receiver is calibrated to the nearest 100kc point on the particular band in use. Ultimate accuracy is limited by the 70E-15 "M" PTO which is also legendary for its end-point error problems. Most R-388s will have around 6kc of EPE which is about the limit that can be adjusted out. Greater EPE requires PTO rework to correct. When the EPE is adjusted out, the receiver is dead-on accurate. Unfortunately, the 70E-15 "M" PTO must be out of the receiver to perform the EPE adjustments. There are some right-angle special tools that have been built by enthusiasts that allow the adjustments to be accomplished without PTO removal but they are difficult to use. Even with the PTO out of the receiver a special tool is required for adjusting the L002 trimmer. Most of the time, if there's the typical >6kc EPE, the PTO has to come out of the receiver anyway.
Stability is solid and drift-free. Sensitivity is competitive. Are there more sensitive receivers? Certainly, but with today's high-level of RFI and EMI that plague most urban areas (and seems to ever be on the increase, even in rural areas) what good is .25uv sensitivity when the background noise runs S-9? Far more important today is the ability of a receiver to operate well in a noisy environment and to have QRM-fighting tools that work. The R-388's selectivity is designed into the conversion schemes and the fixed-IF section but is further enhanced by the use of the Crystal Filter. Why many hams refuse to use Crystal Filters is a mystery. CFs really do a good job at reducing QRM. On AM, select position 1 or 2 and adjust the Phasing for minimum bandwidth. Then tune the AM signal "on the noise." You'll find most of the interference is gone or greatly reduced. The AM audio will sound "muffled" but it will still be intelligible enough for communications. If adjacent frequency SSB activity is a problem then adjust the Phasing slowly and you'll find a point where the dominant audio frequency of the SSB QRM is attenuated substantially. Use the same procedure for SSB reception. For CW, with the CF on you can tune the CW signal off of zero slightly to find the "peak" response. The peak response can also be adjusted with the Phasing control and slight tuning of the signal. The Phasing control can be used to reduce specific audio tone frequencies, that is, to eliminate AM heterodynes. Heterodynes were quite common when there were a lot of AM signals on the ham bands but, nowadays, most AM activity is on nets and one doesn't usually encounter heterodynes too often. On CW, the CF can reduce interference from a nearby CW signal by this heterodyne elimination method. When trying to eliminate CW QRM or an AM heterodyne adjust the Phasing control slowly. The "notch" is very narrow and it can be easy to miss its effect on the interference if the Phasing is adjusted carelessly.
Break-in Set-up and Operation - Set-up for station receiver operation is somewhat involved since the Break-in function does require a separate +12vdc 200mA power supply that is switched with the transmitter's antenna relay auxiliary contacts. I built a solid state +12vdc power supply housed in a very small metal box. It could be switched on by connecting the AC input to the same transmitter terminals that drive the dow-key relay. Upon transmit, the +12vdc is actuated along with the dow-key T-R relay and the receiver is muted. However, I've set my Break-in PS to have the T-R relay auxiliary contacts switch the 120vac line voltage input to the Break-in PS which then has its +12vdc output going to the REMOTE terminals on the receiver. Although the Break-in PS is "plugged in" you don't have to worry about leaving it "plugged in" because the power transformer primary really doesn't have any voltage applied unless the T-R relay is switched to the transmit position and that would require the transmitter to be powered up. (I also have an old Radio Shack Micronta 12vdc power supply that I use for a 51J-4 Break-in supply. I wasn't using this little 2.5A power supply so it was perfect for the 51J-4 Break-in voltage.) Remember to always provide the Break-in function for the R-388 as this disconnects the antenna within the receiver, grounds the receiver input to protect the circuitry from excessive RF levels. Also, the Break-in connection at the rear of the receiver that is marked REMOTE is the ONLY place that you can connect the +12vdc and operate the Break-in relay (it's not actuated if the front panel STAND BY is selected.) There are three terminals provided for the REMOTE connection. Pin 1 is ground. Pins 2 and 3 are the connections to the K101 coil for actuating the Break-in relay. Since pins 2 and 3 are direct connections only to the K101 solenoid coil, observing polarity of the +12vdc when connecting to pins 2 or 3 isn't an issue.
There are a couple of methods that can be used to provide the +12vdc Break-in voltage internal to the receiver. One method is to incorporate the small DC power supply inside the receiver chassis. The very small current requirements will allow very small components that easy can be placed under the chassis. However, an easier method is to incorporate a "voltage doubler circuit" that operates from the 6vac tube heater supply to supply the +12vdc. This only requires two silicon diodes and two electrolytic capacitors, so it's even easier to place under the chassis. Wiring in either case can be installed to allow a "closed" condition from the T-R relay auxiliary contacts during "transmit" to be applied to the REMOTE pins 2 and 3 which would then apply +12vdc to K101 to actuate "Break-in."
Speakers & Antennae - When comparing the two audio output impedances available, the resulting audio quality will be dependent on the loudspeaker used. With a good quality matching transformer, decent audio is available from the 600Z output. For the Collins speakers, if you have the Collins 270G-1 (8" speaker) or 270G-3 (10" speaker,) either of these are fine 4Z loudspeakers to use with the R-388 or any of the 51J receivers. Incidentally, an easy way to connect the loudspeaker is to use the front panel phone jacks. Be sure to note that the PHONES is a standard .250" jack and the SPEAKER is a three-circuit .187" jack (ring and shell are tied together and connected to chassis with tip being the 600Z output.) For 600Z, use SPEAKER and for 4Z use PHONES. The phone jacks are direct connections to the rear 600Z and 4Z terminals. Also, when using a 600Z headset, plug into the SPEAKER jack. There are very low impedance (4Z) headsets available and they can be used by plugging into the PHONES jack. When the receiver was set-up with an external 4Z speaker load a radio operator could temporarily plug-in a HI-Z set of phones into PHONES for monitoring and not affect the speaker output.
Using a Shielded Magnetic Loop Antenna - Though the R-388 will "pick-up" some signals with a minimal antenna the absolute best performance is with a full-size, impedance-matched (or tuned) antenna. This, of course, is true with just about any receiver. With a first-class antenna, the R-388 will hear everything that is on the air with propagation conditions or local EMI/RFI being the only limitation. I've also successfully used a Pixel Loop (a shielded magnetic loop antenna) for general listening and its "broadband" nature (no tuning is required for 100kc to 30mc coverage) makes using it as a receive antenna very convenient. Although the Pixel Loop will do a fine job, the signal level difference between the loop and a full-size "tuned" antenna is usually noticeable. However, in EMI/RFI noisy environments, sometimes a shielded magnetic loop antenna, like the Pixel Loop, can provide some "noise" relief. EMI/RFI can be a problem on 80M and it can be devastating on 160M. Higher frequencies tend to be less impacted by EMI/RFI noise. In using a Pixel Loop for testing of various 51J receivers, I can say that I've never had a signal received on the large wire antenna that I couldn't also hear using the Pixel Loop. The signal might not be as strong on the loop but it will be heard.
If your local QRN and your QTH allows it, using a full-size "tuned" dipole antenna will result in the best reception performance from the 51J receivers when you have general listening in mind. That would be listening to all ham bands but also listening to SW-BC or utilities stations or any interesting signals that the receiver can tune to. The "tuned" dipole antenna allows optimum performance at almost any frequency that the tuner can match to the antenna's impedance at that frequency. At shorter wavelengths (higher frequencies,) like the 20M, 17M, 15M and 10M ham bands, directional antennas with gain, a yagi or a quad, are a real benefit to DX reception on those bands since these antennas themselves add gain to the signal being received. But, don't sell the shielded magnetic loop short, they work quite well and their broadband nature makes them convenient to use.
Some Rebuilds are Never Finished
As restorers, we're never, ever really quite through with a project. We're always keeping an "eye out" for parts that would either enhance our restorations, or, to maybe add a "missing part." That's the case with the "Basket Case" R-388 profiled above. Seven years later and we're still looking for a "CAUTION" tag,...but the receiver still works great.
UPDATE: Finishing the Rebuild? - July 17, 2016 - Our first find was to add the original data plate,...not an original,... THE original. Not too long after I had reassembled the R-388 I was able to talk the fellow who had removed it into trading the original data plate for the reproduction tag I had on the receiver. Now the data plate reflects that this R-388 is from a 1951 contract. The unfortunate part of this trade was that just in the short time this guy had the original tag he had scratched out the original data plate serial number and tried stamping another number. Why? I think he had stamped the Collins SN that was on the rear chassis so the two SNs would match on his R-388 (they shouldn't match!) I removed the number he had stamped and cleaned up the serial number area but since I don't have the correct way to stamp a number, I've just left it blank - even though it IS the original tag. Close examination of the back of the data plate does reveal the number "701" had been stamped originally. I'm going to assume it's correct for this receiver but, in reality, considering where the receiver came from, along with the USN anchor acceptance stamp that was under the Army tag, I can't be sure that this tag even is the "original" data plate.
The second find was probably a year later when a nice condition Burlington carrier level meter showed up on eBay at a reasonable BIN price. It was acquired and restored. Then the Marion Electric meter was removed and the Burlington meter installed.
About six months later at a mid-July (2016) local ham swap meet there was a "FREE" R-388 front panel. It was in very rough condition and missing almost everything but it had nice original grab handles and a perfect escutcheon. These pieces were removed, cleaned up and then installed on my R-388 (note in the 2013 photo above that the grab handles are "close." The jam nuts and the mounting screws were incorrect.)
UPDATE: Feb 28, 2023 - The new 2023 photo to the right shows the R-388 "Basket Case" today, seven years after completion,...well, as complete as it's probably going to get. This R-388 is still fully functional.
UPDATE: May 1, 2023 - SN:701 is now installed in a two-position table rack with SN:33, a R-388 that was converted to a 51J-4 with the installation of the Collins 354A-1 Mechanical Filter Kit. Photo is at the end of Part 3 in the Conclusion section.
1951 R-388/URR - ORDER: 3131-PHILA-51 - Mil
Collins sn:1743 - 2023 photo
CONTINUE to Part 3 GO to Part 1 Return to Home Index
Western Historic Radio Museum
Vintage Radio Communication Equipment Rebuilding & Restoration Articles,
Vintage Radio History and WHRM Radio Photo Galleries
1909 - 1969
- 60 years of Radio Technology -
This website created and maintained by: Henry Rogers - Radio Boulevard, Western Historic Radio Museum © 1997/2023