Radio Boulevard
Western Historic Radio Museum
 

Collins Radio Company

51J Series of Receivers - Part 2
 

51J "Oddities," 70E-15 "M" PTO vs "CR" PTO, BFO Inversions, Break-in Set ups,
Details on Common Problems, Freeing Stuck Trimmers, Restoration Tools,
Repairing the 70E-15 "M" PTO, Rebuilding the 51J-1 SN:652
Rebuilding a 51J-2 Receiver

by: Henry Rogers WA7YBS

51J-2 B&W Artwork from the Manual - 1950

PART 2

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 the R-388, 51J-3 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, so, the poor sensitivity on an antenna but a good CAL response might indicate L-104 damage. 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 coil 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 nor 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.

It's easy (if you use the receiver a lot) to remember that on Odd Bands USB reception will require the BFO index to be to the left of the center mark. If you're on an Even Band, USB will require the BFO index to be to the right of the center mark. Or,...since 80M and 40M are both Odd Bands and LSB is normally used there, have the BFO index to the right of the center mark for those two ham bands. But, 15M is also an Odd Band but USB is normally used there so the BFO index is set to the left of the center mark for 15M. Even Bands are 20M, 17M and 10M and all typically are USB so the BFO index is set to the right of the center mark for those ham bands. The upper portion of 10M is an Odd Band so BFO to the left for USB. It sounds confusing but once you've operated the receiver a few times on SSB with taking into account the BFO inversion between Odd and Even Bands, selecting the proper sideband is easy (see drawing to the right.) IMPORTANT NOTE: You have to be sure that the BFO knob index is mounted correctly on the BFO shaft. Setting up the correct BFO knob position is covered in detail in any of the 51J receiver's alignment procedure. The basic set up is to first rotate the BFO knob one full turn clock-wise, then continuing C-W, set the BFO to the highest frequency (fully unmeshed C) then, without moving the BFO shaft, mount the BFO knob so its index is at 90º C-W from vertical. Then set the BFO index to vertical and align the BFO coil assembly adjustment to 500kc. With this setting, if the BFO is rotated from index-vertical to the right (or C-W) the BFO frequency will increase from 500kc. If the BFO is rotated from index-vertical to the left (or C-C-W) then the BFO frequency will decrease from 500kc.

BFO Position for Sideband Selection

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. Voice transmitters, if their audio response was set up to favor the lower voice frequencies, would have to lower the audio gain level in the speech amp which in turn lowered level of modulation. The voice's low frequencies have a lot of "speech power" that would easily cause over-modulation. Most amateur and military voice transmitters were fairly low power and if they were set up with "bassy, low-modulation level" audio, the signal intelligibility would be lost and copy would be difficult, if not impossible. 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 with an IF bandwidth of about 6kc at -6db and 1.5 watts available audio power at less than 15% distortion. The 51J-4 widened the IF band width a bit more than that, using a 3pf IF coupling C where the R-388 used a 2pf IF coupling C, and added mechanical filters to the 500kc fixed-IF that actually determined the band width and created a steep-sided, flat topped bandwidth that, while great for selectivity, created serious audio issues in the AM mode using the 6kc mechanical filter. Certainly not impressive audio 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 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. Whenever I've used the 600Z output to drive a 600Z/8Z matching transformer and an 8Z loudspeaker, I've found the audio to sound "thin" and not nearly as "clean" as the 4Z output. I've tried several 51J/R-388 receivers and multiple speaker set-ups and I always have the same results.

With the very late R-388 receivers and all but the earliest 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. However, it does depend upon what type of stations are listened to. AM Broadcast from local stations will sound pretty good on a J-4 if the receiver is tuned to the carrier plus one sideband allowing the audio to go up to about 6000hz (theoretically,...and since AM-BC stations are limited to a 10kc bandwidth their transmitted audio should only be up to about 5000hz,...theoretically.) Since there's no fading or phase distortion with ground wave signals, this type of reception might benefit from an external audio amplifier with bass and treble controls to enhance the audio frequency response. Any SW-BC or AM ham transmissions will be subject to skywave propagation conditions and lack-luster signal strength that usually seriously affects audio quality. Limitations on any local AM-BC would be in the program material and that's pretty much the same for any AM-BC or SW-BC. 

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.

General Information on Rebuilding 51J Receivers

If work is required in the receiver's front end, it is extremely difficult to access any of the parts in the crystal oscillator section and most of the other front-end sections 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. An absolute necessary for working on almost any Collins equipment is 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 it's a "must have" to access many of the Bristol set screws deep inside the receiver can't be reached unless you have 99PS-60 set. 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.

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, however this turned out to be from a partially broken Oldham coupler skewing the EPE mechanically. The ease of adjusting the 70E-7A makes finding an excessive 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 seems to be a combination of how the receivers were operated and maintained along with possible ferrite problems within 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. Also, there are reproduction R-388 dials (just the plastic disc) from W3HM Radio Labs, Howard Mills.

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 functionality. 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" and wrecked 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 nice 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.

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.

The filter capacitor assembly plugs into an octal socket (except for the 51J-1.) 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. There are also a couple of tub mount electrolytic capacitors used that should be checked carefully.

Resistors are generally Allen Bradley JAN-types that are extremely stable and never seem to drift in value - but check them anyway.

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.

Source for 51J Series Manuals: The best source for online 51J Series manuals that can be viewed and downloaded for free is from jptronics.org that hosts "K4OZY's Collins Repository." All of the 51J Series receiver manuals can be found on this website. They are PDF formats. Search "K4OZY's Collins Repository" and it will come right up.

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, IF transformers, 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 a complete Crystal Filter assembly or gearbox parts probably would require obtaining a parts set. Some parts are available from W3HM Radio Labs, see References, end of Part 3.

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 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.

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 connected 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 about them 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.   >>>

Garolite Alignment Tools - Repros

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.

The rubber bumper type of feet are found mostly on the 32V transmitter cabinets mainly because of the weight of the transmitter (~90 lbs out of the cabinet.) Some of the receiver cabinets might have 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.

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. 

The photo below-left shows the Crystal Filter after the cover has been dismounted revealing most of the components. The middle photo shows the choke that has to be dismounted. The right photo shows the location of the nut under the choke. This nut has to be removed to allow the top cover to be dismounted.           Photos by: Jan Wrangel SM5MRQ

NOTE: Crystal Filter Alignment - It can be seen from these photos that T101 and T102 have what appears to be a slug adjustment on top and a threaded rod adjustment on the bottom. Each of these IF transformers have only one slug and one adjustment. The top adjustment it easiest to access and is usually the one that's used. The slug also has a brass threaded rod that exits out the bottom and can be accessed from under the chassis, if required. Although it looks like there are two adjustments on both T101 and T102, there's just one slug per transformer that has two methods of adjusting the position of that one slug. T101 is adjusted for peak at 500kc and T102 can be adjusted using a sweep generator to present the IF bandwidth as an oscilloscope pattern. Then T102 is adjusted for the most symmetrical response as the Phasing control is adjusted in position 2. In the USN manual for the URR-23A, an alternate procedure is shown (and it's not in any of the other 51J manuals) that is for adjusting T102 without a sweep generator or 'scope. This procedure has the operator offset the signal generator frequency by 3kc when in position 2 and adjust T102 for the most symmetrical -DCV output on the diode load when the signal generator is alternately adjusted to 497kc and to 503kc or +/- the 3kc. The USN manual does indicate that this is an approximate adjustment of the Crystal Filter.

Thanks to Oskar SP5LKO, who searched through Hollow State Newsletters and found that Dallas Lankford also researched this problem. Lankford found the 8.300mc is the frequency to set the 51J receiver to and that will have all three cam points showing through their alignment holes.

Note in the photo shown to the right, the cam point alignment holes are only on the front plate of the rack assembly. Also, the three holes are very small, probably about 0.125" diameter. In the photo, the lowest frequency cam is pointing to the hole at the lower-right corner of the front plate. The second hole isn't visible in this photo. The third hole can be seen in the background and it can be seen that the highest frequency cam point is aligned with its hole. The receiver is a R-388 and it's set to 8.300mc. These little holes are not very obvious and, from the front of the rack assembly plate, they are difficult to see. Also, the cam points are difficult to see. Requires a flashlight. Perhaps the intent was to use a piece of 12-14 gauge wire bent at a right angle and then insert the bent end into the hole to act as a "feeler gauge." In actuality, this is just a quick check to see if the slug rack is set up correctly.

Thanks Oskar!

70E-15 "M" PTO - Fixing the End-Point Error Problems

Just a reminder,...there isn't really any differences in the "M" or the "CR" versions of the 70E-15. All 70E-15 PTOs are the same. The letter suffixes and prefixes are only indicators of which receiver the particular PTO was installed in. "M" was the identification for a 70E-15 installed in a R-388. "CR" was the identification for a 70E-15 installed in a 51J-3 or 51J-4.

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 sometimes use a 10X oscilloscope probe to isolate the measurement load from the circuit if I'm measuring an oscillator directly.  

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. The object of reducing the L002 inductance is to skew the adjustment range to where it will compensate for the ferrite changes. But, as the range is moved the span of adjustment is greatly reduced. Taking off 1.5 turns reduces L002 to just 2.5 turns and that might narrow the span of the adjustment range to the point where adjustment of the slug has no effect 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  Additionally, Dallas Lankford, in the Hollow State Newsletter, wrote several articles about the 70E-15 PTO and the "Orr fix" problems that he ran into. Basically, Lankford felt that Orr had just ignored the linearity of his modified 70E-15 PTOs since he doesn't mention it in his write-up. Lankford provides a lot of data and graphs on the 70E-15 PTO EPE range and on its linearity after the Orr fix,...more in the next section. The Hollow State Newsletters that relate to the R-388/51J receivers can be found on www.navy-radio.com website.

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" isn't recommended. It can be tedious work that might easily end in disaster. If you feel that the linearity is just too far out and you have the confidence to proceed then 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. Once you remove the PTO cover, all tracking is affected and to check adjustments requires putting the cover back on. Back to the stack,...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 (probably why Orr didn't bring it up.) 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 Newsletter 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 over the entire 1000kc range which is pretty tight.

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. A dental mirror can be used to examine the trimmer if the PTO is installed in the receiver (access plug has to be removed.) 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.

70E-15 Adjustment Tools for L002 Trimmer EPE

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.   >>>

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.

For a real "quickie" test of the EPE, I don't even bother with the KC dial. I use a fine tip pen and mark an index line on the front mount frame and another index mark on the oldham coupler. I first set the PTO to be at 2.500mc output and then mark the index lines. Rotate the oldham coupler one turn, align the index marks and read the frequency output. Mark down the frequency. Go five turns taking readings at each turn with the index lines aligned. Go back to 2.500mc output and then go five turns in the other direction taking readings at each turn with the index lines aligned. You can now see the PTO linearity from 3.000mc to 2.000mc and you can calculate your EPE. If you need to adjust the trimmer inductor, it's easily accessed. This method isn't as accurate as using the KC dial but it's a good "quickie" test.

Try This PTO Set-up Before Digging into a 70E-15 - I suggest first trying to set the PTO to 2.500mc output, check to make sure the MC dial is in the exact center of its span (6.0mc, for example,) then mechanically set the KC dial (loosen the dial's set screws) to 0.00 and carefully snug-up the set screws. Verify that the PTO is still at 2.500mc exactly and the KC dial is exactly 0.00. Now perform the remaining front end alignment. This will result in the best possible tracking taking into account the PTO EPE will only be "dead-on" at the center of the MC dial and the error skews off as the dial is tuned from center. In this method, the noticeable EPE is mostly at the ends of the dial and it's actually only half of the total EPE since the reference is at the center of the PTO span (instead of "end-to-end.") This results in the best tracking where most of the listening is actually done.  

Rebuilding the Collins 51J Series Receivers
 

1949 51J-1 Restoration

When I purchased this 51J-1 it had already been "gone through" electronically in the form of replacing the electrolytics and a few paper dielectric capacitors. It did seem to function pretty well although there were several minor issues. So, this is going to be mainly a cosmetic restoration - hopefully. These restorations are always going to have "surprises" that are found as the receiver is disassembled and even more "surprises" will be found as the receiver goes through the entire electronic process necessary for it to become a fully and correctly operating receiver. Though this 51J-1 is the most original and complete one I've seen, it still has some minor modification issues that are both electronic and cosmetic. The length of this write-up will be determined by the number of "surprises" that I always seem to find as a restoration progresses.

1949 Collins 51J-1
 "As delivered." The front panel seems like a "flat finish" but it's just devoid of any luster. The knobs, likewise, are flat and have no color depth. The S-meter has darken considerably from sun exposure. The sunburned KC dial isn't showing in this photo but it is shown in the next photo down to the right.

1. Sunburned section of the KC dial - It's severely discolored (not shown in photo to the left.) The discoloration is of the middle section of the three-part laminate making up the dial, so it must have been exposed to bright sunlight for decades. I have a spare 51J-2 KC dial but can't find it at the moment (an extensive search has proven futile.) Repro KC dial from W3HM, installation on original hub write-up at the end of this article.
2. Small dent in the dial escutcheon (by "51J.") This is a minor dent that's only noticeable in certain light or viewing angles. Should respond to bodywork techniques (it did.)
3. The "hold down" assembly for the 100kc crystal is missing. I have a spare assembly from the 51J-2 parts set.
4. Missing trim screws below the tuning knob. These screws are in all 51J receivers and were intended for something that apparently was never installed. The screws just "fill" the tapped holes. I have original spare screws.
5. Darkened S-meter scale. This is the same scale that's used on the 75A-1 receiver. It's not bad but I'll keep an eye out for a spare 75A-1 S-meter to show up some day (or maybe I can recondition this one,...a surprise,...that worked!)
6. ZERO adjust knob is a more modern type. I have several spare original types.
7. Tuning knob and Band Change Knob both need attention. The skirt on the tuning knob is "flat black" - might be paint. Band Change knob needs to have a crank fitted. I might have a spare crank.
8. Dial fiduciary (index line) seems to bend and flex with tuning and especially when adjusting the ZERO ADJ. I have to pull the front panel to see what's going on. I have a spare 51J-2 fiduciary, if needed (the problem was that the dial glass had slipped out of its retaining clips and was allowing the index to rub against it.)
9. Although very minor, the power transformer was painted black at sometime in the past. It wasn't removed from the chassis but was painted in place with a brush. The original color was gray. I'll repaint the same way (turned out looking great.)
10. General clean-up required. Front panel is rather "flat" looking. Just needs some reconditioning. Chassis has a lot, a whole lot, of gritty dirty deposits all over. It brushes off pretty easily. Also some minor pitting that might respond to cleaning.

1. The receiver actually works pretty well but there are several minor problems. They don't really affect the actual reception of signals but are in other areas of performance. Bandwidth seems a lot wider than expected.
2. BFO set up incorrectly. Couldn't select sidebands properly. Easy 5 minute fix.
3. The 70E-7A PTO had an EPE that tested at an incredible 12kc off! I did have to calibrate the 100kc oscillator and then I did verify the results using the HP 606B and DFC. I've never run into a 70E-7A that had this much EPE but maybe it has never been adjusted at all,...ever! If the 12kc EPE doesn't adjust out, I do have a spare 70E-7A PTO that I could use as a replacement (all of the 70E-7A PTOs that I've tested before had an EPE of 1kc or 2kc. The actual cause of this unusually high EPE wasn't the PTO. It's covered in "Another PTO Surprise" further down in this write-up.)
4. This is partially cosmetic but I will be replacing the modern molded AC plug type of power cable with a more vintage-correct power cable. The original AC power cable was a two-conductor, black rubber "zip cord" with a period molded type AC plug installed. Original length was six feet long.  
5. I might do something about the re-cap job that, although excellent work, used the intensely-yellow polyfilm caps. Nice caps but terrible in appearance. I usually paint them dark gray and use color code "dots" for a more vintage appearance. Also, the replacement electrolytic caps might be placed inside the original cans since those are still present.
6. And, speaking of capacitors, someone added 3pf capacitors across the IF transformers. These are "coupling" capacitors that are found in later R-388 receivers and in all 51J-4 receivers. It's possible that someone just copied the J-4 schematic not knowing why the caps are there in the J-4 or it could also be that someone was looking for a broader IF bandwidth. The J-2 (and certainly the J-1) had a very narrow IF passband (4kc to 5kc at -6db) and do have a tendency to sound "muffled." I'll have to consider this "originality versus modification for modern usage" but I think I'll probably go with original and remove the caps.

Both cosmetic and functionality inspections were performed on May 1, 2023

51J-1 showing the "sunburned" KC dial

Front Panel Dismount - To clean, touch-up and recondition the front panel required dismounting all of the knobs, controls, S-meter, dial bezel and screws. This then allowed complete access to the entire front surface of the panel. Actually, the panel was relatively clean but it didn't have any luster that the St. James Gray wrinkle paint should have. Another thing is the tint of late-forties St. James Gray is a shade or two lighter than the St. James Gray found on the 75A-4 (as an example of the later, darker St. James Gray.) Touch-up has to be custom matched. It does have some earlier touch-up that probably was pre-mixed St. James Gray since it doesn't match very well. Of course, it's possible that most of the early St. James Gray examples I've seen were just somewhat faded with time and exposure to light. Where necessary, I did a touch-up with my "custom matched" paint (this paint has to be matched after the panel reconditioning since the final color is what the "match" is to.) The final part of reconditioning is to give the panel a rubdown using "3 in 1" oil (or any light-weight machine oil.) This is applied with a clean cotton cloth in a circular pattern and allowed to set for a few minutes. Then the oil is wiped off with another clean dry cloth. This is repeated the next day. This treatment restores the wrinkle finish luster in a subtle manner. Not glossy like Armor-All (which is also very slippery) or possessing that persistent, ever-lingering odor of Boiled Linseed Oil.

Nothing unusual was found in the front panel dismounting. All locking washers were present but, in this case, all of the locking washers were on the inside of the panel (not mixed like the R-388s are.) The S-meter has a small tie-point mounted on the upper right stud that should be noted when dismounting the meter. Also, the S-meter was mounted with locking washers but many other screws and nuts didn't have locking washers. Some of the screws appeared to have "green Loctite" applied to take the place of locking washers. Other nuts just appear to have been removed and then replaced with the locking washer left out.

Dial Escutcheon Bodywork - It's fairly difficult to remove a small dent when the surface is painted and has some sheen. The first step is to not be too aggressive. It takes time working in small steps to remove the dent without damaging either the paint or making the bodywork look worse than the dent did. I protected the paint with a heavy paper (a piece of manila folder.) I made sure I was going to work on a flat surface. I used a single layer of blue masking tape to pad the area on the backside of the dent. I used a small wooden block as an interface between the dent and the body hammer. Then, I only used very light taps. This required checking the progress often. Using this method, I was able to totally remove the dent with no indications of any bodywork. The time required was close to 30 minutes which was expected since I really couldn't "rush" this process.

Chassis Cleanup - Lots of gritty, greasy gunk. It's wide-spread and everywhere on the chassis and components. Luckily, it cleaned up very easily using WD-40 as a solvent applied with a small acid brush. I had to do a section at a time which is normal when the chassis is this dirty. I used a short bristle acid brush with a small amount of WD-40 to cut the gritty-grease that had deposited on all of the trimmer capacitor rotor heads. This had to be followed by a wipe down with Glass Plus using Q-tips. This leaves the trimmers clean on top and easier to find the slot when doing the alignment. I don't think the trimmers for the Crystal Oscillator were ever adjusted since they were covered with grease (I might be finding some "stuck trimmers" during alignment - and I did! - a lot of 'em! I used a heat gun to loosen the stuck trimmers.) Once all of the grit and grime were removed then the chassis was again cleaned using Glass Plus to remove the WD-40 residue. All of the tubes had to be removed during this cleaning to allow better access and to check how much gunk had gotten into the tube sockets (they looked pretty clean.)

Power Transformer Repaint - The black paint on the power transformer wasn't original and had been applied in the past with a brush. I mixed Artist's Acrylic in a gray color that was matched to the color on the audio output transformer. I also applied my paint with a brush. If brushing is carefully done, it doesn't get paint where you don't want it and, if it does, a little Glass Plus and a Q-tip will remove any accidents. Looks original,...well,... from a distance it looks original (it looks much better than gloss black.)

Another Dial Bezel - I found my spare 51J-2 dial bezel and compared it to the 51J-1 dial bezel. The J-2 bezel had a pretty deep scuff on the left side that I couldn't really remove with light polishing. The original J-1 bezel, though not perfect, is in better overall condition and it will be used in reassembly.

Sunburned S-meter Scale

In addition to cleaning the scale, I also cleaned and polished the meter's bakelite housing. Not "glossy" polished but just a slight sheen that looks like a good condition, clean original housing,...which is what it is, now.

S-meter Rejuvenated

Front Panel Remount - Before the panel is remounted the S-meter must be installed. This allows easy access to the two lower nuts and also easy installation of the locking washers. It's easier to mount the dial escutcheon at this time too. The KC dial has to be placed on the tuning shaft.* As mentioned in other restoration sections, since all of the controls and the harness are not mounted, there's really nothing to support the front panel while these controls are being mounted. I used two wires running from the top hole in the side panels down to the top holes in the front panel. This supports the front panel at about a 90º angle down from vertical. Each control and switch has an internal tooth locking washer against the inside of the panel and a dress nut on the outside. Once the controls, switches, nuts and washers are installed (but not fully tightened yet) the front panel can be moved into position. Remove the securing wires as the panel is guided to fit the Crystal Filter controls through the adjustable plate mounted to the back of the front panel. Also, the CAL adjustment has to be guided through its hole (I just loosened the set screws and slid it back for now.) The front panel should slide into position easily but watch the backside for anything that might be interfering with the proper fit, like the NL switch or wires being pinched or similar. If there's no interference then the panel screws can be installed. After that, the control and switch nuts can be snugged-up. I had to watch that the switches remained vertical so that the shaft flat matches the knob index position. Part of the top of the front panel mounting on the J-1 and J-2 is the Collins Winged Emblem. This is mounted with two nuts and locking washers and secures the upper rail above the drum dial.

* Even though this KC dial is sun-damaged it still is functional and I need to proceed on with the electronic portion of the refurbishment, including checking out the 12kc EPE on the 70E-7A PTO. When a R-388 KC dial is found, I'll just drop the front panel and do a R&R. 

Normally, I don't have to be that aggressive in cleaning knobs but all of the knobs on this receiver were in very poor condition from sun over-exposure. Bakelite knobs that have been in normal interior room environments generally clean up easily with Glass Plus or a warm soap and water soak followed by a brushing to remove finger-gunk from the flutes and then a rub-down with a dry cloth.  

Synchronizing the KC dial to the PTO Using a Digital Frequency Counter - The procedures in all of the 51J manuals were written long before Digital Frequency Counters were even around. The entire alignment section is burdened with these lengthy descriptions of how to use the 100kc CAL and harmonics to verify the accuracy of the alignment signal source, be it the RF Signal Generator or the PTO output. All of these multi-step set-up descriptions can be skipped by just using a Digital Frequency Counter as the measuring device. Using the DFC to measure the PTO frequency directly is much easier and more accurate. As expected, the procedure for sync'ing the KC dial is several steps long and assumes the PTO is in calibration. An easy method is to set the PTO output for 2.500mc measured with a DFC. This is the center of the PTO range. Then make sure the MC dial is exactly at mid-scale, e.g., 8.0mc. Now align the fiducial to vertical with the ZERO ADJ. and then set the KC dial to exactly 0.00 and snug-up the set screws. At this point, the end-point error can be measured. Since I had the front panel off and had the CAL adjustment off, I had to recalibrate the 100kc oscillator before the EPE test. But, an easier, direct and more accurate method is to use the DFC on the PTO output and check the frequency out versus the KC dial readout per turn. With this direct and much more accurate approach, the EPE measured 10kc - still, way out but it's a direct and accurate measurement.

With the Triangular Cover Removed C-003 is Exposed

To dismount the PTO, the KC dial has to be removed. Then there are three screws that mount the PTO. One screw is easy to access but the second screw is accessed through a hole in the gearbox plate. The third screw requires tuning the gearbox to align one of the gears that has a hole that will align with another hole in the gearbox plate and allow access to the third screw. Since total removal of the PTO isn't necessary to remove the Oldham coupler, the top-mounted plug on the PTO connector doesn't have to be removed but the KC dial lamp assembly does have to be pulled out of its hole-mount. Now, the PTO can be drawn back and down a bit at the rear, up slightly in front, but, as said, it doesn't have to be totally removed just to change the coupler. When reinstalling, it's pretty easy to insert the center ceramic piece into the PTO-side hub and then guide the PTO into place making sure that its Oldham coupler piece aligns with the other hub nearest the gearbox. Mount the PTO and push the Oldham coupler together. I had to adjust the hub positions a few times to get everything to tune correctly and not bind. The "key" is to be sure all three coupler pieces are pushed together and that the hubs are about centered on the two shafts. Now, the KC dial can be reinstalled and the front panel put back into place. Next, synchronize the PTO,...again.

The Oldham Coupler - the hub on the right is broken

51J-1 Chassis after cleaning. Power transformer is now painted gray, 100kc crystal "hold down" installed, many replaced tube shields.

I performed a new EPE check and now the linearity is very good at about 1kc for a 900kc range. The bottom 100kc skews a bit and adds another 2.5kc. This is actually the high frequency 3.000mc from the PTO (which is the lower end of the tuning range on each band.) I tried several other settings of C-003 but those settings increased the EPE. The best I could achieve was to have 90% of the PTO tracking perfectly and 10% being about 2.5kc off. Not bad for a 75 year old PTO. As to the initial 12kc EPE (that I had measured several times with the same result) I think the broken coupler was creating backlash at the PTO that wasn't visible on the KC dial and wasn't felt while tuning. This probably was enough to really skew the testing and give the unusually high EPE results. Also, since the last conversion relies on the PTO to consistently and accurately mix with the selected Variable IF to provide a stable 500kc, now, with the greatly improved PTO stability and linearity, the overall increase in consistent sensitivity is noticeable.   May 11, 2023

Alignment - I can tell the receiver is pretty close to being in alignment (well,...maybe) but I don't think the Crystal Oscillator trimmers were ever adjusted. The ceramic tops (rotor) were covered with grime and gunk that looked like it hadn't been disturbed in decades. The Crystal Filter isn't working as good as it should. So, like the tube testing, I really don't know what was done earlier as far as alignment. If it was done, I don't know the type of equipment used. So, I'll just go ahead and do a complete alignment.

Four of the Crystal Oscillator trimmers were stuck but a little heat (from a heat gun) got them to easily break loose. I checked and cleaned the crystals and the holder (all of the crystals were date-stamped 12-49.) Everything looked okay. Some of the bands didn't show any output on V-102 pin 7 but I still had a signal present at the correct frequency. I used DeOxit and a small paint brush to clean the rotor contacts on the trimmers. This got all of them indicating voltage on V-102 except for the 30 and 20 - the bands work and receive signals but I don't get any voltage indication on V-102.

500kc Fixed IF Alignment - As mentioned in the "inspection" section above, someone installed coupling capacitors on the IF transformers, probably to broaden the IF bandwidth to make the receiver sound more like a "broadcast receiver." I'm going to remove these because I think one important characteristic of the 51J-1 and J-2 is the very narrow IF bandwidth. The other important difference is where T-101 is located in the Crystal Filter housing. Only in the 51J-1 and J-2, T-101 adjustment access is located through the left hole (when viewed from the front of the receiver.) R-388, 51J-3 and 51J-4 have T-101 access in the right hole. The RC load does have to be used for the alignment and the alignment procedure is basically the same as all other 51J receivers. Initial testing determined that the IF was centered at 508kc and the Crystal Filter crystal frequency was 500.6kc which is probably why the Crystal Filter seemed mostly unresponsive. I adjusted all of the IF transformers for 500.6kc and T-101 was also adjusted for 500.6kc. The gain through the IF section increased noticeably and the bandwidth, as expected, was narrowed to about 4kc at -6db. Also, typical of the 51J-1 or 51J-2, the audio has restricted response to higher audio frequencies sounding somewhat "muffled" and this is normal for the J-1 or J-2. Since the Variable IF was probably aligned for 508kc, further alignment will be required.

The calibration procedure for the Crystal Filter that's in the Collins manuals will require using a sweep generator and an oscilloscope of adjustment of T-102. However, in the USN URR-23A manual (it's the only place I've seen it) there is an alternate procedure for T-102 adjustment without the sweep generator or 'scope. It requires monitoring the diode load voltage which can be accessed at the detector output load resistor in the circuitry. With the IF already aligned to the Crystal Filter crystal frequency, then T-101 is adjusted for maximum negative voltage on the diode load. Then, the Selectivity is set to position 2 and the signal generator is set to 497kc or 3kc lower than the IF and the diode load voltage checked. Then the signal generator is set to 503kc or 3kc higher than the IF and the diode load voltage checked. T-102 is adjusted for the best symmetrical response in Selectivity position 2 measured at the diode load. The URR-23A manual does indicate that this is an approximate adjustment and that the sweep alignment is best. Another note,...the Crystal Filter circuit is identical in all versions of the 51J Series, so using the URR-23A procedure will work fine.

Finishing Up - I installed the black rubber zip cord AC line cord. This was a vintage piece in excellent condition. The round three-conductor cable that was installed had the wrong size strain relief (too small.) The vintage cord I used had its original molded AC plug - it all looks authentic, especially with the correct strain relief. I installed the bottom cover that didn't have a single locking washer with the nuts and didn't even use all of the nuts necessary. I added external-tooth locking washers and all of the nuts. I also replaced a missing screw/nut/lock washer that was part of the internal front-end shield mounting. All side and back screws and lock washers were missing for the shield mount so the proper hardware was found and these were then installed. Final completion will take place when I find a KC dial from a R-388 receiver. But, I can still use the receiver without any issues while I continue to search for a KC dial or a R-388 "parts set."      May 15, 2023

So, the 51J-1 does sound pretty much like my 51J-2 and that's to be expected. Definitely more restricted audio than a R-388 receiver. Sensitivity is the same as any of the 51J Series. As a working example that has been set-up to function like the original versions of the 51J-1 (as they were originally sold,) I can see why there must have been some real excitement at the Signal Corps back around 1948, when Collins provided them with a few examples of their new 51J receiver. The 51J-1 was "miles ahead" of the other receivers that the Signal Corps were testing for RTTY (modified BC-342, modified BC-794, R-274 and the SP-600.) With "to the kilocycle" dial accuracy, super selectivity and no frequency drift, the Signal Corps "had to have 'em" and, eventually, Collins supplied the military and civilians with several types of 51J Series receivers from 1948 up to 1964. 

The dial is mounted to the three-piece hub by a "rolled edge" of the brass material that creates a "lip" that holds the front flange and two rear pieces, a rear flange and the hub, together with the plastic dial in the middle. There is also a locating pin that is driven in to keep the dial and hub orientation correct. Before removing the hub and flange assembly, be sure to note the position of the locating pin in relation to the dial scale. The correct orientation is important so that the set screws are accessible when synchronizing the dial. This KC dial I'm doing has the locating pin aligned with "35" on the black scale of the dial.

To disassemble the hub, first the locating pin has to be driven out. Use the proper size drift pin (.060" dia) and just tap it out from the rear side with the flange supported by slightly open vise jaws. Once the pin is removed than the rolled edge has to be cut off. I used a Dremel tool with a 1.25" diameter flexible cut-off wheel. The hub should be mounted in a vise and then the Dremel tool cut-off wheel can be gently used, a little material at a time, to remove the rolled edge. If carefully done, very little scuffing will show on the flange part when the rolled edge is removed. With the rolled edge gone, the two pieces of the hub should come apart and the old plastic piece removed. Note that the rear hub is actually two pieces, a rear flange and the hub. Next, a small notch has to be filed on the new dial to the inner part of the hole aligned with "35 black" which gives clearance for the locating pin. Try to match the notch in the old dial for size and placement.

Next, clean up the hub pieces as needed and then try a "dry fit" of the repro dial to see if everything goes together. If the fit is good, then proceed with the reassembly. Now, since the rolled edge has been removed there isn't a way to hold the front and rear hub pieces together. But, epoxy can be used to glue the assembly together. Be sure that the locating pin is aligned properly (in my case, with "35 black.") Very little epoxy is needed (a couple of drops is all) since too much will extrude out as the hub pieces are clamped together. To make an easy clamp that will keep the hub together as the epoxy sets up, use a 2" long 1/4 x 20 bolt with one fender washer with a 1/2" diameter hole, a flat washer and a nut/flat washer. This will provide an easy way to clamp the assembly and be able to keep the rim of the flanges in view to allow cleaning off any excess epoxy that might be extruded out. The fender washer goes next to the front flange part of the hub (the 1/2" hole provides clearance for the hub rim that extends just slightly out the front and the fender washer itself pushes against the front flange.) One of the flat washers goes next to the underside of the bolt hex head. The other flat washer and nut push on the rear part of the hub (see drawing #5 below.) Do a "dry run" on this clamp set-up to see how it works before you apply the epoxy. When applying the epoxy, only use a couple of drops. Any more will be extruded out as the clamp is snugged-up. Before the epoxy begins to set up, install the locating pin (just taps in with a small hammer.) Snug-up the nut on the clamp, do not over-tighten. Just a slight bit more than finger-tight is all that's needed. Check the dial to make sure there aren't any epoxy smears, it's easy to clean off before it sets up. Let the epoxy set up for a few hours. Remove the clamp and the KC dial is ready to install.

KC Dial Hub Differences - This procedure describes a KC dial hub from the 51J-1. Later dials use three rivets and the brass hub doesn't have to be a press-fit (and the edge isn't rolled over.) The procedure for fitting a repro KC dial is basically the same idea except the rivets have to be carefully drilled out. The repro dial has to be drilled if rivets are going to be used. Some restorers assemble using epoxy. Be sure to keep the orientation of the dial scale to the set screw locations correct. Also, if you want to use new rivets then you probably wouldn't need the epoxy. I've included a photo of a KC dial from a 51J-2 receiver (photo #6) showing the minor differences. R-388 hubs or 51J-4 hubs are similar to the 51J-2 hub shown.
 

1 Repro dial on the left. Original dial on the right with sunburned section.

2 Front of J-1 hub. Note the locating pin at the 8 o'clock position and the rolled edge.

3 Rear of J-1 hub. Note the hole for driving out the locating pin. Hub and rear flange are separate pieces.

4 J-1 Hub pieces removed. Note the locating notch at 4 o'clock in the KC dial hole.

5 I didn't photograph the clamp, so here's an "exploded view" drawing that should help in visualizing how it works.

6 Front and back of the 51J-2 KC dial hub - Rivets instead of a pin, edge of hub not rolled over

Synchronizing the KC Dial - One More Time!!! - Using a digital frequency counter connected to the output of the PTO makes this a simple operation. Set the MC dial to exactly mid-scale, e.g., 8.000mc. The PTO output should be 2.500mc or the exact mid-range of 2.000mc to 3.000mc output from the PTO. Align the KC dial to read "0" exactly and tighten the set screws. That's it. Pretty simple with a DFC. But, what if the set screws aren't pointing down so you can't access them from the bottom of the chassis when the KC dial is at 0? Then use this procedure,...the KC dial is 100kc for one full revolution as is the PTO. So figuring where to set the dial for the set screws to be accessible isn't too difficult. Set the PTO to 2.500mc output. With the set screws loose, rotate the KC dial until a set screw is accessible from the bottom and note where the KC dial is set. If, for example, the dial now reads 8.022mc, you're going to want to synchronize the KC dial to 8.022mc in order to have at least one set screw accessible. Now, set the PTO to read 2.500mc - .022mc = 2.478mc so set the PTO for 2.478mc on the DFC reading the PTO output. Now set the KC dial to 8.022mc. One set screw should be accessible now so tighten it. Now tune the KC dial to 8.000mc (mid-scale) and the PTO should read 2.500mc on the DFC and the KC dial should read "0." Go back and tune the KC dial to access the other set screw and tighten it. Station Receiver - I've installed the 51J-1 into a Collins A-type cabinet. It's actually a modified 32V-type cabinet but they are very similar looking to an actual 51J-type cabinet. The receiver is paired with a Collins 32V-1 transmitter, an age-appropriate match even though it's a "ham bands" transmitter. This 32V-1 is missing its serial number tag, so I can only guess that it's probably from 1948 or so. It's very easy to set-up the remote standby using a Dow-Key type of T-R relay. The Dow-Key auxiliary contacts can provide the "open" condition necessary for standby when transmitting. Since the remote standby just parallels the front panel standby, I'm just using the front panel switch for that function. The antenna used is the 75M collinear array using a Johnson KW Matchbox as the antenna coupler. The S-meter, like a lot of receivers of the time period, is very generous on 75M with many stations running up to +60db with normal background noise running about S-3. Bandwidth, as expected, is narrow but it allows "zeroing the carrier" for weaker stations for better copy and also "tuning off frequency" about 1.5kc, either upper or lower sideband, for higher frequency audio recovery on strong AM signals. Dial accuracy is right on and frequency drift is not noticeable since the mode is AM. On the Vintage Mil-Rad Net on 75M, all stations are always Q5. I've also monitored SSB transmissions over extended periods of time and have found the frequency drift of the BFO to be negligible.

1949 Collins 51J-1 - Repro KC dial installed

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)

A 51J-2 IF Transformer showing the coil structure beneath the ferrite shields. The typical problem will have the coil securing glue breaking and then the weight of the ferrite shield pushes the IF coils to the bottom of the coil form. This reduces the coupling between the IF coils and the overall IF gain.

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?

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.

photo above: The 51J-2 after rebuilding - Sept 2010

 
1950 51J-2 Installed in a Collins cabinet (2013 photo)

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 may change this back to .01uf in order to maintain the original "51J-2 sound" - Jun2023.) 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.

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

3.

3. 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 eliminates the 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.) 

(All five photos shown were taken in Sept 2010)

4.

4. The underside of the a 51J-2 receiver showing the smaller bottom shield that only covers the front-end of the receiver. Note that the bypass capacitors have been replaced on this 51J-2 while the .01uf tubular ceramic capacitors are still in place.

5.


5. One common problem encountered on the 51J-2 receiver is inside the IF transformers. The entire lower coil and ferrite shield have loosened and have slid down to the bottom of the transformer. This reduces coupling and output level of the transformer. To correct, the IF transformer must be removed, the coils put back into their proper position and then secured with epoxy to prevent future movement. Rough handling of the receiver such as "slamming" in down on a work bench or dropping it from a foot or so down onto the pavement at a swap meet is the probable cause. Out of six J-2 IF transformers inspected, from two different receivers, five transformers had this problem.

 CONTINUE to Part 3                             GO to Part 1                                         Return to Home Index