Radio Boulevard
Western Historic Radio Museum

Collins Radio Company

51J Series of Communications Receivers

PART 1: History, Design and Circuit Details for the 51J-1, 51J-2, R-388/URR,
51J-3, 51J-4, 51J-5 prototype, Lab 51J-4, Beckman Model 7700 (51J-4)
 and several other 51J variations, CY-1260/G Military Receiver Case
PART 2: 51J "Oddities," 70E-15 "M" PTO, BFO Inversions, Break-in Set ups,
How Collins Radio and the 51J were Responsible for the RACAL RA-17,
Details on Common Problems, Repairing the 70E-15 "M" PTO,
Rebuilding the 51J-2, Rebuilding the "Basket Case" R-388

PART 3: Refurbishing R-388/URR in CY-1260/G Case, Refurbishing the Light Gray Panel 51J-4,
 Rebuilding the R-388 w/ 354A-1 Mechanical Filter Kit Installation, Restoring the 51J-1

by: Henry Rogers WA7YBS-WHRM/Radio Boulevard

Artwork from the 51J-4 Advertising Brochure

Many Collins Radio enthusiasts believe that the 51J Series are the best looking receivers the company produced. But, what about 51J performance? Like many pieces of collectible vintage electronic gear, the 51J receivers are now well-over half-a-century old with early examples pushing the three-quarters of century mark. Many users are operating their 51J receivers in "as-found" condition but are they experiencing all that the 51J is capable of? Part 1 of this article details the evolution of 51J design and the differences in the various models. Details on rarely-seen versions, like the earliest version, the 51J-1, or the Beckman/Berkeley equipment that uses the 51J-4 or the Collins Lab Light Gray panel versions. Also, details on the various types of cabinets that could be used. Part 2 starts with the five 51J "oddities" - weird things about the design or operation of the various 51J receivers. Also, a comprehensive look at many of the common and not-so-common problems encountered when delving into any of the 51J receivers including details on how to correct the problematic 70E-15 "M" PTO found in all R-388 receivers. Part 2 finishes with restoration write-ups on the 51J-2 and a Basket Case R-388. Part 3 covers the refurbishing of a R-388 housed in the military CY-1260/G Receiver Case. Also covered is the refurbishing of the Collins Lab Light Gray panel 51J-4, the restoration of a R-388 that has the 354A-1 Mechanical Filter Kit installation and concludes with the restoration of the rare 51J-1 receiver.


51J Series of Communications Receivers - Part 1


1950 ad for the Collins 51J-1 Receiver. One of the early ads to appear in QST didn't necessarily mean the 51J-1 wasn't available much earlier. Or, that if ordered in 1950, the purchaser wouldn't have gotten a 51J-2.

Brief History of the 51J Receivers

Introduced through advertising in 1949, the Collins 51J receiver was intended for commercial-professional users that required an extremely accurate frequency readout and a very stable, drift-free receiver that was especially suited for use in data reception, such as RTTY, but could also provide excellent communications reproduction of AM or CW signals. The 51J was also the perfect receiver for a wealthy ham or SWL enthusiast - if he could afford its nearly $900 price tag. Collins Radio Company had entered into the ham radio receiver market with their ham bands only receiver, the 75A-1, two years earlier. But, according to Collins Radio, the development of a precision general coverage receiver had started (at Collins) towards the end of WWII, in 1944. The project lead was Roy Olsen from 1944 up to 1946 and then Lou Cuillard took over until completion. The new receiver was designated the 51J and the intended market was post-WWII commercial-professional users that could afford an expensive but advanced-design receiver. It's likely that the general coverage 51J was available concurrently with the 1947 75A-1 ham receiver, though the quantity of 51J-1 receivers produced from 1947 through 1949 was very limited. The 51J would provide commercial and possibly military users with general coverage reception from .5mc up to 30.5mc using thirty, 1.0mc wide tuning ranges and, with its crystal oscillator and permeability-tuned oscillator conversions providing better than 1kc dial accuracy and drift-free stability, the receiver itself featured performance that was literally years ahead of the competition. 

Signal Corps and Early RTTY Receivers - During the post-WWII era, the U.S. Army Signal Corps was trying to use some of the WWII vintage receivers for RTTY (Radio Teletype) and were finding the drift to be excessive for RTTY and the dial accuracy along with the rapid tuning rates to be completely inadequate. These receivers were usually the BC-342 in a highly modified form and the BC-779 Super Pro, also highly modified from its WWII form. Crystal control of the LO and the BFO were necessary on these receivers in order to have sufficient stability for RTTY capability. The story goes that Collins sent an early 51J receiver to the Signal Corps to try out. This certainly was before the 51J was commercially released. The Signal Corps' response was positive since the receiver was extremely accurate in its frequency readout, its tuning rate allowed easy tuning of very narrow signals and there was essentially "no drift" in the Collins receiver in stock form. The Signal Corps "had to have 'em" and soon Collins was providing the Signal Corps with the R-381/URR and the R-381A/URR, those receivers being the 51J-1 and 51J-2 respectively. However, there were some non-military aspects to these initial 51J designs that the Signal Corps wanted changed. The initial 51J had a fixed 300Z antenna impedance that really didn't fit many of the military antenna types. It also had the hams bands highlighted in green on the MC tuning dial implying a civilian market. It seemed that Collins had a receiver designed for commercial users or maybe a few wealthy hams but had also considered that the precision tuning and stability of the design would easily meet the military requirements necessary for reliable RTTY. So, although the initial 51J receiver as the R-381 and R-381A provided the Signal Corps with a thoroughly modern design that had the most of the RTTY specifications the had military sought, some minor "tweaking" was necessary for the receiver to become the final evolution that the Signal Corps and other military users needed. That version became the R-388/URR. Many thousands of R-388/URR receivers were built by Collins from 1950 up to about 1962. The R-388 was found in many frequency diversity RTTY set-ups used by the military, many times installed in portable RTTY communications huts.

Hallicrafters R-274

The Signal Corps RTTY Receiver Competition - Though Collins wasn't initially involved with the Signal Corps efforts to develop a sophisticated, post-WWII receiver that was stable enough for RTTY without modifications, two of the largest radio companies were directly involved. Both Hallicrafters and Hammarlund had been working closely with the Signal Corps since 1946 trying to provide a receiver that incorporated specifically designed options the Signal Corps felt were necessary for successful RTTY reception. It wasn't a coincidence that the two receivers from these companies had very similar features since they were both designed to meet those Signal Corps specifications.

Hallicrafters R-274 receiver was first produced in 1949. Of course, the design was almost all Signal Corps work with some Hallicrafters input. The Signal Corps wasn't impressed with the final product (even though it was basically their design) and only one contract was issued for the R-274. A second contract was issued for its updated successor, the R-274D, about 1952. Hallicrafters was probably planning on greater success for the R-274 and had certainly tooled for that type of production. It ended with Hallicrafters offering the R-274D as a ham receiver, the SX-73, but not many hams were willing to pay the $900+ asking price (that's over $11,000 in 2023 dollars.) The R-274 was a fine general purpose receiver,...for 1949,...just not for RTTY.

Hammarlund SP-600JX

Hammarlund was much more successful with their Signal Corps collaboration that produced the SP-600 Series of receivers. Typical of Hammarlund's lethargic engineering pace, though work began on the SP-600 in 1947, it wasn't really even ready when the first contract was signed in 1950. Production didn't really begin until 1951. The initial receivers were designated R-274A and R-274C to indicate their Signal Corps "roots." When built for USN the designation R-274B was used.

The SP-600JX also wasn't very good as a RTTY receiver but the the Army and the Navy (and ultimately the USAF) all ordered SP-600JX receivers by the thousands as a general purpose receiver. The SP-600JX version with the six-channel Crystal Oscillator was intended for increased stability for RTTY operation. The receiver was very difficult to keep on frequency unless the Crystal Oscillator was utilized. That required knowing what frequency was needed and then calculating the crystal frequency and then procuring that crystal. Then that could be repeated five more times to have the full six-channel capability for Crystal Oscillator control. The SP-600 was a fine general purpose receiver and ultimately became popular as a surveillance receiver because of its wide frequency coverage per band. The Hallicrafters R-274 also featured this type of six-channel Crystal Oscillator option.

In both the Hallicrafters R-274 and Hammarlund SP-600 receivers, each tuning range selected covered wide segments of the spectrum and that left the dial resolution vague and the tuning rate fairly rapid. Knowing your tuned frequency left a lot to the imagination and an accurate measurement of the tuned frequency required using a heterodyne frequency meter. While the Crystal Oscillator option did reduce the drift in the LO frequency in both of the receivers, either one still had to be manually tuned to the selected tuned frequency so that the RF and Mixer stages would be properly tuned to the desired frequency as well. It was all such a hassle that when Collins submitted the 51J receiver (and later the R-390 receiver) the difference in technological design superiority was so obvious that the Signal Corps really couldn't make any other choice for successful RTTY, along with all other types of communications and surveillance, than to use the Collins 51J receivers (and the R-390 receivers.)

Other 51J Series Users - Although the initial 51J might not have been conceived as a military receiver, it had evolved into one by 1950. The 51J-3 wasn't produced in great quantities for civilian users with most of Collins' 51J production centered on the R-388. However, in 1954, the 51J-4 was introduced and it was primarily a receiver for commercial users that had a need for increased selectivity along with the precision tuning and frequency stability that the 51J receivers provided. The 51J-4 version, with its stability and dial accuracy along with its mechanical filter selectivity, was found in coastal stations like KPH and KMI. Shown in the photo to the right is Fred Baxter, who was one of the operators at KPH, a Radiomarine Corporation operated coastal station located at Point Reyes, California. Although the RCA CR-88 receivers are present at this operating position, it's also obvious that Baxter is looking at the Collins 51J-4 directly in front of him.

Several 51J-4 receivers were produced for commercial laboratories like Beckman. Beckman produced special Frequency Measuring Systems that had a modified 51J-4 working with other special equipment that Beckman produced. The 51J-4 receivers produced for Beckman have special panels with "Beckman" or "Beckman/Berkeley" in the upper left panel area. Also, most Beckman 51J-4s are identified as "Receiver Model 7700" and have a light-gray finish on the front panel. Additionally, several 51J-4 receivers were supplied to other commercial laboratories and test facilities with light-gray front panels. Even Collins' own laboratories and test set-ups used 51J-4 receivers with the light-gray semi-gloss front panels. 

The 51J-4 was also found in overseas embassies, at universities like Stanford, even some wealthy SWLs (supposedly comedian/actor Jackie Gleason was an avid SWL who owned a 51J-4) and maybe even some ham enthusiasts bought 51J-4 receivers. The 51J-4 was in production until 1964 with a production of at least 7500 receivers (that's about how high the 51J-4 serial numbers go.) All total, with the early 51Js, the R-388s and the 51J-4 included together, the production probably was over 20,000 receivers.

KPH Point Reyes operator Fred Baxter with the 51J-4


51J Circuit and Design Description


Basic Circuit Design of All 51J Receivers

The Front End - All 51J receivers utilize the same basic front end, that is, a permeability tuned, double conversion circuit that starts with the incoming RF signal being mixed with a selectable multiple frequency crystal oscillator to produce an output that can be tuned in a 1mc wide range of either 1.5-2.5mc for Even bands or 2.5-3.5mc for Odd bands. These Odd or Even bands comprise the dual Variable IF of the 51J circuit. The output of the First Mixer combines with the proper selected Odd or Even Variable IF to become an input to the Second Mixer. The other input to the Second Mixer comes from the 3.0mc to 2.0mc VFO (called a Permeability Tuned Oscillator or PTO) and the Second Mixer output produces a fixed 500kc IF signal.

The band select numbers start with .5 to 1.5mc being Band One (odd,) 1.5 to 2.5mc being Band Two (even,) etc., on up to 29.5 to 30.5mc being Band 30 (even.) Band 1 actually is triple conversion, using the "Band One Mixer" tube but that mixer is only used for Band 1 and was only necessary to provide coverage of the AM BC band. Bands 2 and 3 are single conversion since they are essentially the frequency coverage of the dual Variable IF system which is used as the RF amplifier for Bands 2 and 3 with the output going directly to the Second Mixer (mixing with PTO output to produce the 500kc fixed IF.) All of the remaining bands are double conversion and select alternating Even or Odd tunable IF output circuits as required. The selected Crystal Oscillator and RF amplifier band will provide the correct 1.0mc-wide tuning range to the proper selected Variable IF and that output then can mix at the Second Mixer with the 3.0mc to 2.0mc tuning range of the PTO to provide a fixed-500kc output to the IF. By using two selectable and variable IFs, the required number of crystals for the Crystal Oscillator to cover 28 bands (Bands 2 and 3 are single conversion and don't use the Crystal Oscillator) can be reduced to just ten crystals if the fundamentals and harmonics are used. This is the basic 30 band front-end that all 51J receivers employed.

Since the dual Variable IF presents the Even bands lower in frequency than the PTO output and Odd bands higher in frequency than the PTO, an "inversion" of the input to the fixed 500kc IF happens. This is a transparent function when the BFO isn't employed meaning it isn't noticed in the AM mode. Even CW, with its lack of sidebands, wasn't affected. However, as single sideband transmissions developed and started becoming the dominate voice mode, the IF inversion between Even and Odd bands was noticed because it did affect whether the BFO was set higher or lower in frequency related to the 500kc IF as the method to select the proper sideband being transmitted and that BFO position changed depending on whether the band in use was Odd or Even. This "inversion effect" really couldn't be changed since its cause was due to the basic design of the receiver's front end and all 51J Series receivers will behave in this manner.

The 500kc Fixed IF - The 500kc fixed IF remained basically the same for all versions except the 51J-4. Three stages of IF amplification were provided with a Crystal Filter that had four steps of Selectivity and Phasing controls. The 51J-4 receiver added three mechanical filters to the IF with the installation of the 354A-1 Mechanical Filter Kit at the production level. The mechanical filter insertion-loss required an additional IF stage so there were four IF amplifiers in the 51J-4. The IF transformer primary and secondary windings were fairly loose-coupled and that resulted in a narrow IF bandwidth of about 4kc at -6db in the 51J-1 and 51J-2 receivers. The R-388 added 2pf coupling capacitors across the primary and secondary to slightly widen the bandwidth to about 6kc at -6db. The 51J-4 increased the coupling capacitors to 3pf because the mechanical filter selected determined the bandwidth.

The Det, AVC, NL and Audio Sections - In all receiver versions, the detector is a diode-connected dual triode 12AX7 with the second diode-connected triode used as the AVC rectifier. The AVC amplifier was a parallel-connected dual triode 12AX7 in the 51J-1 and J-2 but, when an IF output was required for the R-388, this tube was changed to a 12AU7 and then one triode was the AVC Amplifier and the other triode was the IF output buffer. The Noise Limiter circuit was the same in all versions and so was the BFO (other than a couple of NPO caps added in the R-388 BFO to eliminate any f drift.) The audio output tube was always a 6AQ5 but its location was in the back corner of the chassis on the 51J-1 and early 51J-2. Its location was moved next to the 5V4 rectifier tube during 51J-2 production and remained in that location for all other subsequent 51J versions. The power supply for the 51J-1 and 51J-2 used a 5V4G rectifier and didn't have any voltage regulator. The 0A2 regulator tube was added beginning with the R-388 and 51J-3 and continuing on in the 51J-4. Also, changed with the R-388 was the type of PTO used. 51J-1 and 51J-2 receivers used the 70E-7A PTO with single 6SJ7 tube while the R-388, 51J-3 and J-4 used the 70E-15 PTO with the 6BA6 VFO tube and 6BA6 VFO Buffer stage. The 100kc Calibration Oscillator always a crystal-controlled oscillator that was connected to the grid of the RF Amplifier tube in all versions. All versions had a 4.0Z ohm audio output and a 500Z was provided in the 51J-1 and 51J-2 but was changed to a 600Z ohm audio output in all subsequent versions. The Stand By function in the 51J-1 and 51J-2 parallels the front panel Stand By switch. The R-388, 51J-3 and J-4 use a Break-in relay that isolates the antenna, grounds the receiver input and removes the IF plate voltage.

Details for Each Model


1949 Collins 51J-1

Collins had been developing the 51J general coverage receiver since 1944. It's probable that the receiver was available as early as 1946, concurrent with the introduction of their ham receiver, the 75A (to become the 75A-1 when the noise limiter was added shortly after the introduction.) Certainly, some 51J-1 receivers were being sold by 1947 and the first advertisements appear in 1948. Since the 51J-1 was the introductory version, several changes happened along its somewhat limited production history that ran from possibly as early as 1947 up to the end of 1949 when its successor was introduced, the 51J-2 which was available through all of 1950.    >>>

>>>   Since the initial design was from the end of WWII, the typical receiver antenna inputs at that time were fairly high impedance. All 51J-1 versions had an antenna input impedance that was fixed at 300 ohms. This required the user to "match" the antenna impedance to the receiver input impedance for best performance. This could be using a resonant folded-dipole antenna or using an antenna tuner for other types of antennae. The 51J-1 didn't provide an antenna trimmer circuit to compensate for varying input impedances since the designers expected the end-user to provide the "matching" necessary for best performance. The 300 ohm antenna impedance was due to the ANT/RF coils being actual RF transformers and having primary windings. This provided excellent front-end selectivity but also complicated the types of antennae that some end-users wanted to employ. Vertical whips and dipole antennae were much lower impedance types while random end-fed wires could vary impedance depending on frequency in use but usually presented a very high impedance. For top performance, all of the antennae that the military or commercial users would have employed would have required an extra matching device when using the 51J-1 and 51J-2 receivers.

The PTO used was the 70E-7A that had a single 6SJ7 metal octal tube as the oscillator. The 6SJ7 tube was covered by a screw-mounted cylindrical metal shield. The "easy to access" adjustment for end-point error was located on top of the PTO under a removable triangular metal cover plate. The glass envelope in front of the 6SJ7 contained desiccant crystals that changed color as they absorbed moisture going from blue to pink. When the desiccant was pink it was supposed to be replaced with new desiccant to continue the absorbing moisture action, though the replacement seldom happened. The fixed IF was tuned to 500kc and had three amplifier stages. A Crystal Filter provided four switched steps of selectivity and heterodyne interference reduction using the PHASING control. A standard envelope detector, AVC amplifier, AVC rectifier and a Noise Limiter were also included. 16 tubes were used in the 51J-1.

The 51J-1 front panel featured a black-painted metal dial bezel with "51J RECEIVER" silk-screened in white, the Collins' chrome and orange "winged emblem" and no grab handles. The S-meter was just that,...a S-meter on the early production receivers. The S-meter was illuminated and had a light-yellow scale, though the material was highly photosensitive and rapidly darkened to an amber color. Later, as more 51J-1 receivers were sold to the military (as the R-381) the meter was changed to a dual scale showing both Carrier Level in db and a S-meter scale. These combo CL/S-unit meters had a white scale, a square bezel bakelite housing and were not illuminated (the photo shown at the end of this section is of a meter that I saw mounted in a 51J-1/R-381 receiver but I can't confirm its originality since it was only a photograph and the receiver was "really hacked," in other words, a highly modified example.) The BAND CHANGE knob had a "pull-out" crank handle (shown on the 51J-2 further down) that apparently changed style throughout production. The CALIBRATE nomenclature was originally 100KC CRYSTAL on the initial few 51J-1 receivers but it was quickly changed to just CALIBRATE during the rest of the 51J-1 production. The CAL. was a fine adjustment trimmer for the 100kc crystal oscillator (the wide-range adjustment was on the chassis.) ZERO ADJ. was a mechanical adjustment of the KC dial fiducial index for precise frequency readouts or measurements.

The 51J-1 audio response was "communications grade" with a low end of 200hz (at -6db) and a high end of 2500Hz (at -6db.) A .01uf capacitor was installed on the primary of the audio output transformer for impedance matching and transient suppression and this left the audio sounding somewhat "muffled" when listening to voice or music transmissions. Additionally, the IF had a very narrow bandwidth of 4 to 5kc at -6db and that also adds to the restricted audio high frequencies. Except for changing the value of the .01uf capacitor to 6800pf, which resulted in slightly different specifications (200hz at -3db to 2500hz at -7db,) no significant changes were made to the audio output stage for the entire 51J Series. The 51J-1 and J-2 receivers provided a 4 ohm impedance audio output and a 500 ohm impedance audio output. With the R-388/51J-3 the 500Z was changed to 600Z. The loudspeaker described in the manual is a 270G-1.

The power transformer used for the 51J-1 was a frame-type, vertical mount transformer. The frame-type power transformer was painted gray. This frame-type was changed to a potted power transformer by the 51J-2 production. The audio output transformer also was a frame-type vertical mount that was changed to a potted transformer by the 51J-2 production. The ham bands were high-lighted in green on the megacycle drum dial but, at $875 list price, not many hams could afford a 51J-1 as their station receiver (if you wonder why few hams bought the 51J-1, $875 in 1949 was equivalent to $11,150 today in 2023.) Incidentally, the $875 price did include a table top cabinet for the receiver, if so ordered. Otherwise, a rack mount configuration was supplied.

The 51J-1's chassis layout is slightly different when compared to the later receivers. The most obvious difference is the four tubes in-line at the right-rear corner area of the chassis. On the 51J-1 and early 51J-2, the IF transformers were in-line but were more forward on the chassis. This allowed space for the four in-line tubes that comprised the Detector, AVC, NL and Audio Output functions. The tubes used were three 12AX7 tubes and then the right-most tube was the 6AQ5 audio output tube. Also, this layout resulted in the 1st IF amplifier tube being located directly under the MC drum dial which made removal of that tube difficult. This layout was changed during the 51J-2 production (beginning with 51J-2 sn: 501.) Another difference is the location of the fuse holder next to the 5V4G rectifier tube. This fuse location was changed during the 51J-2 to the more conventional rear chassis apron location. Also, the filter capacitor is a fiber-board mounted, multi-section electrolytic capacitor (the plug-in filter capacitor was introduced with the 51J-2.)

51J-1 Chassis Top - Note the repainted power transformer - this is the original unit but it should be painted gray. The 70E-7A PTO is in the center of the chassis just in back of the MC drum dial. Note how close the 1st IF transformer is to the front of the chassis when compared to later receivers. This made removal of the 1st IF amplifier tube quite difficult since it's almost directly under the drum dial.

Yet another difference was the crystal oscillator tube (V-5, later V105) that originally was a 6BA6 but was changed to a 6AK5 during the 51J-2 production. A close-up photo to the lower left shows the "four tubes in-line" right rear corner of the receiver chassis along with the fuse holder mounted next to the 5V4G tube. Shown to the lower right is the R-381 dual scale meter.

Under the chassis only the RF front-end sections were shielded with the remainder of the chassis left open. This small bottom shield was used in both the 51J-1 and the 51J-2 receivers. The small bottom shield is shown in the photo to the left (this is actually the bottom of a 51J-2 but the cover is the same.) The wiring harness used in the 51J-1 was made up of fabric-insulated wires. The wire-type was changed to vinyl-insulated during the 51J-2 production. The sloping top cover, shown in the 51J-2 section, became one of the instantly recognizable characteristics of the 51J Series. It allowed for easy insertion of the receiver into a rack mounted position. Also, important for rack mounting was the receiver's 35 pound weight. This was an unusually light weight during a time period when most communications receivers weighed double that.

NOTE: The restoration of the 51J-1 shown here is written-up and is in Part 3 of this web-article.

51J-1 - Close-up of the "Four Tubes In-line" Right-Rear Chassis

Possible 51J-1/R-381 Carrier Level/S-meter scale



The 51J-2 was the slightly evolved version of the 51J-1. The 51J-1 had been in production for two or three years but the minimal demand for an expensive general coverage communications receiver had somewhat limited the production levels. By late-1949, when the 51J-2 was introduced, the military had become interested in the 51J and were ordering the R-381 and R-381A versions of the 51J-1 and 51J-2. Despite a production period that was just under one year, the 51J-2 seems to have been produced in larger quantities than its predecessor.

Like the J-1, the 51J-2 versions had an antenna input impedance that was fixed at 300 ohms. The PTO used was the 70E-7 that had a single 6SJ7 metal octal tube as the oscillator. The fixed IF was tuned to 500kc and had three amplifiers. A Crystal Filter was provided along with a standard envelope detector, AVC amplifier/rectifier, Noise Limiter and Audio Output section. 16 tubes were used in the 51J-2.

The 51J-2 receiver had the same black-painted metal dial bezel with "51J RECEIVER" silk-screened in white, the Collins' chrome and orange "winged emblem" and no grab handles. The S-meter was changed to an illuminated, square-front bezel Carrier Level and Audio Level meter. Audio Output level measuring function was actuated by a toggle switch located next to the meter. The new dual scale Carrier Level/Audio Output meter had OUTPUT above the upper scale and INPUT RF below the lower scale. The scale itself was white. This METER switch was the most apparent difference between the 51J-1 and J-2.

Audio response remained the same as the 51J-1 and was definitely not high fidelity, usually producing "muffled" audio highs when receiving AM voice signals. Like the 51J-1, the primary cause was the very selective IF transformers that resulted in the 4kc at -6db IF passband. Audio output impedances were 4Z and 500Z. The loudspeaker described in the J-2 manual fits the 270G-1 parameters for size and impedance.

The REMOTE terminal strip on the back chassis apron provided a stand by function that was in parallel with the STAND BY on the front panel. 

The Collins 51J-2 from 1950 in the standard Collins cabinet
Note the BAND CHANGE knob's "pull-out" crank handle and the green highlighted 20M band on the MC dial

51J-2 Chassis after sn:500

Some 51J-2 receivers may be found with the 70E-15 PTO (round can and two 6BA6 tubes) installed but this is almost certainly an end-user replacement since it required some mechanical modification. It's also possible that the green high-lighting of the ham bands on the megacycle drum dial might have been eliminated in the later 51J-2 receivers since it was unlikely that sales to hams accounted for many purchases. Again, later rebuilds may account for receivers found with later style dial drums installed. The wiring harness changed to vinyl insulated wires during the 51J-2 production. The AC power cord on the 51J-1 and 51J-2 receivers was originally a two-conductor black rubber "zip cord" with a period molded AC plug. Nearly always, the original AC cords have deteriorated long ago and were replaced with the heavy-duty round power cables in both two-conductor or three conductor varieties commonly found on these receivers today. 

The photo to the left shows the chassis of the later 51J-2 (with serial numbers higher than 500) which has a slightly different layout than the first 500 receivers which were like the 51J-1 in that regard. Beginning with 51J-2 serial number 501, the 6AQ5 was relocated next to the 5V4G rectifier tube and the fuse holder moved to the rear chassis apron below the AC power cable location. This location for the 6AQ5 and the fuse holder continued for the remaining J-2 production and all subsequent 51J and R-388 receivers. The filter capacitor was changed from the fiber-board mounted multi-section used in the 51J-1 to an easy-to-replace, plug-in multi-section unit, sometime during the 51J-2 production.  

51J-2 - The characteristic "sloping top cover" of the 51J Series. All 51J receivers will have this type of top cover. The bottom three retaining fasteners will be wing-nuts on the later versions of the receivers. The serial number plate isn't original (it covers non-original SO-239 mounting holes drilled through the original silk-screened ID and SN info.)

The 51J-2 chassis shown above is the basic layout for all of the 51J receivers that came after the 51J-1 and early 51J-2. To the left on the chassis is the Variable IF slug tuning. Middle rear is the ANT/RF/Mixer slug tuning. PTO middle-front. 500kc IF section and BFO with the 51J-2 style dark gray shield-cans. On the right-side-front is the Crystal Filter, middle-right power supply and audio output tube. Along the right-rear is the AVC-NL-1st AF circuits. With the moving of the IF transformers more towards the rear of the chassis, it can be seen in the photo that access to the 1st IF amplifier tube was now much easier than it was before.

The 51J-2 was built through most of 1950 and was replaced with the introduction of the military R-388 and the civilian 51J-3 in late-1950. When the military ordered both 51J-1 and 51J-2 receivers they were designated as the R-381/URR for the 51J-1 and R-381A/URR for the 51J-2. Some 51J-1 and 51J-2 receivers will be found without the Collins' Winged Emblem mounted on the front panel but the receiver panel will be drilled for installation of a data plate, which, of course, is nearly always missing. The implication is that civilian 51J-2 receivers had the Collins WE while the R-381A versions had a military data plate installed instead. This also applies to the 51J-1 versus the R-381/URR military versions of that receiver.

There are indications in Collins manuals and other documentation that the 51J-1 and 51J-2 did carry serial numbers, however, I've never seen a serial number in the location provided on the rear chassis apron (or anywhere else on the receiver.) All examples I've inspected of both the 51J-1 and the 51J-2 receivers always are missing the serial number. Collins did use a lot of "ink-stamped" numbers in their production so it's possible that the serial number was ink-stamped using water-soluble ink and the stamping just hasn't survived.



1952 contract R-388/URR  ORDER: 3362-PHILA-52  Mil SN:161  Collins SN:9108

In late 1950, the R-388/URR was introduced, featuring an 18 tube circuit (adding a voltage regulator 0A2 and VFO buffer 6BA6,) a new version of the PTO (70E-15) and eliminating the fixed 300 Z ohm antenna input (by removing the primary winding on the antenna coils) and redesigning the antenna input to a more flexible design utilizing an Antenna Trim control. This revision was probably at the request of the Signal Corps, who wanted to use the new version of the receiver for their RTTY installations but found old 51J antenna requirements of a fairly Hi-Z, "fixed" (non-adjustable) 300 ohm antenna input impedance difficult to work with since most of the Army installations used either 75 Z ohm dipoles or 50 Z ohm (or less) vertical whip antennas. The new upgraded receiver was designated as the R-388/URR (when going to the Signal Corps) and it was built from 1950 through about 1954 in relatively large quantities. There were also later contracts for 1955, 1956, 1957 and 1962 but these later contract quantities only total about 1000 receivers. From 1950 through 1962 at least 12,000 R-388/URR receivers were produced (the actual total may be somewhat higher.) 

All contracts were built by Collins Radio Company although in the early 1950s Hallicrafters supplied a couple of special types of military cabinets (along with manuals) that were utilized by the Army for some of its R-388 installations. These Hallicrafters cabinets and manuals are probably the source of the erroneous "Hallicrafters-built R-388" myth. Some (possibly all) of the U.S. Navy R-388/URR receivers were designated as AN/URR-23-A (but were identified as "R-388" on the Navy data plate) and this set-up included the receiver installed in a Collins table cabinet (designated as CY-1235/U) and a Collins 270G-3 speaker (designated as LS-199/U.) Additionally, the USAF had Raytheon supply them with a few modified receivers that were designated AF-30 (R-645) and were used for backscatter ionospheric purposes. These USAF receivers had a blue front panel that lacked the standard grab handles. The largest contract quantities for R-388/URR receivers are from 1951, 1952 and 1953 contracts. Careful examination of the various R-388 data plates will show several different "order" numbers within each year. There were at least seven different order numbers used in 1951 and at least five different order numbers used in 1952.

photo above: 1951 R-388 Mil SN:701 chassis showing the typical MFP coating that imparts a gold color to the chassis. Note the cylindrical 70E-15 PTO with two 6BA6 tubes. To the right of the drum dial the 0A2 regulator tube can be seen.

With the R-388 and 51J-3, the side panels were changed to steel and finished in yellow-cadmium type II plating. Grab handles were added to the front panel along with a high quality Burlington Co. sealed meter (some receivers will be found with different makes of meters but they all have similar scaling.) The METER toggle switch was changed to a "spring-return" on the (audio) OUTPUT so that Carrier Level or INPUT was the normal switch position with OUTPUT only reading as long as the switch was held in that position. Two NPO capacitors were added to the BFO (inside the can) to reduce f drift. The conventional "Remote Relay" function that paralleled the front panel "STAND BY" and "ON" functions found on the 51J-1 and 2 was eliminated in the R-388 receiver. Instead a "Break In" function was added that utilized an external +12vdc to actuate an internal relay that disconnected the antenna, grounded the receiver input circuitry and removed the IF plate voltage upon actuation. The new "Break In" function was entirely separate from the front panel STAND BY switch position meaning that the "Break In" relay could only be actuated using the rear chassis REMOTE terminals. Later, there was also a military supplied modification kit for the end-user addition of a solid-state RF-driven relay circuit to protect the receiver's antenna input circuits from high levels of antenna current induced from nearby transmitters while the receiver was in operation (Break-in off.) This replaced the neon bulb antenna static-drain protection device.  

All R-388 or 51J-3 receivers now had the entire underneath of the chassis protected with a full bottom cover rather than the 51J-1 and J-2's small cover that was just over the receiver's front end. Both the top and bottom covers must remain mounted on the receiver for proper shielding during operation (removed only for servicing or alignments.) The metal bezel used on the J-1 and J-2 was replaced with a black bakelite bezel with no receiver designation on the bezel. There are many other changes in component types and placement throughout the R-388 receiver making it quite different from its predecessors, the 51J-1 and J-2.   >>>

 >>>  A "Collins Serial Number" was punch-stamped (or sometimes ink-stamped) on the rear of the chassis. This number was for Collins "in-house" use since the receivers were built and tested, then put into Collins stock. To fill orders, receivers were pulled from stock, sent to QC and then assigned a serial number that was for the end-user, either the Signal Corps/USN or for civilian users for the other 51J receivers. The same serial number that is punch-stamped or ink-stamped on the rear chassis is usually (but not always) white ink-stamped on the front panel but located so that the data plate would cover it. These Collins "in-house" serial numbers won't match the data plate serial number,...and they shouldn't on R-388 receivers.   NOTE: Sometimes an acceptance stamp will be found under the data plate instead of the Collins SN. Also, as the "in-house" numbers approached 10,000 (around 1953,) the numbers returned back to two and three digit numbers. The Collins SNs appear to be consecutive for some contracts but not over the entire production of the R-388 receivers.

Late versions of the R-388, usually the 1953 and later contracts, have a switch to turn the "Break-in" on or off (with external +12vdc supplied.) This switch replaced the 600Z SPEAKER jack and mounted in that location of the panel. Many times a MWO (Military Work Order) will have been incorporated to add the Break-in Switch or other changes to earlier receivers. When the two phone jacks were present they provided 4.0Z audio output on the PHONES jack and 600Z audio output from the SPEAKER jack. The PHONES is a standard .250" jack but the SPEAKER is a three-circuit phone jack, .187" diameter with ring, tip and shell connections (ring and shell are connected together and to chassis-ground.) Sometimes an additional "CAUTION REVIEW MANUAL, etc." data plate was installed to the right of the standard data plate (found mostly on early contract receivers.)

photo above: 1953 contract R-388 with BREAK-IN switch

By the end of the 1953 contracts, a problem was beginning to develop with the 70E-15 PTO. The ferrite used in the core of the PTO seemed to be changing characteristics over time. This "aging" problem took several years to show up but as more and more R-388 receivers were developing difficult to correct end-point errors in their PTO tracking, it became apparent that something was affecting the long-term stability of the ferrite used for the PTO core. Over the past several decades many 51J enthusiasts thought the ferrite core was totally responsible but it appears that the ferrite core in combination with the 24/7 operation and subsequent heat build-up may be the actual cause. All 70E-15 PTOs installed in R-388 receivers will be identified by a "M" prefix to the serial number on the PTO.

The B&W artwork from the USN AN/URR-23A manual is shown below. These receivers have a Navy data plate that looks noticeably different than the Signal Corps data plate. The AN/URR-23-A is identified as "R-388" on the USN data plate (no "/URR") but there's a Navy contract number at the lower part of the tag. Contract number is NObsr-52527 with an issue date of 22 June 1951. In the artwork shown below it can be seen that the USN data plate is different than the Signal Corps data plate. Also, throughout the Navy manual, the receiver is referred to many times as a "R-388" receiver. This artwork also shows the CY-1235/URR cabinet that was a typical Collins cabinet for 1951 and also shows the data plate installed on the lid of the cabinet. The loudspeaker designated as LS-199/U is essentially a Collins 270G-3 loudspeaker and cabinet. 

Most production quantity totals are listed as "for the R-388 receiver" with 51J-3 production not specified. The totals actually may be for both receivers produced and stocked by Collins, later to be used to fulfill contracts to the Signal Corps (as the R-388) or the USN (as the R-388 or AN/URR-23-A) or possibly 51J-3 civilian versions.  

photo above: B&W artwork of the USN URR-23A



Is the 51J-3 a R-388? - The 51J-3 was introduced in late-1950 and it is virtually identical to the R-388/URR receiver but with a few important differences. All 51J-3 receivers will have a Collins data plate not an Army data plate. The Collins data plate will have the "winged emblem" at the top of the tag. "51J-3" will be stamped in the TYPE space with an appropriately "low" serial number stamped in the SERIAL space. However, just to confuse the issue, the Collins 51J-3 manual shows the receiver without any data plate mounted. I've seen 51J-3 receivers with Collins data plates (with appropriately low serial numbers) so it seems that the 51J-3 manual is probably just "artwork" and not representative of production.

Besides the obvious data plate difference, the 51J-3, being that it's a civilian receiver, won't have any military inspection stamps. However, all R-388 receivers will have military inspection stamps from the Signal Corps or USN located in various places on the panel (sometimes under the data plate) and on the chassis and the side panels. The civilian 51J-3 receivers typically won't be MFP treated while all R-388s were MFP treated. The 51J-3 didn't have a front panel mounted "Break-in" switch as the 1953 and later R-388 receivers did. Still, with all of these differences, it's extremely common for a military R-388 receiver to be misidentified as a civilian 51J-3 receiver.

The misidentification problem stems first from the vast quantity of general information radio publications that have been available for decades that show the R-388/URR receiver as the "R-388/51J-3" implying there's no difference between the two types of receivers. Even Collins listed the receivers as R-388/51J-3 because at the Collins production line, there wasn't any difference. The receivers went into stock and when pulled from stock to fill orders then it was determined what the receiver would be (after test, cal and QA.) R-388 receivers filled military contracts and were MFP'd, stamped accordingly and a mil data plate installed identifying the receiver as "R-388." 51J-3 receivers filled civilian orders and just had the Collins tag installed identifying the receiver as "51J-3." For Collins enthusiasts these "final test & QA" differences are significant enough that they consider the R-388/URR is a military receiver for very specific end-users while the 51J-3 is a very similar but civilian receiver that had very different end-users as customers.

This R-388 was sold as a 51J-3 because it has a repro data plate. Close inspection of this receiver found USN inspection stamps and the chassis was MFP'd.

Obfuscation or Innocent Mistakes? - The real problem for Collins enthusiasts today begins with sellers who would rather be selling a scarce "51J-3" than trying to sell a common, military "R-388" and then mislead prospective buyers with erroneous information because the R-388 receiver for sale is missing its data plate (just like the 51J-3 manual shows.) I know of three R-388 receivers that were actually sold as "51J-3 receivers" to friends of mine. One was missing any sort of data plate and was probably just the lack of correct information on the buyer's part causing the confusion. More concerning was the receiver that had a reproduction 51J-3 data plate installed. This receiver sold for a fairly hefty price but, I'm positive the seller was unaware that the data plate was a repro and actually thought the receiver was genuine (so did the buyer,...for a while.) The third receiver actually had a chrome Collins' Winged Emblem mounted in place of the R-388 data plate and was sold as a 51J-3 since there wasn't a data plate to contradict the claim. It also sold for much more than it should have.

A quick check on eBay (12/2022) came up with four 51J-3 receivers being offered. NONE were actual 51J-3 receivers with three being R-388 receivers minus their data plates and one R-388 being identified as a "51J-3/R-388." 

Another R-388 listed on eBay as a 51J-3 (5/2023.) It has an engraved black bakelite tag that has the engraving filled with white paint. Has "COLLINS 51J-3 Receiver" on the tag that's mounted where the mil data plate should be. The seller, not being an expert, probably just used what info he could see for the receiver ID. Clues were the MFP'd chassis, the MFP ink-stamp on the rear chassis and the receiver was installed in 32V transmitter type cabinet. Unfortunately, no photos of the chassis - top or bottom - that would have showed the 70E-15 "M" PTO installed (only in R-388s.)

There have been genuine civilian 51J-3 receivers for sale or auction but they are scarce. Investigate any prospective 51J-3 purchase thoroughly.

A Reproduction 51J-3 Collins-type Data Plate

More Information on Repro 51J-3 tags
Watch Out for 12,000+ Serial Numbers

To further confuse this R-388 versus 51J-3 identification problem there are also reproduction 51J-3 tags out there. I received an e-mail from Tom N5OFF regarding these high serial numbers on 51J-3 receivers. About 20+ years ago, Tom obtained permission from Collins to reproduce the 51J-3 and 51J-4 serial number data plates. To assure Collins and Collins collectors that any informed person would be able to easily identify these high quality, authentic but new data plates as "reproductions," Tom stamped the series of 200 tags with serial numbers beginning with 12000 (see photo left.) The key to this, of course, is being an "informed person."

The existence of quality repro data plates for the 51J-3 puts into question the identification of any supposed 51J-3 that has one of these repro tags installed. Be very careful when inspecting ANY 51J-3 for intended purchase. Thoroughly check the receiver for military inspection stamps. If Signal Corps stamps are found then the receiver IS a R-388. If the receiver has a Break-in switch, it's a R-388. If there are USN stamps then the receiver is an AN/URR-23-A but would have been tagged as "R-388." Be suspicious of a supposed 51J-3 that is MFP'd. And, of course, if the 51J-3 data plate is serialized 12000+, then the receiver is more than likely a R-388.

Always consider that while civilian 51J-3 receivers do exist they are very scarce and seldom encountered.



In late-1954, the 51J-4, with 19 tubes and three mechanical filters (1.4kc, 3.1kc and 6.0kc actual bandwidth,) became available and was offered up to about 1964. The 51J-4 was to be the ultimate evolution of the 51J Series in some regards. Some users feel that the mechanical filter upgrades seriously degraded the audio reproduction capabilities of the receiver, especially for AM-SW-BC users. On the positive side, Collins had recently developed the mechanical filter to provide an IF passband that was defined with steep skirts and a flat top resulting in superior selectivity. Interference from adjacent frequency signals would just disappear as the IF passband was narrowed as more selective mechanical filters were switched in. As HF band congestion increased, so did the need for a more and more selective receiver. The 51J-4 added a fourth stage of IF amplification to compensate for the insertion loss of the mechanical filters. It will be noted when inspecting a 51J-4 chassis that there is a Mechanical Filter Assembly (Collins 354A-1) that is mounted to the chassis. Under the MF Assembly, the chassis is punched for the R-388 type of IF construction and has the silk-screened nomenclature for the R-388 also. The addition of the MF Assembly Unit includes two 6BA6 tubes mounted on top of the unit that actually amplify the input and output of the selected mechanical filter thus bringing the total number of IF amplifiers to four although there are only three "adjustably-tuned IF amplifiers."

The 51J-4 500kc IF transformers have a 3pf capacitor added from the primary to the secondary to increase the coupling and broaden the IF bandwidth. Similar IF coupling caps, with slightly less capacitance of 2pf, were installed in some R-388 receivers but weren't used at all in the 51J-1 or J-2. The slightly wider IF bandwidth assured the mechanical filters determined the IF selectivity along with the Crystal Filter.   

>>>  The mechanical filter selector switch shaft has a chrome lever that is placed behind the BFO knob. The factory 51J-4 receivers will have the MF bandwidth silk screened on the front panel as 1KC, 3KC and 6KC. The Crystal Filter was retained as an additional tool for dealing with interference. Although MFs were excellent for providing a well-defined bandwidth, that factor alone can't eliminate heterodyne interference or enhance certain IF frequencies for better CW reception. Collins knew the combination of MF selectivity along with the Crystal Filter's unique abilities gave the users the best in QRM-fighting tools.

51J-4 SN:2392 from 1957 installed in original type cabinet  (owner: KB6SCO)

photo above: The top of the chassis of 51J-4 SN:2392. Note that the chassis is not MFP coated. The Mechanical Filter Assembly just to the right of the PTO. The original plastic bag that's attached to the Crystal Filter top usually contained spare parts, in this case two PL-259 connectors.  NOTE: This 51J-4 now belongs to KB6SCO

photo above: Under 51J-4 SN:2392 receiver. On the right is the receiver front-end showing the elaborate 30-postion bandswitch and its multiple sections for the Crystal Oscillator, the Variable-IF and the ANT/RF/Mixer sections with all areas shielded. Visible at the bottom is the neon static-drain bulb, the Antenna Trimmer C and K101 (Break-in Relay.) On the left is the IF and Crystal Filter at the upper left and the power supply chokes. The lower left is the filter cap socket and the AVC, NL and Audio stages.  

>>>  The other major difference for the 51J-4 wasn't as obvious as the mechanical filters but it was just as important. The 70E-15 PTOs used in all 51J-4 receivers will be identified with a serial number with the suffix "CR."  It's very unusual to find one of these "CR" 70E-15 PTOs that will have an end-point error that can't be corrected by adjustment of L002 (the EPE L-trimmer compensation.) However, it's very probable that there isn't any internal difference in these "CR" PTOs and their better calibration may be due to how the receivers were used. The EPE problem with the 70E-15 PTO might be a combination of ferrite material and heat build-up from 24/7 operation of the receiver. The R-388 is almost always found with its 70E-15 "M" PTO having excessive EPE and R-388 receivers were usually operated 24/7. The 51J-4 receivers were typically civilian and were much less likely to be operated in a 24/7 environment and therefore the 51J-4 70E-15 "CR" PTOs are often found with very little EPE or with easily correctable EPE.

As far as the rest of the 51J-4 circuit, there weren't any changes made to the RF front-end of the receiver, the simple envelope detector, the AVC amplifier-rectifier circuit or the noise limiter circuits. The audio output circuitry also remained unchanged. Early in production a yellow plastic-body test jack for the AGC line output and a green plastic-body test jack for the Diode Load output were added to the rear chassis apron. Also, added was a printed notice regarding the FCC Part 15 compliance for the receiver that was applied to the rear chassis apron.

It appears from serial number versus reported details (to that the very first few 51J-4 receivers were MFP-coated. This MFP detail only appears in two reported J-4s and they are very early serial numbers. As a general rule, it can be assumed that all 51J-4 receivers won't be MFP-coated.

In the late-fifties, an IF Gain control was added to the circuit. This pot (R187) was located next to the CL Meter Zero Adjust pot on top of the chassis between the MF Assembly and the power transformer. The IF gain adjustment is a 10K pot, cathode to ground adjustment on the 3rd IF amplifier tube. The addition of an IF gain adjustment would allow users to optimize the IF gain versus the noise produced with the end result being better audio with less distortion and lower background noise. The IF Gain circuit is shown in the later edition 51J-4 manuals. Also changed in the very late production receivers was the value of R-155, the cathode resistor for the first AF amplifier. The value was changed from 3.3K to 1.8K in order to increase the available audio output (mentioned as an adendum in the ninth edition manual.)

Federal Procurement catalogs show that the 51J-4 was designated as the R-388A/URR or the R-388B/URR but it's doubtful that any receivers were actually identified as such. No "tagged" R-388A receivers have been found which seems to indicate that although the designation was used in catalogs (perhaps for ordering identification) the actual receivers were probably shipped from Collins with a data plate showing "51J-4."

There are reproduction 51J-4 data plates. When N5OFF made the 51J-3 reproduction data plates over twenty years ago, he also made repro data plates for the 51J-4. It's not nearly as much of a concern because there isn't any ambiguity when identifying a 51J-4. The same "high serial number identification" applies for the repro 51J-4 data plates, that is, serial numbers over 12,000 will identify the data plate as a reproduction. I've only seen one 51J-4 for sale that had one of these data plates installed and from the seller's description it was obvious he was unaware that the serial number identified the data plate as a reproduction.

51J-4 SN:3805 CL Meter date

Approximately 7500 51J-4 receivers were built from 1954 up until 1964. There are two "dated" 51J-4 receiver-meters shown in the photos right and left that have receiver serial numbers tied to component dates stamped on their respective Carrier Level meters. Serial number 3805 has a CL meter date of August 21, 1959. Serial number 4723 has a CL meter date of March 22, 1960. Date codes are not indicators of when the receiver was built but the dates do show that the receiver was certainly built later than that date. These dates indicate that in approximately a seven month period about 900 receivers were built, or about 130 receivers per month. The highest production levels were probably from around 1956 up through 1960 (receiver SNs from about 1500 to 6000.) The 51J-4 was selling for about $1400 in the early 1960s.  NOTE: $1400 in 1962 is equivalent to $13,800 in 2023.

With the introduction of the 51S-1 in 1962 (though advertised earlier,) production levels of the 51J-4 had already been tapering off since 1960 and certainly were greatly reduced after the introduction of the 51S-1. It seems there were some 51J-4 receivers that were still being built even as the 51S-1 was being promoted as the "J-4" replacement.

51J-4 SN:4723 CL Meter date


Project Prototypes Combining the 51J-4 and the 75A-4

There was a 51J-5???   In 1957, Collins division in Canada started working on a version of the 51J-4 that would utilize several of the 74A-4 design features. The J-5 design used the 51J-4 front-end with a couple of tube changes but the PTO was a modified 75A-4 type. The Variable IF remained the same 51J-type with the Odd and Even band switching. The Fixed IF had to remain at 500kc, so the three mechanical filters were the 75A-4 style (round cylinder, nine pin plug-ins, 455kc) but were custom-made for the 500kc IF. From the mechanical filters on back to the audio output the rest of the circuit is basically 75A-4 except that 51J-4 IF transformers were used. The 51J-5 design eliminated the 51J-4 Crystal Filter since the Rejection Tuning of the 75A-4 could be used. The product detector and envelope detector, both used in the 74A-4, were also in the 51J-5. The band change and gearbox was the same as the 51J-4. MC Tuning drum was the same as the 51J-4. The project produced at least three working prototypes, one of which survives in the Hammond Museum in Guelph, Ontario. This particular prototype had been taken home by the head design engineer. When he died, his son donated the prototype receiver to the Hammond Museum. As to the functionality of the 51J-5 receiver,...the prototypes didn't fully function in that the Passband Tuning of the 75A-4 wasn't compatible with the Dual Variable IF of the 51J-4. Because of the alternating inversion at the input of the 500kc fixed IF that takes place going between Odd and Even bands, the Passband Tuning only worked correctly on either all Odd bands or on all Even bands, depending on how it was set up. Before this problem could be worked out, the project was halted. The common belief is that the 51J-5 project was to take advantage of the advances in design that the 75A-4 had over the 51J-4. After all, the 75A-4 was designed in 1954 and the 51J was designed in the mid-1940s. SSB was becoming predominate in military voice comms and the 51J-4 required some experience on the radio operator's part for successful SSB demodulation. Collins marketing thought that the 51J-5 would appeal to the military users and would be priced at about half the cost of the R-390A. It's commonly thought that the 51J-5 project was considered "insurance" if the 51S-1 project wasn't successful. The 51S-1 project was an "unofficial" design exercise perpetrated by a few of the radio engineers at Collins. It was only made "official" after successful operational prototypes were demonstrated to the "higher-ups" in the company. This was about 1959 and probably when the 51J-5 project was halted. The 51J-5 project was written-up in CCA Signal magazine, 3rd quarter, 2013.


The 51J-4's Official Replacement

The 51S-1 is a product of radio receiver design incorporating the "smaller is better" thoughts and trends that started in the late 1950s. The 51S-1 was an unofficial creation from a few of the same design engineers that had worked on the Collins S-line. Until working prototypes were ready, only the few design engineers involved knew about the 51S-1 "unofficial" project. Despite a temporary halting of the design-work when the preliminary work was discovered by an intermediate manager, the engineers continued to "unofficially" work on the 51S-1. Fortunately for the design engineers, once the higher-ups were told about the operational prototypes of the 51S-1, they approved of the new receiver's performance and it went into further refinement for production. Most of the design work was from 1960. Since several of the design engineers involved in the 51S-1 development had worked on the Collins S-line, the 51S-1 ended up looking pretty much like the S-line receivers. But, inside it was a significantly different circuit than the 75S-1 receiver, mainly because the 51S-1 had to provide continuous coverage from 2.0-30mc. SSB product detector was provided. Mechanical digital readout for megacycles. Rejection Tuning replaced the 51J Crystal Filter. Mechanical Filter switching was tied to the Emission Switch. Audio 4Z and two separate 600Z outputs were provided. The feeble dial illumination and the intentionally darkened CL meter imply the receivers were intended for "dimly lighted rooms." 

Though the 51S-1 performs fine as a shortwave receiver, it has an entirely different approach for the radio operator's tactile interface with the receiver when compared to the earlier 51J receivers. Much of the operator's enjoyment of "switching this" or "adjusting that" as the receiver is being used is lost with the severe reduction of the number of controls found on the 51S-1. NOTE: There are 16 controls or switches on the 51J-4 and only 9 on the 51S-1. It's obvious that the 51S-1 was "geared-for" commercial operators that didn't want to "fuss" with a multitude of controls and just wanted to "set the frequency and hear the signals." The other thought is that in some installations, like in embassies or similar installations where "less-than-knowledgeable" personnel might be acting as the radio operator, the simple "tune and listen" approach was probably an advantage.

The 51S-1 was available from 1962 through about 1975.

The "winged-emblem" 51S-1 shown above is an early version without the tuning dial brake. The earliest 51S-1 receivers had white scale CL meters. Early in production, the meter scale was changed a fiberglass material that was intentionally a dark amber color so the meter scales would only show with illumination. The early MEGACYCLES knob had five lobes but that was soon changed to just four lobes. The 51S-1 cabinet is unique with a different layout of the rear panel when compared to the 75S series receivers.

More 51J-4 Receivers

Collins 354A-1 Mechanical Filter Kit

Shortly after the 51J-4 became available, Collins offered the 354A-1 Mechanical Filter Kit. This was exactly the same assembly that was installed in all 51J-4 receivers. A close look under the chassis of any 51J-4 and one can see that the chassis holes are punched and the nomenclature is silk-screened for the assembly of 51J-3 or R-388 receivers. So, essentially the 354A-1 kit allowed the owner of a 51J-3 to upgrade that receiver to a 51J-4. At the time, owners of R-388 receivers would have been the military and there is quite a bit of evidence that some 354A-1 installations into R-388 receivers were military depot jobs. It was also possible to upgrade a late-version 51J-2 into a receiver that had mechanical filters for improved selectivity. The 51J-2 serial number had to be 501 or higher.

The 354A-1 kit installation wasn't for beginners. Its successful installation required an electronics technician background and experience. The 28 page instruction manual contained 78 numerated steps (with check-off boxes) to guide the installer. The instructions also contained a parts list, schematic, drawings of the installation, lists of many components and parts that needed removal and finally, IF alignment instructions. Besides an electronics background and experience, the installer would have required a good mechanical aptitude for the correct assembly so that everything would function correctly, not only electronically, but mechanically.   

The receiver shown is a 1951 R-388/URR with Order 3096-PHILA-51 and SN:33 on its data plate and the Collins manufacturing SN:5282 punch-stamped on the rear chassis. Also, the receiver has a MFP date of "OCT 1952" stamped on the rear chassis. Besides the 354A-1 MF Kit there's also a newer 70E-15 replacement PTO installed. This can be determined by the PTO serial number "11277," indicating this PTO was probably built in the early 1960s.

The refurbishment and repair of the 1951 R-388 with 354A-1 Mechanical Filter Kit installation is written-up in my typical OCD excruciating detail style and is in Part 3 of this web-article (link to Part 3 is at the bottom of this page.)

1951 R-388 with the 354A-1 Mechanical Filter Kit Installed

Are 51J-4 Receivers with Light-Gray
 Front Panels Laboratory Receivers?

Some 51J-4 receivers will have light-gray panels with black nomenclature. It's generally believed that these were laboratory or test facility receivers. Some sources say the light-gray panels were used at Collins in-house laboratories and test set-ups.

Some light-gray panel J-4s were used in Beckman test equipment. It's likely that Beckman produced the panels since the J-4 receivers also required some Beckman-implemented circuitry modification to function as necessary for the Beckman equipment. The Beckman panels will have special nomenclature that included Beckman or Beckman/Berkeley logos, the panels aren't drilled for a Collins data plate and also sometimes elimination of the CALIBRATE function and its nomenclature was necessary. All of the light-gray panel Beckman receivers seem to be marked as Model 7700 with various suffixes to distinguish some of the circuit modifications required for their various uses. It seems likely that Beckman bought standard 51J-4 receivers and performed all of the modification work themselves.

The other 51J-4 receivers with light-gray panels are standard Collins receivers with the exception of the panel. These panels are drilled for data plates which are sometimes installed. Some examples will have a mixed variety of Collins-type knobs. There are two types of nomenclature fonts seen on the light-gray panel 51J-4 receivers. The standard is the small letter-number font that's common to both the St. James gray and the light-gray panels but also seen is a larger letter-number font as seen on receivers #2 and #3 shown further below.

51J-4 SN:4723 from 1960

Note the asset number shown on the bar code tag. This receiver has the asset sn:146 while my receiver has the asset sn:00142. Note the Collins-Rockwell sticker upper left. Data plate removal is a common occurrence with the lab receivers.    photo from: Universal Radio

Light-Gray Panel 51J-4  sn: 4723

Making the Case for a Collins Lab Receiver

From some of the features found on this receiver one might infer that it was used at Collins. For example, it has a Collins-built plug-in RC coupling adapter in place of the 6kc mechanical filter. Also, the 1.4kc MF was replaced with an 800hz MF (J-4 type.) But wait,... here's more definitive evidence. 

I suspect that the number "029 1059 00142" engraved at the upper right corner of the panel on SN:4723 is an asset number that identifies the receiver as property of a particular company or laboratory. By examining other light-gray panel 51J-4 receivers, it seems that "029 1059" is the Collins' asset number with "00142" indicating the particular receiver by an "asset serial number." Three other receivers are shown with identical "029 1059" numbers and varying serial numbers. The website  has a photo of a light-gray panel 51J-4 receiver with the asset number is "029 1059 00" which, if the serial number of 105 was added after the "00," becoming "029 1059 00105," then the format would be the same and with nearly the same numbers that SN:4723 has. Does that imply that 4723's asset number identifies the receiver as sn:142 (for the asset records anyway?) Shown to the left is a photo from Universal Radio of a light-gray panel 51J-4 they sold. The sticker shows basically the same format for the asset number, dropping the "00" ahead of the asset serial number, and a Collins-Rockwell sticker. Both sn 146 and sn 105 don't have data plates installed. Missing data plates are common on light-gray panel receivers and may have been part of the deposition of the receiver or maybe they were never installed.

Close-up of the light-gray panel 51J-4 from with same engraved asset number but with the serial number separated. Note the toggle-boot

The third receiver is shown below in the collection of light-gray panel 51J-4 receiver photos. Receiver #3 has the same style engraved asset number in a similar location. This receiver appears to still have its data plate. Interestingly, all receivers shown seem to have the same style toggle boot installed. Unfortunately the poor resolution of the photograph prevents reading the asset number.

The most significant "Collins-clue" comes from the "Collins - Product Support Operations" sticker. The big question is, what does the sticker indicate? "Product Support Operations" does sound like an engineering lab. "Processed" also sounds like an "in house" thing,...but, then,...why bother with the sticker? Unless the sticker indicates which division of Collins the receiver belonged to. Center photo below shows the Collins sticker on the rear chassis of sn:4723. Also, of note is the photo of "029 1059 146" above with its Collins-Rockwell sticker.

Also of interest on sn:4723 is the single digit "production" serial number "2" that's ink-stamped on the back of the chassis - below left. Note the FCC Part 15 compliance stamp. And, below to the far right, a close-up of the data plate.

Other Light-Gray Panel 51J-4 Receivers

The photos below show a few other variations of the 51J-4 receiver with light-gray smooth-finish painted panel. Note that most have the standard Daka-Ware knobs installed. Receiver #2 has S-line knobs installed along with the standard Daka-Ware tuning and band change knobs.

The Beckman Model 7700/3 shown in photo #1 is very different from the Beckman/Berkeley system receiver shown further below. Beckman #1 appears from the front to be almost a standard J-4 receiver. But, who knows what's behind the front panel? Both Beckman 7700s use slotted OH screws to mount the dial bezel and the "Berkeley" uses all slotted OHs on the panel.

Note the engraved asset number above the CL meter on #3 receiver. Note the larger nomenclature on #2 and #3 receivers. I wonder when the 4:1 vernier tuning knobs were added to #1 and #3 receivers.

Receiver #4 is an unusual Beckman 7700/3 that's under restoration. The receiver was found by Tom W3TA in nearly complete condition but seriously suffering from "outdoor over-exposure." This photo shows the very different greenish-gray wrinkle finish Beckman panel after its clean-up.

The Beckman Frequency Measuring System photo and photos #1, #2 and #3 are from Bear - WB2GCR    Photo #4 is from Tom - W3TA





Beckman/Berkeley Frequency Measuring System

Description - Beckman/Berkeley supplied these Frequency Measuring Systems to allow accurate measurement of any RF signal in the frequency range of the 51J-4 receiver, or .5mc up to 30.5mc. Signal levels could be as low as 1uv and they could still be measured. There were three basic units, the 51J-4 receiver, the Beckman Translator that converted signals from the 51J-4 into various types of signal information to interface with the Beckman Digital Frequency Counter, referred to as an "events per unit-time meter." In the rack shown to the left, the Frequency Counter/Meter is at the top. The next device is a separate Frequency Meter. Next down appears to be a power supply. Next is the 51J-4 receiver and below it is the Beckman/Berkeley Translator. This set-up is somewhat different and larger than the smaller "three unit" type of Frequency Measuring System.

Modifications - The 51J-4 did have some modifications made to it. First was to bring out the PTO signal to a connector on the rear chassis of the receiver. Also, an output for the 2nd IF was brought out to the rear of the chassis. The 51J-4 was powered up with a connection to the Frequency Counter/Meter. The 51J-4 shown in the photo left doesn't have a 100kc calibrator installed so there's no switch for it on the front panel (a calibrator would be superfluous on a frequency measuring device.) Also, there's a vernier frequency control to the right of the RF Gain control. It's difficult to read the dymo-lables for the mechanical filter selector but the 1kc is covered with a blue dymo-lable that appears to have "0" on it. The 3kc is a green label and appears to have 7 on it and the 6kc is also a green label that appears to have "9" on it.

Procedure - To measure frequency all that was necessary was to tune the 51J-4 to the signal that was to be measured for frequency. The operator watched the oscilloscope on the Translator and tuned for a 1:1 Lissajous figure, which is basically a "O" 'scope pattern. Then the frequency was read out on the Frequency Meter. The Frequency Meter had several vertical columns of lamps that each illuminate a digit, e.g., bottom was 0, next up was 1, next up after that was 2, on up to 9. Each vertical column represented from right to left, 1s, 10s, 100s, 1000s, etc. The columns were arranged so the two left-most were megacycles. The entire eight columns were "00mc  000.00kc" and which lamp/numeral was illuminated in a column represented that digit for the total number that represented the measured frequency. 

Users - Beckman indicated that some of the possible users would be for the military to monitor various types of broadcasts. Also, design studies for transmitters and receivers was a possibility. Monitoring Commercial Broadcast transmitters. Apparently, sixty years ago there were many laboratory businesses that would be used by broadcasters for monitoring the station transmitter frequency. The broadcasters probably felt it was less expensive to pay the lab than to own the equipment themselves. Beckman also thought the system could be used as a secondary "frequency standard" or it could be used to check signal generators or crystal oscillators. One well-known user was KPH, the Radiomarine Corporation Coastal Station at Point Reyes, California that had two Beckman/Berkeley frequency measuring systems set-up at the station. 


photo left: Beckman/Berkeley Frequency Measuring System at KPH

Close-up of the Beckman/Berkeley Model 7700/2-1

Swiss Army 51J-4 Receivers - Martin Bosch from Switzerland... sent me a photo of his 51J-4 that had a career in the Swiss Army. The 51J-4 receivers were maintained by the Army depots and had certain characteristics in common. All of the 51J-4 receivers purchased were very late versions (SN 7000+ on Martin's) and these receivers were fitted with "S-line" knobs except for the TUNING, BAND CHANGE, that appear to actually be somewhat smaller versions of the original Daka-Ware style. The ZERO ADJ and the ANT TRIM knobs appear to be the original style. The Swiss "Signal's Shop" (Army Depot equivalent) changed the position of the METER ZERO adjustment from a chassis location to the front panel. Incorporation of this modification involved removal of the toggle switch for changing the meter from Carrier Level to Audio Output, leaving the meter as just a Carrier Level indicator. The Swiss Army and the Swiss Embassies used and maintained the 51J-4 receivers up into the 1990s. The Swiss purchasing history shows that they purchased two 51J-3 receivers and 82 51J-4 receivers. Martin indicated that because these receivers were always maintained by the Signal's Shop and that they were "in use" into the 1990s, many have been found in "as new" condition.

Martin also has a modified R-388 that was used by the Swiss Army that has the 354A-1 mechanical filter installation.


I've noticed that Martin's photo of his 51J-4 is very popular on the Internet, but he did send me this photo/link via an e-mail plus he included all of the history of these later 51J-4s as used by the Swiss Army.

51J Series Summary - Dial accuracy and drift-free stability - both necessary for RTTY work - were the 51J selling points to the Signal Corps and for most of the commercial users. As the reputation of the 51J became known, other uses for "a receiver that doubles as a frequency meter"*  brought on more commercial users and, with the 51J-4, complete frequency measuring systems were being built that had the 51J-4 at the core. These are still the 51J's primary attributes today,...frequency readout accuracy, drift-free stability and dependability. Although the stock audio is a restricted communications-grade, the 51J receivers are great performers providing they have been thoroughly serviced or rebuilt and, most importantly, that they have been fully aligned. Although the 51J-4 and R-388/URR are considered the ultimate design level for the series, the 51J-1 and 51J-2 have their appeal and can also provide great reception. The 51J Series, especially the R-388 and the 51J-4, are very popular receivers for vintage ham stations and for SWL enthusiasts providing great performance with fabulous visual appeal. Sensitivity is very competitive after a full servicing and dial accuracy (for an analog readout) can't be beat if the PTO is in good shape. Drift is non-existent. The fact that the audio reproduction is limited doesn't affect ham performance very much since most amateur voice transmitters are already limited to 300hz to 3000hz audio bandwidth anyway. Even many of the Shortwave Broadcasters have somewhat limited audio and then, with the lack of any quality program material and variable propagation affecting reception, the 51J's audio limitations really aren't all that noticeable when SWLing. The 51J receivers aren't for audiophiles,...they were designed for accurate frequency readout, stability and reliable communications. 

* This paraphrased quote is from a Technical Materiel Corporation's GPR-90 Sales Service Information (Sup-1 GPR 9-56) and it expresses the frustration that TMC had with users comparing the vague frequency readout  of the GPR-90 to a precise Collins receiver frequency readout. The complete quote was "People who require frequency meters should buy frequency meters - not make them double as communications receivers."


General Information on the Various Types of 51J Receivers

Quick Reference Guide to the Standard Versions

51J-1 - 1947? through 1949 - Officially available in 1949 but perhaps supplied to the Signal Corps and a few other commercial users somewhat earlier. Collins says the receivers were available in 1945 but this must have been in prototype-form when the design was nearing completion. The basic electronic design concept is certainly obvious in the 1947 75A-1 ham receiver, so it's certainly possible that the general coverage 51J was concurrently available with the 75A-1 although the sparse quantity of 51J-1 receivers seen today indicates that very few were produced (usually the 51J-2 is seen more often.) Most sources indicate 1949 as the date of availability to the civilian commercial market but this might have been when advertising first appears. When a complete and original 51J model is found it will be equipped with an illuminated amber scale S-meter, the Collins "winged emblem", a metal dial bezel with "51J RECEIVER" silk-screened above the kilocycle dial, the 70E-7 PTO, the megacycle dial featured green highlighted amateur bands, "100 KC CRYSTAL" nomenclature was used for the Calibration Oscillator switch (on the very earliest production only,) the bottom of chassis had a small cover for the RF sections only with the rest of the chassis bottom uncovered, no grab handles were used, no skirt was on Megacycle Change knob but it did feature a pull-out and retractable crank (although different types of cranks are seen in artwork and ads,) a single phone jack on front panel and the circuit used 16 tubes. The antenna input impedance was fixed at 300 ohms nominal Z with no Antenna Trim control used. Each Antenna coil had a primary winding on the J-1 and J-2 receivers. Unfortunately, of the 51J-1 receivers I've seen, all examples had some modifications and, unfortunately, nearly all of the examples I've seen have been extensively and destructively modified beyond any restoration.

51J-2 - 1950 - The 51J-2 models had essentially the same characteristics as the J-1 with these following exceptions; "CALIBRATE" replaced "100 KC CRYSTAL" on panel nomenclature (only the first few 51J-1 receivers had "100KC CRYSTAL" on the front panel,) the meter was changed to a Carrier Level/Audio Output Leverl meter with appropriate scaling but meter was still the square front bakelite housing model with illumination. A switch was added to the meter circuit to allow measuring either carrier level or audio output, marked "METER - INPUT - OUTPUT." Beginning with 51J-2 serial number 501, the IF transformers were moved further back towards the rear of the chassis. This required moving the three 12AX7 tubes to the right and moving the 6AQ5 tube next to the 5V4 rectifier. All subsequent 51J receivers have this same tube-chassis layout. Some versions of the 51J-2 might be found with the 70E-15 PTO installed, certainly a post-production end-user modification. It's possible that the green highlighted amateur bands were removed from the megacycle drum dial sometime during the 51J-2 production. Very few of the 51J-2 receivers have survived in totally original condition. All examples found seem to be modified in some manner.

R-388/URR - late-1950 through 1962 - This is the military version 51J receiver that featured major changes to the original 51J receiver design. The antenna input impedance changed from fixed 300 Z ohms to low-Z 50 ohms adjustable by eliminating the primary winding on the antenna coils and adding an antenna trimmer capacitor with front panel control. The dial bezel was changed to a black bakelite piece and "51J RECEIVER" engraving eliminated, the megacycle dial drum no longer had the green highlighted amateur bands, the carrier level meter was now a non-illuminated, sealed unit made by Burlington Company (field replacement meters made by Marion Electric and other companies, DeJur and Simpson are often found) audio outputs on the front panel allowed for phone (4Z) or speaker (600Z) connections (pre-1953,) grab handles were now installed on the front panel, a skirted-knob used for megacycle change, an 0A2 voltage regulator tube was installed, the entire bottom of receiver chassis was now protected with a slide-in aluminum bottom cover, the side panels are made of steel and finished with gold color cadmium-based chemical treatment, the schematic was usually applied to the inside of the top cover, all receivers were MFP coated (some very late contract versions might not be MFP'd,) remote standby or Break-in now required +12vdc to be applied to a relay via rear terminals. All R-388 receivers used the new but problematic 70E-15 PTO which added the PTO buffer output tube. This PTO was cylindrical in shape and has two tubes. All R-388 PTOs have "M" as a prefix for the serial number. An SO-239 connector on rear chassis, "IF OUTPUT," was provided for driving RTTY TUs and other data devices. 1953 and later receivers have a "Break-in" on-off switch added to the front panel in the same location as the SPEAKER jack which was removed. Diode Load and AVC pin jacks are added to the rear chassis on last of production. The R-388 was supplied to Army Signal Corps at various times from 1950 up into the early 1960s. Most (all?) USN R-388 receivers are considered as AN/URR-23-A but data plate has "R-388" as the receiver ID. Finding all-original, complete and unmodified R-388 receivers is becoming more and more of a rare occurrence. Many of the R-388s found today suffer from incompetent workmanship that seems to have been perpetrated ever since the receivers became easily available on the surplus market. But, even now in 2023, finding an "all original" example is still not impossibly difficult.

51J-3 - 1951 through 1954 - The 51J-3 is virtually identical to the R-388/URR but the ID tag will show that the receiver is a 51J-3 rather than R-388/URR. It's probable that the civilian 51J-3 receivers are exactly the same as the R-388/URR but the 51J-3 receivers are usually installed in a Collins cabinet and the chassis will be lacking the MFP coating. Also, a true 51J-3 shouldn't have any military inspection stamps. 51J-3 serial numbers in the 12000 range indicate that the data plate is a reproduction and calls into question the authenticity of the receiver as a true 51J-3. Sometimes R-388 receivers will have had their military ID plate removed and then (later) are merely misidentified as the 51J-3. Check for Signal Corps acceptance stamps or other indicators to verify the correct identification for "tag-less" receivers. Basically, the civilian 51J-3 will have "51J-3" on their ID tag while ALL Signal Corps R-388/URR receivers will have "R-388/URR" as the receiver identification. Navy stamps indicate the receiver is the AN/URR-23-A but it would have been tagged as "R-388" on a Navy data plate. While many "radiophiles" consider the R-388 is a 51J-3, the subtle differences are very apparent to Collins enthusiasts. Authentic civilian 51J-3 receivers are scarce.

51J-4 - 1954 through 1964 - Civilian, Commercial, Military versions are all the same with the receiver itself being very similar to the R-388 with the following exceptions. The 51J-4 added three mechanical filters to the IF by installing the Collins 354A-1 Mechanical Filter Kit into the R-388/51J-3 chassis. Examine the 51J-4 chassis and it will be seen that the chassis is punched for the standard IF strip of the R-388 but that a mechanical filter assembly package has been installed which converts the receiver to a 51J-4. The front panels were also changed to add 1KC, 3KC and 6KC nomenclature for the mechanical filter selector switch lever which is located behind the BFO knob. The MF assembly adds an additional IF amplifier tube bringing the tube total to 19. All 51J-4 receivers have a 70E-15 PTOs with "CR" as a suffix for the PTO serial number. Majority of production receivers have St. James Gray wrinkle finish front panels with white silk-screened nomenclature. A small quantity of 51J-4 receivers were produced with front panels painted light-gray smooth finish with black nomenclature, possibly built for laboratory or test facility users. Some 51J-4 light-gray panel receivers are marked "Beckman - Receiver Model 7700" or sometimes "Beckman/Berkeley." The Beckman/Berkeley receivers were a component of a Frequency Measuring System that was a rack with several other components that functioned with the Model 7700/51J-4. The light-gray panel 51J-4 receivers are sometimes found with a variety of Collins-type knobs installed. 51J-4 receivers are not usually MFP coated. AVC and Diode Load pin jacks are on the rear chassis. IF Gain control is on later receivers and is mounted on upper chassis between MF assembly and power transformer. "FCC Part 15" information is silk-screened on the rear of the chassis on later production receivers. When supplied to the military as the R-388A/URR or R-388B/URR it's possible that the ID plate had "51J-4" stamped on it. Good examples of "all original and complete" 51J-4 receivers are still very easy to find but, of course, "good ones" will be expensive. There are several J-4 variations that might be considered rare or scarce when compared to the standard J-4.

Special Versions

R-388A/URR and R-388B/URR - With this designation, these receivers should have been Signal Corps receivers. These are versions of the 51J-4 but it's unknown if the data plate actually identified the receiver as a "R-388A/URR." Same with the R-388B. At this time, no examples of a receiver identified by the data plate as "R-388A/URR" are known to exist ( also indicates the lack of any known examples.) The designation is shown as "R-388A/URR" in Federal Procurement Books. It could be that this was just an identification for ordering the 51J-4 receiver and the receivers shipped from Collins may have actually had the "51J-4" designation on the ID tag. Since the R-388A/URR was shown in Federal Procurement Books as a purchasable receiver (purchase price was about $1200,) the implication would be that a military depot conversion of a standard R-388 receiver using the 354A-1 Mechanical Filter Kit making the receiver into a 51J-4 really wasn't a "R-388A,"...or was it? The known examples of mil-converted R-388s to 51J-4 all have their original R-388 data plates still installed. But, after the mil-conversion, how would the mil-users have identified the receiver since it wasn't a standard R-388 anymore?

AN/URR-23 and AN/URR-23-A - These are U. S. Navy designations for the 51J-2 and 51J-3 installed in a Collins cabinet designated as CY-1235/U with Collins speaker designated as LS-199/U. According to the USN manual for the AN/URR-23-A, the receiver IS a R-388 and this is shown many times throughout the manual. Also, three artwork pictures show the data plate which identifies the receiver as "R-388" only, no "/URR" and the data plate has a USN contract number to the lower left of the tag. It's possible that the AN/URR-23-A identification was on the data plate that was mounted on the CY-1235/U cabinet. The LS-199/U loudspeaker also had a data plate installed. The contract was issued for the AN/URR-23-A on June 22, 1951.

R-381/URR and R-381A/URR - These are Signal Corps designations for the 51J-1 and 51J-2. These receivers don't have the Collins Winged Emblem but have typical mil-style data plate installed.

AF-30 or R-645 - Special modified version of the R-388/URR used for ionospheric backscatter research. Modified by Raytheon for the USAF or USMC. Has a blue panel and no grab handles. Supposedly a very small contact of only seven receivers. Several photos on

Light-Gray Panel 51J-4 - The most often heard reason for the light-gray panel 51J-4 were that they were for use in laboratories or in test facilities. Some say that they were for Collins "in-house" labs. But, did Collins  supply light-gray panels to other end-user laboratories? Were light-gray panels ever advertised as an option?

Receiver Model 7700 - This identification is on the Beckman-versions of the 51J-4 with light-gray, smooth-finish painted front panel. One Beckman/Berkeley 51J-4 has been found with a greenish-gray wrinkle finish panel with identification as Model 7700/3. Various "/xx" suffix to most Model IDs, e.g., Model 7700/2-1. Some of the Model 7700 receivers were components of a Beckman Frequency Measuring System. The Model 7700 receivers had several modifications incorporated into the circuitry so they could function as needed with the other Beckman equipment. Beckman did all of the modifications to the receivers and probably made the panels themselves. It seems probable that Beckman purchased standard 51J-4 receivers from Collins and then incorporated all of the modifications themselves.

Gloss-Black Panel 51J-4 - Two are shown on One is in good shape having been restored, the other is in rough condition with modifications. Another original condition one showed up on an Antique Radio Internet forum. The black panel was a replacement panel in black glossy finish and the top and bottom corners of the panel were notched and the grab handles were mounted low on the panel and had ejector buttons. There are no phone jacks on the front panel. There are two fuse mounts above the Crystal Filter controls. The nomenclature for the Crystal Filter was relocated to below the controls. There is a green indicator lamp by the "ON" nomenclature of the power switch. Speculation was that it was the forerunner to the LTV-G133F, a modified 51S-1, for airborne intelligence use. Several photos on   NOTE: Mar 2023 - One of these variations of the 51J-4 has been found with a black wrinkle finish front panel. The receiver appears to be a R-388 with similar modifications that the gloss black 51J-4 versions had but without a 354A-1 mechanical filter kit. This receiver has been modified over the years and many parts are missing so authenticity is difficult to establish (just examining photographs.)

R-648/ARR-41 - The Airborne R-390A/51J-4

While the popular nickname "Airborne R-390A" is certainly appropriate for the R-648, much of the circuit design borrows heavily from the Collins' 51J series of receivers, especially the dual variable "odd-even" IF with reduced number of crystals in the Crystal Oscillator and the fixed 500kc IF. Even the 500kc IF Mechanical Filters are physically a very close match to those found in the 51J-4 (except they're soldered in place.) The R-648 is more like a 51J-4 with modular construction and a mechanical-digital frequency readout.

The R-648 was for use in Navy aircraft (doing RTTY in some airborne installations.) Seventeen tubes are used in the receiver providing two RF amplifiers, double conversion for most bands, three 500kc fixed-frequency IF amplifiers with two mechanical filters (1.4kc and 9.4kc, later models have 1.4kc and 6.0kc MFs) and an audio amplifier with three stages of amplification. When looking at the chassis, one sees the familiar slug racks and slugs, a PTO, modular construction with seven modules and, of course, a mechanical-digital frequency dial. One feature the R-648 has that neither the R-390A or 51J-4 has is tuning from 190kc up to 550kc. The remaining frequency coverage is 2.0mc up to 25.0mc. The R-648 shown is from 1957.

On board the aircraft, the power was usually +28vdc that was provided by the battery-charger buss. This powers the R-648 via an onboard dynamotor with an output of +250vdc at 100mA. The +250vdc is also routed to an 0A2 regulator tube to provide +150vdc. Additionally, a divider network provides about +31vdc for AVC bias. Tube heaters are wired in series-parallel to run on +28vdc and the dial lamps are in parallel on the +28vdc line in the receiver (#327 lamps.)

The GAIN control functions as the RF gain when the receiver is in the CW mode with the AF gain automatically set to maximum. The GAIN control functions as an AF Gain control when the receiver is the VOICE mode with the RF gain controlled by the AVC line. The Sensitivity control is provided to set the maximum available sensitivity and is a slotted-shaft pot behind the "toilet seat" cover marked SENS ADJ." The audio output level is set with a pot adjustment located at the rear of the chassis. This setting was to act as the "maximum" limit so that the headset used would not be over-driven. The audio output impedance is not critical and anything over 300Z ohms was considered appropriate. The audio output has ample volume if a 600Z ohm load is provided. This can be a loudspeaker with a 600Z ohm matching transformer.

In actual use the R-648 is a very sensitive receiver that provides an accurate frequency readout and excellent stability. The selectivity is steep-sided as expected with mechanical filters but the early receivers had a 9.4kc MF for VOICE reception and many times that's just a little too wide in a crowded ham band. The audio filter can be switched in but it's easier and more effective in the AM mode to just tune up or down a couple of kc to reduce QRM. CW is fine with the 1.4kc filter and SSB also sounds just fine with the narrow bandwidth. The CW audio filter is very narrow. There is some vibration from the dynamotor and although it can be felt it doesn't seem to affect the receiver in any way. Noise from the dynamotor might seem a loud at first but, after the audio comes up, the dynamotor noise is pretty much masked by the receiver audio. A nice performing, small, light-weight receiver that's easy to find room for in any vintage ham shack. The shock mount shown isn't original but it does the job of elevating the receiver off of the bench and providing vibration isolation via its shock feet.



Civilian 51J Cabinets - The cabinets for the 51J receivers were optionally available and are sometimes found with the civilian receivers, especially 51J-4 receivers. The cabinet is similar to any of the late-forties and early-fifties Collins' cabinets that were used on the 75A-1, 75A-2, 75A-3 receivers but with some noticeable differences compared to the 32V-1 and 32V-2 transmitter cabinets. Receiver cabinets generally didn't have the lower rows of side vents that extended almost to the bottom of each side. These lower side vents were used in the 32V-1 and 32V-2 transmitter cabinets. The standard 75A-type Collins receiver cabinet had a rear rectangular cutout that was about 3" in height and 17" wide. A true 51J cabinet will have a very large rectangular opening at the rear of the cabinet to allow access to the three wing-nuts that secure the receiver's top cover and allow its easy removal. The rear cut-out height must go up past the two lowest row of side vents leaving only four rows of rear vents. The width of the cutout must extend past the vents by about .375" on each side. The size of the opening is 7.5"H x 17.75"W. Some cabinets might be found where the cut is just below the lowest row of vents or it might be inline with the width of the vents. In either case, the top cover can't be removed unless the receiver is taken out of the cabinet. See the Collins' catalog artwork shown to the right for the appearance of the proper cutout height and width relative to the vent cutouts.

I've had two 51J-4 receivers that were in what appeared to be their original cabinets but, with both receivers, the top cover couldn't be removed when the receiver was installed in the cabinet because the rear cut-out height wasn't above the second row of side vents and the cutout's width wasn't >17.5". If you can't remove the top cover out the back opening, then suspect an amateur-modified cabinet. To remove the 51J top cover with the receiver mounted in an original Collins 51J cabinet,...lift the cabinet lid to be able to see the wing nuts, then reach around the sides of the cabinet to loosen the wing nuts, then lift the rear part of the top cover up to clear the wing nuts, then slide the top cover back and down and out the rear cabinet opening.

51J Cabinet Artwork
from the 1959 Collins Catalog

If you're installing a 51J receiver into any type of cabinet do not remove the actual top and bottom covers from the receiver. These covers provide proper shielding, especially the bottom cover, and must remain installed during operation even if the receiver is in a cabinet. The top cover should be removable with the receiver in the cabinet for servicing, like testing tubes.

Later versions of this cabinet, as found on some 51J-4 receivers and probably on those cabinets sold separately by Collins, had a top lid latch. Also, the tapped panel mounting holes were replaced with drilled holes and clip-nuts. The top lid hinge was changed to a "piano hinge" so it's visible with the lid closed. Rubber feet mounted with aluminum spacers were used on these later receiver cabinets.

1952 R-388/URR - Order:3362-PHILA-52 - Mil SN:161 - Collins SN:9108
installed in a Hallicrafters CY-1260/G Receiver Case

More details on R-388 SN:161 and on the CY-1260/G case in the restoration section in Part 3 of this web-article.

Military Cabinets aka CASE, RECEIVER - Hallicrafters supplied a CY-1260/G cabinet, shown in the photo to the left, that was used with the R-388/URR in some applications. Hallicrafters also supplied a manual when the receiver was installed in this or similar Hallicrafters' cabinets. Also, there was another Hallicrafters cabinet made in a similar style to the CY-1260/G that would hold two "stacked" R-388 receivers. Hallicrafters actually made several CY-version cabinets that were available for several different types of receivers including the SP-600 and the R-274 and R-274D. These Hallicrafters-built cabinets and the accompanying manuals have probably led to the myth that Hallicrafters built R-388 receivers which, of course, wasn't the case. Collins wouldn't even let RACAL build 51J receivers!

The Hallicrafters-built military cabinets are fabricated with heavy steel and welded seams. Shock-type (rubber cushioned) feet are bolted to the bottom of the cabinet and the feet centers are drilled and tapped for secure mounting to a table. There's a rectangular opening in the cabinet-bottom that must have been for ease of manufacture since it can't be for access underneath the receiver because the receiver's bottom cover must remain installed during operation. Inside, an elaborate guide/glide set-up was installed on each inner-wall to mate with accessory metal rolling wheels that are mounted on each of the rear sides of the R-388 receiver. This allowed very easy installation and removal of a receiver from the cabinet since sliding friction was practically eliminated. The cabinet's rear cutout allows easy access to the receiver's rear terminal strips and SO-239 fittings. Six small louver vents are on each side of the cabinet but no grab handles or slots were provided which makes moving this very heavy (~35 lbs. empty!) cabinet somewhat difficult. Note in the photo to the left that the receiver is raised off the table by about two inches when using the CY-1260 cabinet. Total cabinet height is 13.5 inches. Contract date for CY-1260/G cabinets and manuals was in 1951.
photo left: 1952 R-388 SN:161 - installed in the Hallicrafters CY-1260/G military cabinet. These cabinets are built from heavy-gauge steel with elaborate glide rails on each inner side, also made of steel. There aren't any grab slots on the sides so moving these cabinets can be difficult with the receiver installed (total weight is 70+lbs.)

photo right: Close-up of the data plate on top of the cabinet.


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