Radio Boulevard
Western Historic Radio Museum


Collins Radio Company

51J Series of Communications Receivers

Includes: 51J-1, 51J-2, 51J-3, 51J-4, R-388/URR
and several other 51J Series variations

History, Versions, Circuits, Rebuilding


by: Henry Rogers WA7YBS

Artwork from the Collins 51J-4 Manual

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. Many users are operating their 51J receivers in "as-found" condition but are they experiencing all that the 51J is capable of? The following article details the evolution of design and the differences in the various models. Details on Beckman/Berkeley uses for the 51J-4. Also, how to get the most out of your 51J receiver and a look at some of the common problems encountered when delving into any of the 51J receivers. I've included lots of photos and detailed descriptions for servicing and restoring or rebuilding the 51J-2 model, the R-388/URR model and the 51J-4 model.


1950 ad for the Collins 51J-1 Receiver. One of the first ads to appear in QST didn't necessarily mean the 51J-1 wasn't available much earlier.

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 civilian 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 receiver, the 75A-1, two years earlier. But, according to Collins, the development of a precision general coverage receiver had started (at Collins Radio Co.) 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 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 certainly 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 and also probably 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 1947 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 an excellent general purpose receiver and ultimately was 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 other than to use the Collins 51J receivers (and the R-390 receivers.)

Other 51J Series Users - The initial 51J wasn't conceived as a military receiver although 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 1955, the 51J-4 was introduced and it was primarily a receiver for commercial users that had a need for 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

51J-1 and 51J-2

The 51J receiver utilized a permeability tuned, double conversion circuit using the 70E-7 PTO in a dual, tuned-IF system and a multiple frequency crystal oscillator to cover .5mc to 30.5mc in thirty (1mc wide) bands. The tunable dual IF system selects either 1.5 - 2.5mc (even) or 2.5 - 3.5mc (odd) depending on whether the band number selected is odd or even. 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 but only to allow coverage of the AM BC band. Bands 2 and 3 are single conversion since they are essentially the frequency coverage of the tunable dual IF system. All of the remaining bands are double conversion and select either the even or odd tunable IF as required in combination with the Crystal Oscillator and PTO to provide the correct tuning range. The antenna input impedance was fixed at 300 ohms. The fixed IF is tuned to 500kc and has three amplifiers. A Crystal Filter is provided for adjustable selectivity and heterodyne interference reduction. A standard envelope detector, separate AVC amplifier/rectifier tubes and a Noise Limiter are also included. 16 tubes are used in the 51J-1 and J-2. The ham bands are high-lighted in green on the megacycle drum dial but, at $875 list price, not many hams could afford a 51J as their station receiver. 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 receiver had a black-painted metal dial bezel with "51J RECEIVER" silk-screened in white, the 70E-7 PTO (with the 6SJ7 metal octal tube located under a screw-mounted shield cap,) the Collins' chrome and orange "winged emblem," no grab handles and an illuminated, square-front bezel S-meter (the 51J-1 had a S-meter but a dual scale Carrier Level/Audio Output meter was used on 51J-2.) Audio response is restricted by design at 200 to 2500Hz and is definitely not high fidelity, usually sounding somewhat lacking bass and "muffled" audio highs when receiving AM voice signals. The most apparent difference between the 51J-1 and 51J-2 is that the latter added an Audio Output function to the new dual scale Carrier Level/Audio Output meter that was actuated by a toggle switch located next to the meter. Another difference is the nomenclature for the 100KC Crystal Calibrator, was designated as "100KC CRYSTAL" on the early 51J-1 and merely as "CALIBRATE" on most 51J-1s and all 51J-2s. Some later production 51J-2 receivers may be found with the 70E-15 PTO (round can and two 6BA6 tubes) installed but whether this was a later retrofit as part of a repair or if it was an official Collins-upgrade is unknown.

photo above: The Collins 51J-2 from 1950 in the standard Collins cabinet
Note the green highlighting on the 40M band on the MC scale


It's also possible that the green high-lighting of the ham bands on the megacycle drum dial was eliminated in the later 51J-2 receivers since it was unlikely that sales to hams accounted for many purchases. Again, later repairs may account for receivers found with later style dial drums installed. Both the 70E-15 PTO and the standard megacycle scale dial drum are used on all of the standard production of the 51J-2's successors, the 51J-3 and R-388/URR and, ultimately, the 51J-4.

The 51J-1 was built in a very small quantity from around 1947 through 1949 and by the end of that year it had been replaced with the 51J-2. The J-2 was built through most of 1950. The military ordered both 51J-1 and 51J-2 receivers and they were designated as the R-381/URR and R-381A/URR. Only a relatively small quantity of 51J-2 receivers were produced before the upgraded version 51J-3 and R-388/URR replaced it.

The photo to the left shows the chassis of the 51J-2. Note the "square" 70E-7 PTO with the 6SJ7 tube on top but underneath the cylindrical shield. This chassis shows the basic layout for all of the 51J receivers. 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 early-style dark gray shield-cans. On the right-side-front is the Crystal Filter, middle-right power supply and along the right-rear is the AVC-NL-1st AF circuits. Later versions of the 51J added the "cylindrical" 70E-15 PTO and the 0A2 regulator tube to the left of the PTO. The 51J-4 added the Mechanical Filter assembly where the front-most IF transformer is located.

photo above: The characteristic "sloping top cover" of the 51J Series. All 51J receivers will have this type of top cover. The bottom three retaining fasteners should be wing-nuts on the later versions of the receivers.



photo above: 1951 contract R-388 with standard Burlington carrier level meter

In late 1950, the 51J-3/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 ohm Z 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 cabinets (along with manuals) that were utilized by the Army for some of it's R-388 installations. These Hallicrafters cabinets and manuals are probably the source of the "Hallicrafters-built R-388" myth. Some of the U.S. Navy R-388/URR receivers were designated as AN/URR-23-A (but were tagged as "R-388") 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 grab handles. The largest contract quantities for R-388/URR receivers are from 1951, 1952 and 1953 contracts.

photo above: R-388 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. 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" was entirely separate from the front panel STAND BY switch position meaning that the "Break In" relay can 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 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 a different receiver from its predecessors, the 51J-1 and J-2.

Though the standard carrier level meter was supplied by Burlington, it isn't unusual to find that the original meter will have been replaced during a field repair. Marion Electric meters have a convex plastic cover and are sometimes found installed. There were also other meters sometimes installed that are similar in appearance to the Burlington style, DeJur, for example. The 1953 R-388 shown below has the MWOs listed on the front panel. Also, note the Signal Corps acceptance stamp near the meter.

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

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. 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.)

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 is virtually identical to the R-388/URR receiver but with a couple of important differences. 51J-3 receivers will have a Collins data plate (looks like the 51J-4 data plate) with "51J-3" stamped on the plate with an appropriately "low" serial number issued. Besides the obvious data plate difference, all R-388 receivers will have military inspection stamps from the Signal Corps or USN while the civilian 51J-3 shouldn't have any military inspection stamps. The civilian 51J-3 receivers won't be MFP treated. The 51J-3 didn't have a front panel mounted "Break-in" switch as did the 1953 and later R-388 receivers. Still, 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 many 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. But, for Collins enthusiasts there are differences between the two receivers 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.

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

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.

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." 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 scarce and very seldom encountered.



In 1955, 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 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 "fixed-tuned IF amplifiers." 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. There weren't any changes made to the simple envelope detector, the AVC amplifier-rectifier circuit or the noise limiter circuit. The audio output circuitry also remained unchanged.  

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

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

photo above: Under the 51J-4 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 Power Supply section and the AVC, NL and Audio stages.  

Towards the end of the 1950s, 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 later 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. Approximately 7500 51J-4 receivers were built from 1955 up until 1964. 

U.S. Navy 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 Navy catalogs, the actual receivers were identified on the data plate as "51J-4."

Are the 51J-4 Receivers with Light-Gray Front Panels Lab 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 supplied to Beckman with special nomenclature and also some circuit changes. 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. Some light-gray panel examples will have a mixed variety of Collins-type knobs. Variations in nomenclature fonts are frequently seen with the light-gray panel J-4s.


51J-4 SN:4723

Collins "Product Support Operations" Sticker

The PTO in SN:4723 showing the "CR" suffix
for the serial number. Collins Radio?

Light-Gray Panel 51J-4  sn: 4723  - Collins Lab Receiver?

This 51J-4 has a Collins "Product Support Operations" sticker on the rear chassis apron that's dated Mar 19, 1971 and indicates the receiver was "Processed by Product Repair and Modification."

From some of this features found on this receiver one could infer that it was used at Collins. 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.)  The PTO serial number is 6667CR (does the CR suffix indicate Collins Radio? Or, was it just the initials of the builder?) I suspect that the engraved numbers at the upper right corner of the panel, "029 1059 00142" are an asset number that identifies the receiver as property of a particular company or laboratory. Who knows? Maybe it's Collins' asset number. Note that the receiver further down (#3) also has similar numbers engraved in the same location and it also has a similar toggle switch boot installed.

Of course, 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.

Date Stamp on the DeJur CL Meter


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


Other Light-Gray Panel 51J-4 Receivers

Showing 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. This Beckman 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. It was found by Tom W3TA in nearly complete condition but seriously suffering from over-exposure outdoors. This photo shows the very different greenish-gray wrinkle finish 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





51J Series Summary - Dial accuracy and 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, other complete systems were being built that had the 51J-4 at the core. These are still the 51J's primary attributes today,...frequency readout accuracy, stability and dependability. Although the stock audio is somewhat restricted, 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 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 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 - Official release date is 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 just be when advertising first appears. When a complete and original 51J model is found it will be equipped with an illuminated 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 earliest production only,) the bottom of chassis had 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 retractable crank, 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.

51J-2 - 1950 - The second 51J models had essentially the same characteristics as the J-1 with these following exceptions; "CALIBRATE" replaced "100 KC CRYSTAL" on panel nomenclature (late 51J-1s also had this change,) the meter was changed to a Carrier Level 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." Very late versions of the 51J-2 might be found with the 70E-15 PTO although certainly later rework would be a more likely explanation. It's also possible that the green highlighted amateur bands were removed from the megacycle drum dial sometime during the 51J-2 production. Again, later rework is a possibility with an R-388 drum replacement of a damaged original J-2 drum dial.

R-388/URR - 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 are sometimes found installed,) 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, a PTO buffer output tube was installed (brought total tube count to 18,) the entire bottom of receiver chassis was now protected with a slide-in aluminum bottom cover, many of the aluminum parts now had an irridite finish (gold color,) the side panels are made of steel and finished with gold color chemical treatment, the schematic was usually applied to the inside of the top cover, most (all?) receivers were MFP coated, remote standby or Break-in now required +12vdc to be applied to a relay via rear terminals. 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 ID.

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. The actual "civilian" 51J-3 is rare. It's probable that the civilian receivers are exactly the same as the R-388/URR but 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.

51J-4 - 1955 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 a 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. Majority of production receivers have St. James Gray wrinkle finish front panels with white silk-screened nomenclature. Some production versions have 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 usually not 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 USN as the R-388A/URR or R-388B/URR but it's possible that the ID plate had "51J-4" stamped on it. 

Special Versions

R-388A/URR and R-388B/URR - U. S. Navy version of 51J-4 - it is unknown if this model was actually identified as such even though it's listed in the Navy books. Same with the R-388B. These receivers may actually have the 51J-4 designation on the ID tag.

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 a data plate that was mounted on the CY-1235/U cabinet and on the LS-199/U speaker. 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.

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 standard grab handles but has ejector button type of grab handles mounted low on the panel. Buttons release the locks on the side glides. 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, Collins did supply light-gray panels to other end-user laboratories? Was this upon-request or by special order? The Beckman versions appear to be special order panels for their Model 7700 receivers but were the panels installed at Collins or by Beckman? Also, some Beckman 7700s appear to have several circuit modifications, so did Beckman incorporate the mods themselves? It appears so.

Receiver Model 7700 - This identification is on the Beckman-versions of the 51J-4 with light-gray, smooth-finish painted front panel. 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.

Black Panel 51J-4 - Two are shown on One is in good shape having been restored, the other is in rough condition with modifications. The black panel was a replacement panel in black glossy finish and the top and bottom corners of the panel were notched and no grab handles were used. 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

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

While the popular nickname "Airborne R-390A" is 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. The R-648 was for use in Navy aircraft (doing RTTY in some airborne installations) and it uses a lot of the same mechanical design approaches as found in it's bigger brothers, the R-390A and the 51J-4

17 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 series 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.)


Accessories and Miscellaneous Info

1952 R-388 installed in the Hallicrafters CY-1260/G military cabinet
. These types of cabinets had special guides on each side and the receivers had specially installed metal wheels mounted to the rear chassis sides to allow a "rolling" of the receiver to remove it from the cabinet. These cabinets are built of heavy-gauge steel and their weight is formidable.

Civilian Cabinets - The cabinet for the 51J receivers were optionally available and are found with many of 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 and the 32V-1 and 32V-2 transmitters. 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 removal, with the lid up, out the top. The rear cut-out should go up past the lowest row of vents. Some cabinets are found where the cut is just below the lowest row of vents and in that case the top cover can't be removed unless the receiver is taken out of the cabinet. I believe the cabinets modified at Collins will be correct and later "collector modified" cabinets may be incorrect. With the proper height back cut-out, the top cover wing-nuts are removed, then the cover is slid back and down a sufficient distance to allow tilting the front of the cover up and lifting it out the cabinet top opening, lid up, of course. When operating the receiver, the top and the bottom covers (both) must remain mounted on the 51J receivers for proper shielding even if the receiver is installed in a cabinet.

Military Cabinets - Hallicrafters supplied a CY-1260/G cabinet, shown in the photo 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 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 have probably led to the myth that Hallicrafters built R-388 receivers which, of course, wasn't the case.

The Hallicrafters-built cabinets are fabricated with heavy steel and welded seams. Inside an elaborate guide set-up was installed to mate with accessory metal rolling wheels mounted on 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 greatly reduced. The rear cutout allows easy access to the rear terminal strips and SO-239 fittings. Louver vents are on both sides of the cabinet but no grab handles or slots were provided which makes moving this very heavy 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 which increases the total height to about 13.5 inches. The cabinet data plate is shown to the right.

Break-in Operation for the R-388, 51J-3 and 51J-4 - An Oddity - Perhaps the most inconvenient requirement for the R-388, 51J-3 or the 51J-4 is how to provide the Break-in function. Break-in requires an external +12vdc 200mA source that is switched "on" when in the "transmit" mode to isolate the receiver input and to mute the audio output. Inside the receiver, a relay is operated by the Break-in voltage and that relay disconnects the antenna input line, grounds the receiver input circuitry and also removes the voltage from the IF amplifier plates to mute the receiver. There is also a neon bulb as a protection device on the antenna line but this is mainly a static voltage drain and not intended to handle large amounts of RF energy. Break-in must be used when the receiver is paired with a transmitter and both are using the same antenna. Even with separate transmit and receive antennas, employing the Break-in function will protect the receiver input circuitry from excessive RF voltage. In addition to Break-in, when using a single antenna, the standard T-R relay is necessary to switch the antenna itself from receiver to transmitter. Usually, this does provide some isolation and many Dow-Key relays had an additional spring-loaded contact switch inside the receiver-side coaxial fitting barrel that further increased receiver isolation. But, the Break-in provides positive disconnect of the antenna line and the grounding protects the receiver input just in case there's a T-R relay failure or a mistaken connection happens. The front panel STAND BY position does not actuate the Break-in relay. The Break-in relay can only be operated by applying +12vdc Break-in voltage to pins 2 and 3 of the REMOTE terminal strip (pin 1 provides a ground for a cable shield.)

The 51J-1 and 51J-2 have conventional stand-by provisions providing external relay connections that parallel the front panel STAND BY switch. The auxiliary contracts on the T-R relay can provide the remote Stand By function. There isn't any antenna input protection circuitry provided in these early version receivers.

NOTE: There are a couple of methods that provide the +12vdc "Break-in" within the receiver. Either a small power supply mounted under the chassis of the receiver or a voltage doubler circuit that runs off of the 6vac tube heater supply. See "R-388 Expected Performance and Operation" further down in the "Rebuilding the R-388/URR Receiver" section.

Loudspeakers and External Audio Amplifiers - All of the advertising or the manuals never specified a particular matching speaker for any of the early 51J series of receivers. This seems to imply that the standard 8" speaker in the 270G-1 was used with the early 51J receivers and the 270G-2 with 10" speaker was used with the 51J-4 (the 51J-4 manual actually specifies a 270G-3 loudspeaker which is identical to the 270G-2 except that each side of the cabinet has three holes for installing rack mounting brackets.) R-388 receivers were generally used for specific military set-ups such as RTTY or other data transmissions where a loud speaker would not be necessary (a headset would be used for monitoring signals.) There was a rack mounted dual speaker system available for diversity RTTY set-ups. There was a USN version of the R-388 referred to as the AN/URR-23-A that came in a standard Collins cabinet, CY-1235/U and included the 270G-2/3 speaker (mil ID: LS-199/U.) The 51J-2 was also supplied in a similar fashion and designated as the AN/URR-23 (no -A.)

For a vintage mil-rad station, perhaps the easiest speakers to find (that are period and manufacturer correct) would be either the Collins 270G-1, an 8" speaker, or the Collins 270G-2 or 3, a 10" speaker. The 270G-1 was typically supplied with the 75A-1 receivers and had the Collins "winged" emblem on the front with chrome trim strips with a perf-metal grille (usually a sort of maroon color.) The 270G-2 was typically supplied with the 75A-2, 75A-3 and 75A-4 receivers and these don't have the Collins WE and have black trim strips with a beige color felt-flocked grille. Any of the three types are fine sounding, communications loudspeakers and, though usually outrageously expensive, they are fairly easy to find. However, any 4.0Z speaker will function fine with the 51J receivers. There's not too much of an advantage to using a high-quality, large diameter speaker installed in a bass reflex cabinet because the audio output circuitry of any of the 51J Series receivers was always "communications grade" in quality. The audio range available on early 51J receivers lacks bass since the audio output was restricted to 200hz on the low end and the upper limit was 2500hz and can only produce about 1.5 watts maximum. The R-388 seems to have somewhat better audio and, although the specs are the same, it might benefit from a larger speaker. Even the 51J-4 audio was still specified as 200hz to 2500hz +/- 3db in the last of the manuals with the 1.5 watts output power maximum (at <15% distortion.) These less-than-impressive specifications will become more and more apparent with better speaker systems. With the very late R-388 receivers and most of the 51J-4 receivers, the Diode Load was brought out to the rear chassis apron as a test point jack. It would be possible to use a shielded cable connected to the Diode Load and run that to the input of a High Fidelity audio amplifier (using the aux. input or some other fairly Hi-Z input) to drive a quality loudspeaker. You would lose the Noise Limiter function but it was designed for repetitive pulse noise that isn't heard very much nowadays. Also, the 6kc mechanical filter will still be limiting any quality AM reproduction so very few enthusiasts have bothered with the external Hi-Fi amp set-up. Usually the standard Collins communications-grade loudspeakers will give the best results when used with any of the 51J receivers.


How Collins and the 51J were Responsible for the RACAL RA-17

British Homage to the 51J - RACAL Engineering began as a small electronics company in England. RACAL started operations in 1950. The founders were two friends, Raymond F. Brown and George Calder Cunningham, who had been working together at Pessley Electronics. The RACAL name was derived from RAy and CALder, the founder's names. The small RACAL company built almost anything at first from golf clubs to shelving units along with some electronic devices.

Things changed for RACAL in 1953 when the British Royal Navy became interested in having RACAL build a couple hundred Collins 51J receivers for Royal Navy use. The 51J was being successfully used for RTTY communications in the USA military. It provided frequency-solid stability along with a dial readout that was "to the kilocycle" accurate. The 51J was the first receiver used by the US military that provided reliable RTTY reception without modifications. The US military R-388/URR was the most successful 51J version and was produced in the greatest quantity. Certainly the Royal Navy was aware of the success the 51J/R-388 afforded US military RTTY users and they needed the same type of reliability for their RATT comms (British for RTTY.)

If RACAL was going become involved in British-built Collins' 51J receivers, they would have to be licensed by Collins to do so. Naturally, RACAL wanted to use mostly British parts (probably so did the Royal Navy) but Collins Radio Company insisted that only parts from the USA could be used and only receiver assembly could be performed by RACAL. Ultimately, the proposal ended up with a group from Collins doing an inspection of the (then) small RACAL manufacturing facility. Apparently, Collins wasn't impressed and simply refused to license the manufacture of the 51J by RACAL. The only option left was for RACAL to design their own receiver that would meet the requirements for the Royal Navy contract.

RACAL contacted Dr. Trevor Wadley, a well-known South African electronics engineer, to help with the project and the result was the famous RA-17 receiver. Though entirely different in electronic design concept, using the famous "Wadley Loop" to eliminate drift, the RA-17 met or exceeded the 51J specifications and went on to be produced in many variations over the next 20 years. In looking at the RA-17 receiver to the right, it's immediately apparent that the physical appearance of the RA-17, especially the dual-dial arrangement and the slanted top (can't be seen in the photo,) paid homage to the Collins 51J receivers. 

RACAL RA-17C-12 from 1961. The "C" versions of the RA-17 were built in England but were intended for North American users and for NATO. Mostly USA-types of tubes, hardware and nomenclature were used for the "C" versions. The RA-17C-12 versions were also designated as the R-501/URR by the USA mil-users.


General Information on Reworking 51J Receivers

Rebuilding Difficulty - The 51J Series is a fairly difficult receiver to work on because it is not modular in mechanical design and needs to be substantially disassembled to work on the commonly encountered problems, including those problems involving the PTO. The front panel always has to be taken off to remove the PTO. Working on the PTO requires careful attention to the delicate parts inside. The PTO usually must be calibrated for minimum end-point error outside of the receiver on the 51J-3, 51J-4 and R-388. This requires that a jig be made to accurately measure the turns to better than one degree of rotation. If work is required in the receiver's front end, it is extremely difficult to access any of the parts in the crystal oscillator and most of the other front-end stages aren't much easier to get into. Additionally, the wires from the various coils are very fine and easy to accidentally break when doing rework in the front-end of the receiver. All R-388 receivers will be MFP'd which adds to the difficulty of rework. The 51J gearbox is complex and rework is difficult. To take on a poor condition 51J receiver, you should be experienced in complete disassembly and reassembly of advanced communications equipment. You should have professional soldering equipment, possess a good soldering technique and use only real SnPb solder. Your test equipment should be laboratory-type although your skill at RF/IF alignment will determine the ultimate quality of the receiver's performance. The 51J Series is certainly within the capabilities of most restorers who have some communications receiver experience. Don't be in a hurry and always be thorough. Some Thoughts on Rebuilding - Most of the Collins 51J Series receivers will operate as found. However, these receivers are probably not operating to their full capabilities and generally give the new owner a feeling that the 51Js are over-rated. Although you won't find handfuls of bad capacitors (as in the early Hammarlund SP-600s) there are several common problems with the 51J receivers now that they have aged over five decades. Also, since the receivers were used extensively and then were probably stored poorly, you will usually find some mechanical problems that will need to be repaired. Once all of the circuits has been gone-through and rebuilt if necessary and the mechanical issues addressed, a full alignment is necessary. Though not essential, the original alignment tools will make many of the adjustments easier (repros were available and they worked great. Can't find anyone selling them now, in 2023.) Early 51J receivers that use the 70E-7 type PTO align easily. Later receivers with the 70E-15 PTO will almost always require a rebuild of the PTO to function at their design level of performance. When completed, the 51J Series receivers are fully competitive with any other mid-fifties communications receiver. Highly sensitive, very accurate dial resolution and great selectivity. Note, that high fidelity audio was not included. The 51J receivers have highly restricted audio since they were primarily designed for communications, specifically data reception.
Commonly Found Problems and Component Issues - A thought for consideration is that most of the earlier 51J receivers, the 51J-1 and J-2, have had thousands of hours put on them by former commercial/military users. These receivers were sometimes roughly treated and are usually well-worn, needing much more than the usual re-cap and alignment to function at their design level of performance. The early 51J-1 & 2 gear boxes have brass drive gears where the later R-388/51J-3 and J-4 versions have steel drive gears. Sometimes the wear on early gearboxes is so severe that binding and jamming are experienced when changing the Megacycle position.

The early IF transformers often seem to sustain internal damage with rough handling of the receiver requiring disassembly and re-gluing of the coils and ferrite shields back into their proper position for correct operation.

The R-388 and 51J-4 receivers seem to be in much better overall condition and usually don't require extensive rebuilding, although there certainly are exceptions. Most component problems and certainly all mechanical problems seem to be aggravated by poor storage conditions. However, there are some component quality issues with some types of capacitors that might affect the AVC circuit performance. This seems to be a problem mostly encountered with the earlier receivers and is seldom found in the R-388 or 51J-4.

Most of the 51J problems will be mechanical in nature and brought on by poor storage, rough handling, careless or indifferent technicians or the most commonly encountered problem,..."hamster" repairs and modifications.

Manuals - If you are working on a 51J-2 receiver be aware that the 1950 version of the 51J-2 manual is fraught with errors in almost every section - almost on every page - almost in every paragraph. The alignment section, circuit description section, the component designations and component identification layouts seem to contain the greatest number of errors. Misidentified components are the most common errors but some alignment procedure errors also exist. How this manual got through the proof-readers is a mystery. The 51J-1 manual is probably just as bad. The later Signal Corps R-388 and 51J-4 manuals are excellent with very few errors found. However, the R-388 schematic found in some manuals has several component identification numbers transposed which makes correct ID'ing difficult. The schematic that is glued inside the top cover is correct. The Navy manual for the AN/URR-23A (R-388) is excellent but very difficult to find.

Capacitors - Nearly all of the capacitors used in the 51J front-end and IF sections are tubular ceramic caps which are extremely reliable and never seem to fail. There are some issues with the small silver mica caps (the little red ones) in that sometimes they will short (silver migration, most likely) but this isn't very common. The bypass capacitors are usually paper dielectric types that should be replaced in early versions. Also, the variable trimmer capacitors are integral to their fiber mounting boards in the front-end. These can and do "stick." Do not force any stuck trimmer. First apply some De-Oxit to see if that frees the trimmer. If De-Oxit doesn't loosen the stuck trimmer than try a bit of heat on the metal slotted head. Use a small soldering iron to apply the heat. This will sometimes melt and loosen material that won't dissolve in the De-Oxit. You don't want to break the trimmer since they can't be replaced without special modifications.

The filter capacitor assembly plugs into an octal socket. These are dual electrolytic capacitors with 25uf per section used in the 51J-1 and 51J-2 receivers. The R-388 and 51J-4 use either 35uf per section or 40uf per section depending on if the capacitor was ever replaced. Working voltage is usually 450vdc on all types. These are very reliable, well-sealed capacitors that seldom fail. Check for leakage current before powering up the receiver. Capacitors that haven't had voltage applied for decades should be reformed. Leakage current on a reformed electrolytic should be less than 100 uA at full working voltage.

Unnecessary Modifications - There are plenty of modifications that have been published for the 51J Series. A search on the web will result in several to choose from. Nearly all of them are concerned with two areas of the receiver circuit. First is the AVC circuit which many users feel has too short of a release time. However, Collins was very specific as to why the AVC time-constant is relatively short and that was for quick receiver recovery when going from transmit back to receive. In a commercial-military set-up with break-in being used, quick exchanges would have been the norm and Collins anticipated that with the AVC time-constant they selected.

The second is the standard diode detector used in the stock receiver which many users want to replace with a Product Detector. The one 51J-4 that I used with the Product Detector mod (and AVC mod) still distorted SSB signals if the RF Gain was advanced too far. I thought the receiver functioned much better after I removed these two mods.    >>>

>>>  The usual "ham mod" caveats should be seriously considered before actually corrupting the original receiver circuitry. When the receiver is functioning correctly and in good alignment, the AVC does work fine and so does the diode detector. However, Collins designed the receiver in the late forties, long before SSB transmissions became the standard voice communications mode. When operated as a typical late-forties communications receiver, no serious problems will be encountered with the 51J Series receiver. This means that you will have to reduce the RF Gain when receiving SSB or CW signals. If signal quality is not an issue, the AVC can be left on but, in either case (AVC on or off,) the RF Gain will have to be reduced to about 8 or less for minimum distortion of a typical SSB signal. Of course, signal strength will affect the RF Gain setting for best SSB demodulation. The AVC, the BFO injection and the diode detector circuits are the primary reason that input signal level, determined by the RF Gain, must be reduced so the proper ratio of BFO injection to signal level will result in good SSB demodulation. Additionally, when receiving CW signals with an older receiver, excessive BFO injection into the detector can "mask" very weak CW signals. Maximum sensitivity in CW was always achieved by "riding" the RF Gain control with the receiver out of AVC and the Audio Gain near fully advanced. Additionally, the Carrier Level meter will no longer be useable for relative signal strength measurements when the AVC is in the off position.
More on Mods - The most common modification is to replace the 6BA6 BFO tube with a 6BE6 mixer tube and create a Product Detector for operation with the BFO on - that is SSB and CW signal reception. This is a fairly simple modification that if done carefully will function fine for SSB or CW but it can be relatively easy to reverse and put the receiver back to stock, if that's desired. I had a 51J-4 receiver that had this modification and its performance wasn't very good. I felt the receiver still distorted the SSB signal with the RF Gain at maximum (the object of the mod.) I ended up removing the mod and returning the receiver to stock configuration with much better performance operating the receiver as a typical "late-forties" communications receiver. Additionally, the 51J-4 had the AVC mod that adds capacitance to increase the release time. This was also returned to stock configuration. The person who installed the mods did an excellent job with no damage to the circuit components that remained. I'm fairly sure the mod was working as intended because it did change how SSB and CW could be demodulated but "improvement" would be a subjective judgment. Once I had the mods removed, as a stock 51J-4, the receiver was one of my favorites but I unfortunately had to sell it in 2002. Subsequently, in 2012, I was able to purchase another 51J-4 receiver. This one was stock and had never been modified in its past. It's performance was spectacular with plenty of sensitivity and, with mechanical filters AND a crystal filter, QRM was not an issue. After a complete IF/RF alignment it became one of my favorite receivers, even though decent audio quality for AM reproduction required tuning just one sideband plus carrier. But, once again, I was tempted by an irresistible trade and that receiver also departed. Now, I have a Collins' lab, light-gray panel 51J-4 sn:4723, so we'll see how long this one stays around.

Certainly how you intend to use your 51J Series receiver will determine your interest in any of the published mods. Remember that most ham modifications will enhance one area of performance at the expense of another. The product detector mods may work fine for SSB or CW and the AVC mods might allow for better SSB response but learning how to operate the stock receiver in the manner in which it was designed will also give you great performance in all modes of reception. Also, check "Collins' Audio Fix" in the "51J-4 Receiver Performance Evaluation" section towards the end of this web-article. But, if you want the best in SSB reproduction how about TMC's MSR-8 SSB Adapter  >>>

The Technical Materiel Corp. - MSR-6 SSB Adapter

Technical Materiel Corp. - MSR-8 - SSB Adapter for the 51-J - Technical Materiel Corporation made several variations of their popular CV-591 SSB Adapter. Nine production versions were given the prefix designation of MSR (Mode Selector - Receiving.)

Only one of the MSR SSB Adapters was set-up to work with a 500kc IF output like the 51J receivers have. That model is the MSR-8.

There were nine MSR models. Two were for 200kc IF, one for 500kc and the remaining six versions all operated on 455kc IF outputs. The MSR-6 shown above is for 455kc. All of the MSR versions are very similar in design and appearance to the CV-591. Any of the TMC MSR SSB adapters are difficult to find since they weren't on military contracts. The MSR-8 is probably the most difficult to find.  

My MSR-6 came set-up with a GPR-90RXD receiver and both units have matching serial numbers.

A Simple Mod for the 51J-1& J-2 - There is one very simple modification that does improve the upper audio frequency roll off. Across the primary of the audio output transformer a .01uf capacitor is installed to keep the high frequency transients down which also reduces the high frequency audio response to about 2500Hz. That capacitor can be replaced with a .0047uf cap to increase the audio frequency response and still protect the primary from transients. With the .0047uf capacitor installed, you'll find the 51J-1 or J-2 receiver will sound a little more like a standard communications receiver, that is, not high fidelity but not as "muffled" as it did before.

NOTE: The R-388, the 51J-3 and 51J-4 all use a 6800pf (.0068uf) capacitor in this application and don't really benefit from any change to a smaller value capacitor.

The Alignment Tools - Most of the time these two Garolite tools are missing from the R-388 and the 51J-4 receivers, which is unfortunate. They work really well, especially the tool for adjusting the ceramic trimmer capacitors. The tool for aligning the IF transformers also is very helpful. At one time, these tools were being reproduced very accurately and were easily available. However, a recent search (2023) on the Internet didn't find anyone selling these reproductions anymore. They may show up on eBay sometimes but I've haven't really seen that happen either. If you find a set, buy them. They are worth having. Unfortunately, I only have one set. I should have bought a couple of more sets when they were available.

Rubber Feet for Collins A-line Cabinets - If your Collins cabinet has really worn or missing feet don't replace them with the typical "hole-mounted" rubber feet. The correct rubber feet are actually rubber bumpers. The originals were 1" in diameter and 0.5" tall with about a 1" long 1/4" x 20 threaded stud secured with an external tooth washer and 1/4" x 20 nut. McMaster-Carr carries rubber bumpers listed as "Threaded Stud Bumpers" but their 1/4" x 20 studs are too short. However, the mounting hole in the Collins cabinets is 5/16" diameter and McMaster-Carr does carry a 1" x .5" rubber bumper with a 5/16" x 18 stud 1" long that will work fine.

Reducing "High Line" AC using a Line Bucking Transformer - The 51J Series of receivers were designed to run on 115vac input voltage. Tube heater voltages and the non-regulated B+ voltage is dependent on 115vac being the line voltage used. Today, line voltages are nearly always minimum 120vac and often run up to nearly 125vac. Short term operation, such as testing or alignment, isn't usually affected by "high line" operation. Long term operation however will affect tube life and the higher non-regulated B+ can cause an increase in overall heat generated. It's very easy to add an external Line Bucking Transformer to lower the "high line" down closer to the specified input AC voltage. Using a 6.3vac filament transformer (that was designed for 115vac primary voltage) will lower the AC line by about 7vac to 8vac, e.g., lower a 124vac line down to 116-117vac. Our AC line here in Dayton is close to 124vac (123.8vac measured 7/29/22) and Line Bucking with a 6.3vac filament transformer lowers the AC line to 116vac. For receivers I use 6.3vac rated at about 3A minimum transformers and for medium power transmitters I use 6.3vac at about 8A minimum. It's not critical since the VA is only for the small voltage drop, not the entire VA the equipment requires. You can always use a larger current-rated filament transformer, especially if you want to use it to supply 115vac to a power strip, just be sure to have a primary switch on the bucking transformer. There are lots of easy hook-ups shown on the Internet (search on "line bucking transformer.") Of course, an autotransformer (Variac or Powerstat) could be used just as easily but filament transformers are plentiful and cheap. Variacs are also plentiful, just usually not cheap. Besides, the Variac belongs on the test bench.

Easy Access to the Crystal Filter - How many times have you wanted to get into the Crystal Filter assembly on the 51J receiver but were intimidated by its lack of obvious and easily removable covers? Well, Jan Wrangel SM5MRQ, decided to get into his 51J-4 crystal filter and took photos to document just how easy it is to gain access to the Crystal Filter components. If the receiver is in a cabinet it has to be extracted out of the cabinet. Then remove the top cover and the bottom cover. From the top of the Crystal Filter assembly a single screw has to be removed. From the bottom, the choke nearest the front of the chassis has to be dismounted (just unbolt the choke - no need to unsolder the wires) to allow access to a 6-32 nut that is underneath the choke (between silk-screened IDs T101 and T102.) This nut mounts the bottom of the Crystal Filter top plate. Once the nut is removed now the top cover of the Crystal Filter can be removed from the top. This allows access to most of the circuitry inside. Reassemble in reverse order when checking, repairs or cleaning have been completed. If further access is required such as removal of the IF transformers inside, then more disassembly will be required. Complete extraction of the Crystal Filter assembly will require some unsoldering work. This procedure is only for simple checking, cleaning and minor repair work to the Crystal Filter. 

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

70E-15 PTO - Reworking the Problems

Excessive End-Point Error in the 70E-15 PTO - The 70E-15 PTO tunes from 3.0mc to 2.0mc in ten turns. There is a trimmer inductance (L002) provided to adjust the end-point error. The end-points on nearly every R-388 PTO are excessive and beyond the range of the trimmer inductor. Usually, if the EPE is more than 6.0kc, it will be out of the range of the adjustment. All 70E-15 PTO end-point errors (EPE) are similar in that the tuned range, which should be exactly 1.000mc change in exactly ten turns of the PTO, has decreased. I've never encountered a 70E-15 PTO where the EPE actually has an increased range. The EPE issues are supposedly related to the questionable quality of the ferrite slug material used in the 70E-15. However, I think the problem is a combination of the poor ferrite quality combined with continuous operation with the resulting heat. This opinion is based on the acquisition of a NOS 70E-15 PTO. This PTO had virtually no EPE, probably because it had spent the last 50 years stored in a box and had never been subjected to the rigors of continuous operation. However, it's also common to find the 51J-4 receivers with virtually no EPE in their PTO and, of course, 51J-4 receivers post-date most of the R-388 receivers. Maybe the ferrite problems were corrected in the later 70E-15 PTOs and the NOS example was from this later manufacture. Once in a while 51J-4s will be found with excessive EPE but these were probably in an environment that required continuous operation, or they might be very early models or maybe a replacement PTO was installed that was from an earlier receiver. If your 51J-4 or R-388 has excessive EPE, that is >6kc EPE, correction will require removing one coil turn from the internal PTO trimmer coil (L002.) This requires disassembly of the PTO. When modifying the trimmer coil, be sure to only remove one turn. If more than one turn removal seems necessary in order to get the EPE in spec, you can perhaps remove another half of a turn - but no more. Excessive turns removal will reduce the span of adjustability of L002 to the point where it doesn't affect the EPE at all. Bill Orr wrote extensively about the R-388 and correcting the 70E-15 PTO EPE problems in the form of an article in CQ magazine in the December 1969 issue. This detailed article should be read before attempting to rework your first 70E-15 PTO. Orr's article can be found online in PDF form on the Collins Collector Association website -
The PTO Test Jig for the 70E-15 - The earlier PTO, the 70E-7A used in the 51J-1 and J-2, has the end-point adjustment inductor easily accessible on the top of the PTO can. It is rare that an early PTO will have end-point errors that are not within the range of this adjustment. The same cannot be said for the 70E-15 PTO. This later PTO has chronic end-point error problems that are difficult to correct. Additionally, the end point adjustment is in front of the PTO and is not accessible unless the PTO is removed from the receiver. The end-point adjustment is behind a hex-head plug that has to be removed and then there is a locking nut that also has to be loosened before the trimmer inductor adjustment can be moved. There are some restorers that have built special right-angle tools for accessing the locking nut and the trimmer inductor but these do require a lot patience to use. Generally, the PTO has to be removed from the receiver anyway for modification of the trimmer inductor for end-point correction. I find it easier to just remove the PTO, do the rework and do the entire EPE adjustment on the bench with a simple test jig. 

The test jig should be simple and easy to make. Orr's CQ article shows his fixture using the KC dial but you don't really need to be that elaborate. Also, using the KC dial covers the access to L002, so the PTO has to be dismounted from the fixture for adjustment and then remounted for testing. Orr may have used the special angled EPE tool but these are difficult to use if you can't see the slot of L002. A fixed reference index line and a very narrow pointer is really all that is necessary. A very narrow pointer is attached to the PTO shaft. It can be made out of solid TC wire. A fixed, small, transparent plastic index scale that has a scribed "zero" line is mounted to the PTO case. Be sure mounting your small plastic index doesn't cover access to L002 EPE adjustment. Mark several other lines on each side of "zero" to allow you to see if the end-point error adjustment is proceeding in the right direction. You'll have to count the ten turns but that's not difficult. How accurate you observe your pointer and the index zero line during the adjustments will determine your overall accuracy when the PTO is installed back in the receiver. You just need to determine a 1000kc change in frequency for exactly ten revolutions of the PTO shaft and a simple pointer and index does that. The PTO should tune exactly 3.000mc to 2.000mc in exactly ten turns of the PTO shaft.   >>>

>>>  I use a bench power supply connected to the PTO wires with clip leads. A digital frequency counter monitors the PTO output. In this way, the PTO can be completely removed from the receiver. It makes any rework (like modifying the trimmer inductor) much easier since you don't have to have the harness connected to the receiver. When measuring the PTO output frequency for EPE adjustment be sure to have the cover in place as its proximity to the PTO circuitry greatly affects the output frequency. Also, don't worry about the seals for the PTO cover. At the factory, when a PTO was completed, that is EPE adjusted and it was "ready to go," the PTO was heated, then, while still hot, the EPE plug was installed. As the PTO cooled, it formed a slight vacuum inside which protected the components. But, the first time someone decided to adjust the EPE, air leaked in and the vacuum was lost (that's why Collins didn't want anyone working on their PTOs since, at Collins, after any adjustment or rework, the PTO again went through the process to create the slight vacuum inside.) Nowadays, nobody worries about the vacuum loss in the PTO because we all take excellent care of our receivers and the environment they are kept in is usually very clean and temperature-controlled,...isn't it? 

If all you need to do is to adjust the EPE, it's fairly easy to dismount the PTO but leave the PTO wiring connected to the receiver to supply the voltages. Attach a shielded test cable to the PTO output coax and connect that to a digital frequency counter. The simple test jig fixed index should be mounted to the PTO and the pointer to the PTO shaft. Not too difficult and relatively fast if all you need to do is an EPE adjustment.


Rebuilding the Collins 51J Series Receivers


Rebuilding a 51J-2 Receiver

Origin of the Primary 51J-2 - I was initially given a cosmetically very nice 51J-2 by my old ham friend, W7ZCA, Paul Eisenbarth (now SK,) in exchange for re-capping his Collins 75A-4 receiver. Paul had been given the 51J-2 at sometime in the past and had done some rework on it. At one point, Paul had the receiver setting on the floor of his workshop with the bottom RF shield off, exposing the ten crystals of the Crystal Oscillator circuit. Paul's young grandson, who was probably about 4 years old then, found the temptation of the small shiny and removable crystals irresistible and pulled all of them out of the holder and hid them around Paul's workshop. This removal and hiding was all accomplished while Paul was off in another room. In fact, it was several days before Paul even noticed that the crystals were missing. Of course that was enough time for his grandson to forget where he had hidden all ten crystals. Paul was able to find a few of the crystals but seven were still missing. A telephone call to International Crystal Manufacturing Company had surprising results. After Paul told the sales person the story of how the crystals came to be missing, they found the story so charming that they sent an entire set of ten crystals for the 51J-2 to Paul - free of charge. Note: International Crystal Mfg is no longer in business, 2022.

The crystals got the 51J-2 working again but there were other problems that seemed unfixable. The gearbox had severe wear that allowed the Megacycle control to be advanced maybe three or four bands before the gearbox would bind and jam up. You then had to rotate the Megacycle control back a band or two to undo the binding and then proceed forward three or four more bands. It was a cumbersome method but it did allow eventually getting to the desired band. Additionally, the Antenna Coil primary on Band 2 was open due to an excessive amount of RF accidentally injected into the receiver while operating on 160 meters. Paul had obtained another Antenna Coil but it was from an R-388 receiver (which doesn't have the primary winding on the coil.) 

So, that was the condition of the 51J-2 receiver when it was given to me. I tried to use the receiver with the worn gearbox but it seemed like sooner or later (probably sooner) it was going to break something in the gear train. I decided to look for a parts set to rob the gearbox from to correct the problem.

photo above: The 1950 Collins 51J-2 showing the metal dial bezel, the square illuminated Carrier Level meter, the green "ham band" highlighting on the megacycle dial scale and the Collins' winged emblem - all characteristics of the early 51J receivers.

Finding a Suitable Parts Set - After a few months, a suitable 51J-2 showed up on eBay. Nobody seemed interested in it since it was in fairly rough cosmetic condition. The operational condition was described as a "fun radio" - whatever that meant. So, two hundred dollars later I was the owner of another 51J-2 receiver. When the second 51J-2 arrived, I found out what a "fun radio" was - totally and incompetently reworked beside being non-functional with a repainted front panel in black paint with white rub-on lettering. I swear that the eBay photos were of a different front panel (probably a different receiver) but by that time the seller had pulled the auction photos. It really didn't matter because all I wanted was the gearbox and that was in good shape.

The plan was to use two 51J-2 receivers to build up one nice condition, fully functional receiver. In checking over the two receivers, it seemed easier to use the chassis of the second receiver as the starting point and rebuild the entire receiver using the best parts from both units. This allowed me to skip the tedious removal of the gearbox which certainly would have resulted in many synchronizing problems.   >>>

>>>   The second receiver had been incompetently reworked and several of the front end coils had broken wires that were their connections to the trimmer capacitors. Also, all of the high quality tubular ceramic capacitors had been replaced with cheap disk caps. I decided to strip out the receiver from the front of the IF section back to the audio output and start over. I reinstalled the high-quality tubular ceramic capacitors harvested from the first 51J-2. I rebuilt the entire front-end of the receiver to repair the many broken coil leads. I had to replace the third conversion input coil because it looked like it had been burned. All parts used were either new parts or good ones harvested from the first receiver.

Once the electronic rework was complete, the front panel, the knobs, the megacycle drum dial, the meter and many other parts were transferred from the first 51J-2 to the new rebuilt receiver. I ended up with a great looking, complete 51J-2 - but how did it work?

Troubleshooting - Upon power-up the new rebuilt 51J-2 seemed to be working fine. I performed an alignment and the receiver worked fine - or did it? After about 30 minutes of operation the sensitivity dropped down to where signals barely moved the carrier level meter. From the start, the audio would distort if the RF Gain control was higher than 8 when in AVC. Clearly, there were a few more bugs to work out before the receiver could be called "complete and working." However, other projects came into the shop and the 51J-2 was put on the shelf and temporarily forgotten.

In fact, "temporary" ended up being about three years before I got back to the receiver. I had recently been told that Paul W7ZCA had become an SK and that had me reflecting about all of the "deals" we had exchanged over the years. I remembered the 51J-2 and thought that I should probably finish that receiver since it was such a nice looking example of a fairly rare Collins.

The strangest problem was the erratic variable gain of the receiver. When on the bench in the normal position the gain was down but when the receiver was placed up on its side to troubleshoot the gain would then be somewhat normal. I could even tilt the receiver about 20 degrees and get the gain to go up and down with just a slight change of the angle - weird. >>>

>>>To narrow the problem down I used a clip lead type scope probe and measured at various points in the receiver while tilting the receiver back and forth on the bench. I had to inject a fixed-level signal into the receiver's antenna input so I knew the variations in the gain were occurring because of a fault and not the signal level changing. I could see the gain change problem was occurring just past the first IF amplifier tube. All components and the tubes had already either been checked or replaced except the IF transformer itself. I removed the IF transformer cover and discovered that the coil and the ferrite shields were at the bottom of the mount and loose. Apparently, moving the receiver around changed the position of the IF coil which changed the coupling and resultant gain. Upon removing all of the IF transformer covers, it was discovered that every IF transformer had loose coils, loose ferrite shields or both. Complete removal and rebuilding of all of the IF transformers was necessary. I used epoxy to secure the ferrite shields in position. I actually used the IF transformers from the first 51J-2 because they were in better overall condition but still required securing of the ferrite shields to prevent future problems. Upon power-up, I had more gain through the 51J-2 than ever before and an IF alignment only improved the response.

photo above: A 51J-2 IF Transformer showing the coil structure beneath the ferrite shields. Note that this transformer had the mica caps replaced but nothing was done to secure the ferrite shields.

photo above: The 51J-2 after rebuilding

Although I thought that the IF transformer problem solved everything that was wrong with the 51J-2, I soon discovered that the AVC would cause distortion after several minutes of operation. I had already replaced all of the tubes in the receiver with a set that checked fine on the TV-7 tube tester. However, you can't find all tube problems with a tube tester. Once the AVC started to cause the distortion, I replaced the AVC amplifier tube with no change in the problem but changing the Dectector/AVC Rectifier tube cleared up the problem. I had already rebuilt that entire section of the receiver so I was sure the problem had to be tube related.

An additional problem was only on the AM BC band or Band 1. Low gain was the result of a defective mixer coil that appeared to have gotten very hot. Fortunately, the first 51J-2 receiver provided a good condition coil. These coils are easy to replace as they are only held in place by two tangs that have to be pushed inward to dismount a coil and "snap" into place when installing the coil into the mounting hole. The wire leads are the same diameter as the coil wire itself and the connections to the trimmer capacitors is fairly delicate so care must be taken in the removal and installation.

A full IF/RF alignment had already been performed early on the receiver and a quick touch-up was all that was necessary for top performance that is at or better than the original specs. Audio is fairly nice as I did replace the .01uf across the audio output transformer with a .0047uf to increase the high frequency audio response. I usually run the Collins 270G-1 loudspeaker with this 51J-2 and that gives just a little bass discernable on strong SW BC stations or AM BC stations. Reception of ham AM stations benefit with the increased audio highs with better intelligibility. But, the 51J-2's audio is still communications-grade, not high fidelity. 

Photographic Details on the 51J-2

photo left: The top of the chassis of a 51J-2 receiver showing the earlier 70E-7A PTO. Under the metal shield on the PTO is a metal octal 6SJ7 tube. In front of the PTO tube is the glass holder for the desiccant that protects the PTO from moisture ingression. Otherwise the receiver is very similar to the later 51J-3 and R-388 receivers.     Photo above: Close-up of the 70E-7A PTO

photo above: The back of the 51J-2 showing the unique slanted top cover that is used on all of the 51J Series receivers. The top cover must be installed even if the receiver is installed into a cabinet. The top cover shielding eliminates the spurious oscillations that will be tuned at various frequencies throughout the tuning range ("birdies.") Note the ID plate - this is NOT standard. The ID information is typically silk-screened to the rear of the chassis. This particular 51J-2 had holes drilled thru the information so a suitable "data plate" was made from a derelict "parts set" 51J-2 chassis. 

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

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


Rebuilding the R-388/URR Receiver

The R-388 is the most commonly encountered version of the 51J series of receivers. About 12,000 were built for the military over a period from 1950 up to 1962. As a result, parts are fairly easy to find which makes a rebuild easier to accomplish. However, just because there are lots of spare parts and parts sets around doesn't mean that the parts needed will be cheap - they aren't. Mainly because of the "Collins" name but also because any parts taken from any R-388 almost relegates that receiver to "not restorable" status which can be a difficult decision for the owner to make (but not a dealer.) Most e-Bay dealer-sellers seem to know that no matter how bad the receiver's condition is, if it has "Collins" on it, it will sell,...even if it isn't priced as a "parts set." Nowadays it's difficult to find "parts sets" anymore - everything can be restored (at least, according to the sellers.)

The following "basket case" is not typical of that status since most of the parts were present. It was just that the receiver was mostly disassembled with no intention of future reassembly.

The Basket Case - A friend of mine had the luck to find three R-388 receivers locally. They had been listed on Craig's List. His telephone call to me was mainly for advice on how to "power-up" the three receivers. I told him that typically R-388s will pretty much work "as found" but it would be a good idea to thoroughly check the filter capacitor and to test all of the tubes before applying AC. This was accomplished but he found that none of the receivers seemed to work. I was asked if I would "take a look" at them. We made a deal that I would service and align the two best receivers in trade for the worst of the bunch which, by this time, had been disassembled for parts needed in the other two receivers.

I delivered the two serviced and aligned R-388s and picked up the "parts set" R-388 while at a ham swap meet in Reno. What I got was the R-388 chassis, the front panel and a box of parts. The gearbox and the main band switch had become de-synchronized during some of the disassembly. Also, about half of the tubes were missing as were nearly all of the screws. I was able to have the screws sent to me through the mail. I had plenty of tubes so that wasn't a problem. Really, all that was necessary was to put the R-388 back together and go from there.

The front panel was in exceptional condition and had been cleaned by the former owner. The kilocycle glass was missing but I had a glass piece from a "parts set" 51J-2 that fit correctly. When I purchased a "parts set" 51J-2 to complete the rebuild of another 51J-2 receiver I was lucky enough that the "parts set" 51J-2 that had an almost perfect R-388 megacycle drum installed. Fortunately, the original 51J-2 megacycle drum was excellent so I really didn't need the R-388 one that came with the "part set." It was so nice though I saved it and wrapped it up in plastic to protect it. So, here it was a few years later and I was rebuilding an R-388 that had a very well-worn megacycle drum. Obviously, that "saved" megacycle drum had "saved the day."

photo above: The R-388 chassis after restoration. Note the megacycle drum - this is an original drum scale, not an overlay. Note the plastic bag piece on the Crystal Filter housing. These bags usually contained some spare parts, connectors, etc. The alignment tools are reproductions.

Synchronizing the Band Switch to the Gear Box - This is really much easier than it sounds. Since the gear box has mechanical stops at each end all that is necessary is to know which way the Megacycle knob is rotated to change bands in the proper direction. Since counterclockwise rotation increases the frequency, rotation clockwise to the end stop would be the .5 to 1.5 mc band or Band 1. Once Band 1 is in position then a check of S-106 can be visually accomplished to see where the arm of the switch is. By checking the schematic versus looking at S-106 it was determined that the switch was actually on Band 2 and all that was necessary was to loosen the set screws on the coupler from the gear box to the band switch and then rotate the switch shaft until Band 1 was in position and then tighten the coupler set screws. Rotating through all thirty band positions did verify that the switch rotated from position 1 to position 16 and then from Band 17 to Band 30 the switch shaft did not change position. Then when returning from Band 30 down to Band 17 no change in the switch shaft position but with Band 16 on down to Band 1 the switch shaft did rotate correctly and stop in position one. While doing these tests all of the other movements of the slugs and slug racks were verified and no problems were noted.

Dial Drum Drive Cable and MC Dial Pointer Cable - These are special metal cables that have a plastic covering over them. Luckily, both cables were present but were loose in the box of parts. I think the cable material is available from various Collins suppliers and the dimensions and installation instructions are in the manual if your R-388 is missing these cables.

A Different Carrier Level Meter - Most R-388 receivers will have a Burlington sealed carrier level meter. These high quality meters have a metal housing and metal scale. The meter installed in this R-388 was built by Marion Electric Instrument Company. Probably a field replacement. It has a convex face with gloss finish mounting flange with a sealed metal housing.

Repro ID Tag - As can be seen, this receiver originally had two data plates installed that had been removed in the past. The center tag was the serial number identification plate and the tag to the right was a "Caution" tag that informed the user to refer to the TM manual. Luckily, there are a few repro tags available from Mike Chanter (Collins Radio Association.) He supplies a 1950, 1953-54 and 1957 type tags. Since this R-388 was probably built on a 1951 contract, I should have gotten the 1954 tag but instead I bought the 1950 version. The thinking was that the MFP date of  "JAN 52" would have to be from an earlier build, thus the 1950 tag. However, knowing that Collins built R-388 receivers an put them into inventory for later contract sales, maybe the 1953-54 tag would have been a better choice. Knobs and Tubes - Since this R-388 was a "basket case" it naturally didn't include the knobs or many of the tubes. Luckily, I had a complete set of Daka-Ware knobs that had come with the "parts set" 51J-2 receiver. I did have to search through the junk box to find a set of Daka-Ware kilocycle and megacycle skirted knobs since those weren't on the 51J-2 "parts set." Most of the tubes missing were the 6BA6 tubes. In fact, the R-388 uses seven 6BA6 tubes. Also, missing were the two 6AK5 tubes, the three 6BE6 tubes, the 6AQ5 and the 5V4 rectifier. All tubes were found in the tube "junk" boxes. All tubes used in the R-388 should test well above "minimum acceptable" for best performance.
Restoring the Dial Bezel - The dial bezel had been broken at one time and glued together to repair. The break was at the bottom of the bezel - an area hidden by the skirt of the main tuning dial. There was also a chip in the opening for the kilocycle dial glass. I decided to use an epoxy fill to repair the break because there were some missing sections. Tape dams were used and epoxy was layered to complete the fills. After the epoxy had set-up the areas were leveled by filing and then polished. Usually I would have used black power coloring for the epoxy but I didn't have any around so I carefully applied jet black nitrocellulose lacquer and polished this to match the bezel bakelite. To finish the bezel required filling the engraving "MEGACYCLES" and "KILOCYCLES" along with the "tick marks" for the kilocycle index. I used Artist's Acrylic mixed to a manila color and applied with a brush. The fill paint is left to set up for one minute and then a paper towel piece dampened with Glass Plus is used to remove the excess fill paint. The end result was a bezel that looked great and had repairs that were very difficult to see. (see the Update for July 17, 2016 at the end of this section regarding an original replacement bezel for this receiver.)

New Power Cord - The original power cord had been cut on this R-388/URR. The plastic strain relief was broken when trying to remove it. Fortunately, most larger hardware stores carry the correct type of power cable and also the same style of plastic strain relief.

photo left: The R-388 immediately after restoration. Those aren't the correct grab handles. The meter is a field replacement. See below for updated photo of this receiver

IF and RF Alignment - The alignment procedure in the TM has a few steps that are outdated. This is because today we have easy access to digital frequency counters that are extremely accurate. When the TM directs you to use the R-388's Crystal Calibrator to "beat" against the signal generator's 500kc input to be sure that it is accurate, it's because back in 1950 that was the most accurate way to assure that 500kc was the input signal. Today's digital frequency counter (or synthesized signal generator) is just as accurate and much easier to use. I like to check the Crystal Filter's crystal frequency just to be sure. Usually that crystal is within 0.01kc or so of 500kc but I use that frequency of the crystal as the IF. This assures that the Crystal Filter works great (and they will - if you take care in the IF alignment.) The TM directs you to "de-tune" the IF transformers with a series RC load consisting of a .01uf capacitor and a 4.7K resistor (a simple shunt-LP filter.) The instructions are very specific as to where to connect the RC load for each IF transformer adjustment.*  Since the load is to chassis-ground, only one end has to be moved each time and using short clip-lead connections works fine. Though the special alignment tools (that are always missing) will help with the alignment they are not strictly necessary. Reproduction tools used to be available and if you can find a set they are worth having. I think the original type alignment tools make the trimmer caps easier to adjust but not the slugs. The IF transformers are also easier adjust with the proper alignment tool.

* I believe the RC load is to isolate the 500kc IF and prevent it from mixing or interacting with the crystal oscillator or the PTO during adjustment since the bottom cover, thus the normal shielding, has to be removed to adjust the bottom slug in the each of the IF transformers. Many IF alignment procedures of single conversion superheterodynes instructed the technician to remove the Local Oscillator tube when performing the IF alignment for the same reason.

Variable IF and RF Alignment - Signal is injected into the Antenna Input SO-239 connector. Align the Variable IF sections first. There are two sections, odd and even, requiring different frequency inputs. Alignment is standard in that the trimmer caps adjust the high end of the range and the slugs (inductance) adjusts the lower end.

RF tracking is straight forward but you will only have to adjust six ranges as the remainder of the bands are tracked by oscillator heterodyne action.

Mechanical set-up of the kilocycle dial and the PTO should have been performed before the tracking alignments. All 70E-15 PTOs will have some end-point error unless they are removed and calibrated. If you are satisfied with the EPE and it happens to be a few kilocycles then be sure to "rock" the signal generator frequency at each calibration dial frequency rather than changing the actual dial readout. For example, if you have to align to 7.40kc but the actual peak output is on 7.402kc, then use 7.402kc on the signal generator and 7.40kc input on the receiver and then adjust the proper trimmers or slugs for peak output. Always set the receiver frequency and then "rock" the signal generator for peak diode load, then adjust the slug or trimmer. The more end-point error there is in the PTO the further out this procedure becomes but for just a couple of kilocycles of error it works fine. The correct procedure would be to rework the PTO for zero error and then all of the tracking will be accurate. Remember that the tracking accuracy is in the PTO and the crystals in the crystal oscillator. The slugs merely align the tracking of the Ant, RF, Mixers and the Variable IF and affect the overall gain of the receiver.

R-388/URR - Expected Performance and Operation

I've worked on many R-388s and this of course includes many full alignments. Probably the most important improvement that can be performed on the R-388 receiver is to do a full alignment. It's surprising how far out the alignment can be and the receiver still seems to be performing fine. So, this evaluation of R-388 performance is for receivers that are in good condition and have had a recent full IF and RF alignment.

The R-388 is probably the overall best performer of the entire 51J Series for vintage AM ham stations. This is because it doesn't have the mechanical filters that were installed in the 51J-4 receiver. Though the MFs are great for SSB and CW, the 6.0kc filter is not particularly ideal for AM reception. This is because Collins' engineers were thinking "communications" (and primarily data communications) and not wide-bandwidth audio reproduction. The R-388 relies on the designed-in bandwidth of the fixed IF section of the receiver which is probably around 8.0kc at 3db down or perhaps a bit wider. However, the IF passband curve is a little more bell-shaped rather than the steep slopes and flat tops of the Collins' mechanical filters. So, the first "plus" is the IF bandwidth of the R-388 for AM reception. The audio output 6AQ5 does have a capacitor from plate to B+ for impedance matching which on the earlier J-1 and J-2 receivers seriously limited the upper audio response but the R-388 changed the value of this capacitor to 6800pf which doesn't affect the audio response. So, you should expect your R-388 to sound just a little limited in the upper end of the audio range but most listeners find this audio to be "pleasingly mellow."

Dial accuracy is legendary and is plus or minus one kilocycle if the receiver is calibrated to the nearest 100kc point on the particular band in use. Ultimate accuracy is limited by the 70E-15 PTO which is also legendary for its end-point error problems. Most R-388s will have around 6kc of EPE which is about the limit that can be adjusted out. Greater EPE requires PTO rework to correct. When the EPE is adjusted out, the receiver is dead-on accurate. Unfortunately, the 70E-15 PTO must be out of the receiver to perform the EPE adjustments. There are some special tools that have been built by enthusiasts that allow the adjustments to be accomplished without PTO removal but they are difficult to use. Most of the time, if there's the typical >6kc EPE, the PTO has to come out of the receiver anyway.

Stability is solid and drift-free. Sensitivity is competitive. Are there more sensitive receivers? Certainly, but with today's high-level of RFI and EMI that plague most urban areas (and seems to ever be on the increase, even in rural areas) what good is .25uv sensitivity when the background noise runs S-9? Far more important today is the ability of a receiver to operate well in a noisy environment and to have QRM-fighting tools that work. The R-388's selectivity is designed into the conversion schemes and the fixed-IF section but is further enhanced by the use of the Crystal Filter. Why many hams refuse to use Crystal Filters is a mystery. CFs really do a good job at reducing QRM. On AM, select position 1 or 2 and adjust the Phasing for minimum bandwidth. Then tune the AM signal "on the noise." You'll find most of the interference is gone or greatly reduced. The AM audio will sound "muffled" but it will still be intelligible enough for communications. If adjacent frequency SSB activity is a problem then adjust the Phasing slowly and you'll find a point where the dominant audio frequency of the SSB QRM is attenuated substantially. Use the same procedure for SSB reception. For CW, with the CF on you can tune the CW signal off of zero slightly to find the "peak" response. The peak response can also be adjusted with the Phasing control and slight tuning of the signal. The Phasing control can be used to reduce specific audio tone frequencies, that is, to eliminate AM heterodynes. Heterodynes were quite common when there were a lot of AM signals on the ham bands but, nowadays, most AM activity is on nets and one doesn't usually encounter heterodynes too often. On CW, the CF can reduce interference from a nearby CW signal by this heterodyne elimination method. When trying to eliminate CW QRM or an AM heterodyne adjust the Phasing control slowly. The "notch" is very narrow and it can be easy to miss its effect on the interference if the Phasing is adjusted carelessly.

Break-in Set-up and Operation - Set-up for station receiver operation is somewhat involved since the Break-in function does require a separate +12vdc 200mA power supply that is switched with the transmitter's antenna relay auxiliary contacts. I built a solid state +12vdc power supply housed in a very small metal box. It can be switched on by connecting the AC input to the same transmitter terminals that drive the dow-key relay. Upon transmit, the +12vdc is actuated along with the dow-key and the receiver is muted. Or, if the transmitter has a built-in antenna relay with only auxiliary contacts available (like the ART-13) then the +12vdc power supply's AC input can be connected to the 120vac line and the +12vdc output connected in series to the auxiliary NO contacts of the transmitter relay and to the receiver Break-in. Closure of the contacts connects +12vdc to the receiver to actuate the Break-in relay. When using a Break-in power supply connected in this manner, some method should be set up to allow the 120vac to the small power supply to be switched off when the station is not in use. Powering the AC supply along with the other station equipment from an AC power strip that has a ON-OFF switch is probably the easiest method. It's probably even easier to just use a +12vdc 250mA "wall-wart" type of power supply for the Break-in rather than homebrewing a small power supply. It's probably best to avoid the new small "charger" types and look for an older wall-wart designed to operate a 12vdc device. The newer types tend to be RFI noisy. But, in either case, remember to always provide the Break-in function for the R-388 as this disconnects the antenna within the receiver, grounds the receiver input to protect circuitry from excessive RF levels. Also, the Break-in connection at the rear of the receiver that is marked REMOTE is the ONLY place that you can connect the +12vdc and operate the Break-in relay (it's not actuated if the front panel STAND BY is selected.) There are three terminals provided for the REMOTE connection. Pin 1 is ground. Pins 2 and 3 are the connections to the K101 coil for actuating the Break-in relay.

There are a couple of methods that can be used to provide the +12vdc Break-in voltage internal to the receiver. One method is to incorporate the small DC power supply inside the receiver chassis. The very small current requirements will allow very small components that easy can be placed under the chassis. It's also possible to incorporate a "voltage doubler circuit" that operates from the 6vac tube heater supply to supply the +12vdc. This only requires a few components and no transformer, so it's even easier to place under the chassis. Wiring in either case can be installed to allow a NC on transmit applied to REMOTE pins 2 and 3 will apply +12vdc to K101.

Speakers & Antennae - When comparing the two audio output impedances available, better audio is usually obtained from the 4Z ohm output with a matching Z speaker. The 600Z ohm output was originally for driving data devices and requires a matching transformer to operate a standard Z speaker. I've always thought the 600Z line had a "roughness" to the audio that isn't present on the 4Z output. If you have the Collins 270G-1 (8" speaker) or 270G-2 or 3 (10" speaker,) either of these are fine 4Z loudspeakers to use with the R-388 or any of the 51J receivers. Incidentally, an easy way to connect the loudspeaker is to use the front panel phone jacks. Be sure to note that the PHONES is a standard .250" jack and the SPEAKER is a three-circuit .187" jack (ring and shell are tied together and connected to chassis with tip being the 600Z output.) For 600Z, use SPEAKER and for 4Z use PHONES. The phone jacks are direct connections to the rear 600Z and 4Z terminals. Also, when using a headset, for 600Z phones, plug into the SPEAKER jack. There are very low impedance (4Z) headsets available and they can be used by plugging into the PHONES jack. When the receiver was set-up with an external 4Z speaker load a radio operator could temporarily plug-in a hi-Z set of phones for monitoring and not affect the speaker output.

Though the R-388 will "pick-up" some signals with a minimal antenna the absolute best performance is with a full-size, impedance-matched (or tuned) antenna. This, of course, is true with just about any receiver. With a first-class antenna, the R-388 will hear everything that is on the air with conditions or local EMI/RFI being the only limitation. I've successfully used a Pixel Loop antenna for general listening and it will function fine but the difference between the loop and a full-size "tuned" antenna is quite noticeable. In RFI noisy environments, sometimes a shielded magnetic loop antenna, like the Pixel Loop, will provide some relief. The RFI is more of a problem on 80M and it can be devastating on 160M. Higher frequencies tend to be less impacted by RFI noise. However, if your local QRN and your QTH allows it, using a full-size "tuned" antenna will result in the best reception performance from the 51J receivers when you have general listening in mind. That would be listening to all ham bands but also listening to SW-BC or utilities stations or any interesting signals that the receiver can tune to. The "tuned" antenna allows optimum performance at almost any frequency that the tuner can match. At shorter wavelengths (higher frequencies,) like the 20M, 17M, 15M and 10M ham bands, directional antennas with gain, a yagi or a quad, are a real benefit to DX reception on those bands since the antennas themselves add gain to the signal being received.

UPDATE: July 17, 2016 -  As restorers, we're never, ever quite through with a project. We're always keeping an "eye out" for parts that would either enhance our restorations, or, to maybe add a "missing part." This was the case with the R-388/URR profiled above.

Our first find was to add the original data plate,...not an original,... THE original. I was able to talk the fellow who had removed it into trading the original data plate for the reproduction tag I had on the receiver. Now the data plate reflects that this R-388 is from a 1951 contract. The unfortunate part of this trade was that just in the short time this guy had the original tag he had scratched out the serial number and tried stamping some other number. The reason? Unknown. I removed the number he had stamped and cleaned up the serial number area but since I don't have the correct way to stamp a number, I've just left it blank - even though it IS the original tag.

The second find was probably a year later when a nice condition Burlington carrier level meter showed up on eBay at a reasonable BIN price. It was acquired, restored and installed.

About six months later at a local ham swap meet there was a "FREE" R-388 front panel. It was in very rough condition and missing almost everything but it had nice original grab handles and a perfect escutcheon. These pieces were removed, cleaned up and then installed on my R-388 (note in the earlier photo above that the grab handles are "close" but not correct.)

The photo to the right shows the R-388 after these parts were installed - July 17, 2016.

Now, if I can just find a "CAUTION" tag to mount next to the data plate.

1951 R-388/URR


51J-4 Receiver  SN:4723

 Some Servicing, Some Restoration and a Performance Evaluation

As the final version of the 51J Series one would expect the best performance to be from the 51J-4. The original market consisted of the commercial communications or laboratories users who had specific requirements with stability, dial accuracy and selectivity being at the forefront. Great audio wasn't a consideration since the bandwidth selectivity had to be kept narrow. The users today are all either hams, SWLs or collectors and this group definitely have different performance desires than the original purchasers had. 

51J-4 Stock Performance - For dial accuracy, stability and selectivity, it's hard to beat the 51J-4. Mechanical filters provide a superior selective ability to all but eliminate most QRM and the Crystal Filter can take care of heterodyne interference. Sensitivity is very good or at least competitive. Where the 51J-4 has problems is in its audio reproduction when listening to an AM signal. To say the receiver sounds awful on AM wouldn't be too much of an exaggeration for an audio purist (I'm not one but the stock J-4 audio still sounds pretty bad on AM, AM-BC or SW-BC.) I've owned two stock 51J-4 receivers and both of them sounded pretty much the same on AM. The AM signals would normally be tuned with the 6.0 kc mechanical filter selected. What is noticed is that on either side of the passband center the audio range is pretty good but at passband center the audio is "muffled." This is because the 6.0 kc filter is limiting the audio response to 3.0 kc at the highest. On either side of passband center you are tuning to one sideband or the other of the AM signal and the audio response can go up to 6kc or so and thus the audio highs sound better. This is normal AM reception for the 51J-4. In fact, Collins recommended that when tuning an AM signal to always use one sideband or the other depending on receiving conditions.

The drawing shown to the right is from the 51J-4 manual. Drawing (A) shows how in a typical "bell-curve" type of IF the AM signal has the carrier placed at the center of the passband and both upper and lower sideband information is detected and reproduced. If one tries to tune "off frequency" to recover more audio highs it can be seen that the carrier would be reduced as the signal is tuned further from the passband center and eventually distortion will develop. Drawing (B) shows the 6kc mechanical filter's "flat top" passband curve and how it allows tuning one sideband and the carrier without loss of carrier strength. This allows better reproduction of the sideband information plus the carrier because the ratio of carrier to audio modulation is preserved.

It all looks good on paper and certainly works fine with strong, constant-level AM signals. But, that's not how 90% of the AM signals are - except for local AM-BC stations. Typical SW-BC stations fade, sometimes producing phase distortion, ratios of carrier to modulation change and cause distortion, sometimes there are heterodynes from other SW-BC stations. With ham AM signals the same problems can exist in addition to QRM, carrier frequency drift and low modulation levels. In the real world, tuning the 51J-4 for an AM signal using the 6kc mechanical filter as described in the manual will work well sometimes, especially on the AM-BC band, but with SW-BC or ham AM signals, much of time distortion and restricted audio are very noticeable. If you get used to tuning AM signals in this one sideband plus carrier manner (and conditions allow for it) the audio will have more highs and will generally sound like decent communications-grade audio,...most of the time.

On SSB, drawing (C) shows how the sideband information should fit within the 3.1kc passband without any losses. The stock 51J-4 works quite well on SSB if it's operated as a typical early fifties receiver without a product detector. The same goes for CW reception. Of course, the 3.1 kc and the 1.4 kc filters can be used in those modes for greater selectivity. You can get away with using the 3.1 kc filter for an AM signal if QRM is particularly bad by selecting one sideband or the other depending on where the QRM is. Those who are 75A-4 users probably wonder why the Crystal Filter continued to be used when MFs are provided. The Crystal Filter is actually better for certain kinds of QRM, especially heterodynes or adjacent frequency SSB signals. The Crystal Filter Phasing can eliminate that kind of interference where the MFs can't.

As for the stock 51J-4, it was primarily for communications, not for aural-pleasure derived from band cruising using wide-bandwidth for high fidelity reproduction from SW-BC stations (like PRC's Firedrake!) The basic 51J design was for data reception, not particularly for voice and certainly not for music reproduction. It's also important to remember that AM signals on the 51J-4 should be tuned to one sideband or the other. If you center the AM signal within the 6.0 kc MF passband then the audio will be restricted to 3kc and the audio will sound "muffled." The 51J-4 dial accuracy meant laboratories didn't have to have frequency meters anymore. Same with many of the military installations. Dial accuracy, frequency stability and excellent selectivity were what most users were interested in when purchasing a 51J-4. Great audio reproduction was not even considered since the passband had to be kept narrow for communications selectivity.

Having sold or traded off at different times my two formerly owned stock 51J-4 receivers, I swore I'd never buy another one,...ever! All it took to change my mind was seeing a light-gray panel laboratory version 51J-4. This receiver has turned out to be very interesting and it has a Collins-installed "audio fix" for the 6kc MF position that allows decent AM reception.

Collins' Audio Fix - The R-388 receiver has much better AM audio reproduction,'s not high fidelity but it is an easily listenable, mellow-sounding reproduction. The only difference between the R-388 and the 51J-4 are the mechanical filters and the mechanical filter two-tube assembly that compensated for the insertion loss caused by the mechanical filter. All of the current ham modifications that might be found in 51J-4 receivers are for SSB reception rather than to improve SW-BC or ham AM reproduction. Since the R-388 has the exact same post-500kc-IF circuitry as the 51J-4 and sounds fine, the AM audio problems aren't with the AVC or the AM detector. There have been several published modifications to replace various types of mechanical filters with RC coupler circuits. Mostly this was either to replace a missing MF or a defective MF. Also, many of the mods wanted a wider passband than the available MFs could provide.

An interesting discovery was made when I purchased the light-gray panel 51J-4 sn: 4723. This receiver has a Collins Product Support Operations sticker on the rear chassis showing that sn: 4723 underwent repair, reworking or modification in March 1971. One thing very apparent during inspection of the receiver was that Collins (apparently) had removed the 6kc mechanical filter and replaced it with a plug-in RC coupler.

The coupler consists of a 4.7k input load resistor, a 47pf silver mica capacitor (the red body types Collins loved to use) and a 470K output load resistor. The components are vintage, the solder joints have age-patina and the workmanship is first-class with a garolite insulating base and proper pins for "plug-in" installation. Interestingly, on VE7CA's website, he has a section on 51J-4 receivers where he shows the exact same type of RC coupler that he found installed in his light-gray panel 51J-4. VE7CA also believes it to be a Collins-installed RC coupler. Photo left shows the RC coupler VE7CA found in his 51J-4. The photo is from VE7CA's website but is edited to show just the RC coupler. For VE7CA's write-up on his 51J-4 experiences go to and click on "51J-4 PTO Rebuild."

As to the RC coupler operation, when selecting 6kc the actual bandwidth is about 12kc and rather than the steep-skirted passband edges the passband edges are more like the R-388 receiver. The audio sounds pretty good and it's a noticeable improvement when using the 51J-4 listening to an AM signal with 6kc selected. Can it be that simple? Apparently, it can. Although, this receiver is of late-enough production to have the IF Gain control which, if adjusted correctly, would reduce background noise and possibly reduce distortion due to excessive IF gain and could also contribute to the improved audio from this 51J-4 (I never checked the IF Gain setting on my formerly owned J-4s.)

Is that Collins' 51J-4 Audio Fix?
Apparently many 51J-4 users find the 6kc mechanical filter a problem for AM reception. There is another adjustable example shown on

Modification Caveat - I wouldn't normally advocate this RC coupler as a "ham mod" except this seems to be a Collins "fix" and it does a good job in a very simple, "no holes drilled" manner. And, most importantly, it can easily be unplugged and the 6kc MF reinstalled, if desired. Well, I don't have that option but I already know how the J4 sounds with a 6kc MF! I can say this Collins "fix" gives the user a noticeable improvement in the quality of audio for AM signals. But, although the audio highs are improved there's still that 200hz roll-off in the bass, so don't expect miracles, but the AM audio has better intelligibility and that's a big help. Besides, there's still the 3.1kc MF for good SSB performance given that there's still the stock AVC, the envelope detector and BFO. Additionally, the 3.1kc MF can be used on AM by just tuning to one sideband or the other to avoid severe QRM. NOTE: I do most listening with the 3.1kc MF selected and only switch to the 12kc RC coupler if the AM signal is relatively strong and has good modulation levels. With the 800hz MF, a very narrow CW bandwidth is available (also works fine for 170hz shift RTTY.) Underneath, the receiver still appears totally stock (except for the new AC power cable installation with super-long wire lengths - I did a correct reinstallation of the AC power cable using the correct-type vintage strain relief and much shorter wire runs.) I have to say that sn:4723 has proven to be a very interesting 51J-4.

51J-4 sn:4723 showing the top chassis and underneath.

Servicing SN: 4723 - Jan 5, 2023 - This 51J-4 did function but it was obvious that it hadn't been thoroughly checked out in a very long time and that the sensitivity and gain were down quite a bit. The seller had indicated that he had tested the receiver before listing it and it picked up WWV at 5, 10, 15 and 20mc. So, since it had been very recently operated, I didn't really need to do a "pre-power-up" test. I could just go right to seeing how well the receiver worked. When operating, I could pick up many signals but the receiver didn't seem very sensitive, the CL meter wasn't moving and the receiver took a very, very, very long time to "warm-up."

Usually, a long warm-up time indicates a few to many weak tubes so the first thing to do was to test all of the tubes. All of the tubes tested very good BUT in the Mechanical Filter assembly one 6BA6 position actually had a 12BA6 installed. No wonder it took a long time to warm up. I momentarily thought the 12BA6 in the MF IF amplifier might have been some type of mod. Something like "install a low gain tube for reduced distortion" but I think it's more likely that was the closest "BA6" tube a former owner could find. With all good condition tubes, SN:4723 had much better sensitivity. I set the zero on the CL meter and could just get about a 10db increase by tuning in WWV, so sensitivity is not up to spec, yet. But, the Calibrator signal will drive the CL meter up to +60db so the problem might be the limitations of using a Pixel Loop antenna indoors (heavy wet snow took down my 135' Inv-vee Tuned antenna.) There wasn't any change in the audio quality, so I'm pretty sure the 12BA6 was installed because that was the only "BA6" easily available (out of the junk box?)   >>>

>>>   I did a quick end-point check on the PTO and got 4.5kc change over 1.0mc span. That's not too bad considering most 70E-15 PTOs have EPEs that are >6kc and they're so far out they aren't even adjustable. 4.5kc should be able to be adjusted down to <1kc EPE using the compensation adjustment on the front of the PTO. 

There was a lot of "crackling" in the audio output that seemed to originate in the MF assembly caused by a dirty tube socket (the one that had the 12BA6 installed.) Cleaned the tube socket with DeOxit.

The MF selector switch shaft slips a little and it must have been a long-term problem because the bristol screw that tightens the bell-crank lever to shaft clamp was virtually stripped of its splines so the clamp couldn't be fully tightened. Easy to replace, so I installed a good condition spline-head screw but, when tightened, the apparent "slipping" still occurred. The problem was in the bell-crank lever on the MF assembly switch shaft was slipping on that shaft because the set-screws weren't tight. After tightening the set screws the MF selector switch level worked like it should.

I'll run SN:4723 for a few more days to get some hours on the tubes before I do an alignment.

Detailing - Jan 8, 2023 - Although this 51J-4 didn't really look it, it was pretty dirty. I was amazed at how much gunk came off with Glass Plus. Someone in the past had polished the panel with something that left a white power residue. It couldn't have been too abrasive since the silk-screened nomenclature looks fine. Also, the index lines on the knobs were crudely redone as were the index marks and nomenclature on the bezel. I dismounted the knobs, cleaned the panel and then individually cleaned each knob. I had to carefully use a razor blade to shave off the excess white paint on the index lines before reinstalling. I removed the bezel and cleaned it with Glass Plus and then polished it lightly with Wenol's. The two glass windows were very dirty but cleaned easily with Glass Plus. The bezel and windows were then reinstalled. I had to clean and darken the bezel screws as they had lightened somewhat. I used a black marker (not a Sharpie - they're purple - try "pen+GEAR" very black markers) to darken the screw heads. The panel is in very good shape. The paint has a semi-gloss sheen to it when clean. Jan 12, 2023 - AC Power Cable - I really didn't like the way the AC power cable had been installed. It might have been part of the Collins rework but it sure didn't look like it. The power cable was just stuck through the chassis hole (not even a grommet) and a plastic clamp was installed on the inside (and that was after a new hole was drilled for the 6-32 mounting screw for the clamp.) The proper and original method of installation required using a molded black plastic strain-relief that will clamp the AC cable and mount into the chassis "double-D" hole. I looked at new strain-reliefs at the hardware store and they are pretty cheap looking. Back home, I checked all of the parts bins and managed to find a vintage strain-relief that was the correct size. These are difficult to install unless you have the correct tool. I haven't seen one of the proper strain-relief installation tools in probably 25 years. But, I used 16 gauge wire twisted to compress the strain-relief to get it into the chassis hole. Then small channel-locks can achieve the right angle to compress and push the strain-relief into position with the cable inside. I had to cut and then remove the original connections by unsoldering and also had to clean the joints with SolderWick for new soldering. I then trimmed and routed the three AC wires correctly, stripped and tinned, then mounted and soldered the AC connections to finish the installation,...correctly. The photo of the underside of SN:4723 above shows the new AC power cable installation.

Missing Screws - A pet-peeve, I guess. I bought all new 4-40 x .375" pan head, phillips, SS machine screws along with flat washers and locking washers to correctly mount the bottom cover with all the same type of screws. Bought three 6-32 wing-nuts and flat washers for securing the top cover correctly.

IF Alignment with Mechanical Filters - The manual indicates the IF adjustments should be performed with the 3.1kc MF selected. Monitor the Diode Load output with an analog VTVM and keep the signal generator output at a level that produces about -3.0vdc on the Diode Load. Connect the signal generator through a .01uf capacitor to the grid of the last Mixer V-107. The manual indicates that a RC load consisting of a .01uf cap in series with a 4.7K resistor needs to be connected to the IF transformers as they are adjusted (a simple shunt-LP filter that keeps the IF from mixing with the crystal oscillator and PTO since you have the bottom cover removed.) Peak all of the IF transformers top and bottom while moving the shunt load as directed in the procedure. There's also the adjustment on the mechanical filter assembly. Recheck to be sure that all adjustments are at peak at the correct input frequency. Disconnect the signal generator and the VTVM.

Test the IF alignment by tuning through a broad AM signal with the 6.0kc MC filter selected. Carefully listen to the signal as you tune from one end to the other. You should hear the AVC quickly attack at the low end of the passband, then the signal will quickly clear up and as you tune through about 2 to 3kc or so, the signal should stay constant and clean with fairly high audio frequency response. You will hear the AM signal narrow in audio bandwidth at the center of the 6.0kc filter but the signal level should remain constant with no distortion although with reduced higher frequency audio response. As you tune out of the high end of the passband, the audio frequency highs will increase for 2 to 3kc and then the AVC will react to the noise as the signal is quickly reduced by the steep slope of the mechanical filter.

This should be repeated for the 3.1kc filter and the 1.4kc filter also.

IF Gain Adjustment - One other thing though, if you have a later version 51J-4, it should be equipped with an IF Gain adjustment. This should be set last after the completed IF/RF alignment. The proper IF Gain setting was first accomplished at the factory, but that was over 50 years ago! Different tubes and component aging requires that the setting should be at least checked. The Collins procedure requires injecting 3uv into the antenna input at 2.1mc and then setting the IF gain pot for 270mv output on the IF OUT coax connector on the rear chassis. At the time this procedure was written measuring RF levels was commonly accomplished using a VTVM with RF probe and measuring the RMS voltage values specified.

NOTE: This IF Alignment section is general information on how I align 51J-4 receiver fixed IF sections, not specifically for SN:4723.

Unneeded Alignment of SN:4723 - Jan 20, 2023 - After using SN:4723 for a few weeks, I felt the receiver was already pretty close in alignment. The fixed-IF was especially nicely adjusted because the Crystal Filter seemed to function just as it should. The date in 2013 that was written on a paper stick-on label stuck on the multi-section plug-in filter capacitor was probably when the receiver was last serviced and probably aligned at that time. There were a couple of checks that I performed however. First, I pulled all of the crystals in the Crystal Oscillator to check the sockets and pins for any corrosion and none was found. I then measured the Crystal Oscillator output and adjusted all of the trimmers for just under 2 volts. All of the trimmers had been set a bit lower in voltage but that was probably okay. Next, I aligned the Variable IF. Again, nothing was very far off from being in alignment and only needed slight peaking. Then the RF section was adjusted. This was only slightly off on a few adjustments. Last was the .5 to 1.5mc band and, again, nothing too far off, just a slight peaking of the adjustments. There weren't any problems with stuck trimmers or slugs, everything adjusted easily. A quick test using a ten foot wire "test" antenna on the AM-BC band and KOH 780kc pushed the CL meter to +80db. On the Pixel Loop, WWV 15mc pushed the CL meter to +40db (even WWV 20mc pushed the CL meter to +40db!) So, I've done alignments like this before. It's where you turn an adjustment only to put the adjustment back where it was. There were one or two adjustments needed but most were just "move and return" types.

UPDATE: Jan 28, 2023 - I was wondering why I wasn't hearing very many 40M hams or any SW-BC on the 31M SW band. It seemed like below about 12mc, the receiver sensitivity just dropped. WWV 10mc would barely move the CL meter. During a quick check of another receiver, rather than connect the Pixel Loop to it, I connected a 25ft length of wire as a "test antenna." This was just a test to confirm that this receiver, that had been disassembled for photos and was now reassembled, still functioned fine. With this test set-up, the receiver picked-up several 40M hams and SW-BC stations in that area of the spectrum that were quite strong. Hmmmm. I decided to hook-up this test antenna to the 51J-4 just to see what would happen. The results were a busy 40M ham band, quite a few SW-BC stations and WWV 10mc pushing the CL meter up over +60db. Quite a difference from the Pixel Loop. I have been suspecting that there's a problem in the low noise amplifier for the Pixel Loop as, even on MW, the signals seem scarce and those that are present are pretty weak. Unfortunately, my standard 135' tuned inv-vee antenna that I normally use for HF test listening was destroyed by a falling tree brought down in a heavy snow storm Dec 31, 2022. The plan is to re-route the feedline from the "two half-waves in-phase" (270' CF tuned dipole) from the shop over to the house. Will require 125' of feedline and using a different antenna coupler. Work to commence whenever the snow melts. As to the Pixel Loop,...more detailed testing is going to be necessary but I don't think it's working correctly.

Conclusion - All Collins enthusiasts have their favorite models. For me, the 51J Series has the best of what I like - good performance characteristics with an incredible "bench presence." Add to that, superior build-quality resulting in excellent reliability and you end up with a really great receiver. The earlier 51J-1 and J-2 have desirable features and performance. I have a 51J-2 and it's an amazing performer considering it's from 1950. The 51J-4 also has unique features that allow reliable communications in crowded band conditions. I also have a 51J-4 and it has the ability to deal effectively with almost any type of QRM. But, lucky for those amateurs and collectors who want the very best, the most common version really has those features that work best for most users. Of course, that receiver version is the R-388/URR. Thousands were built in the early fifties - a time when vacuum tube design was peaking and the military demanded the very best radio gear. The R-388/URR is probably the easiest version of the 51J Series to find (I have two of them and they are excellent all around receivers.) The best communications equipment design and manufacturing company at the time, Collins Radio Company, provided a real "winner" in the 51J Series and especially with the R-388/URR. A well-cared-for and recently aligned 51J receiver of any version is a pleasure to operate.

1960 Collins Lab 51J-4  SN:4723


1. Nick England's great website has a very detailed section on the R-388/URR receiver and all of the variations. Also includes a lot of info on the 51J-4 and many of the numerous 51J variations. Lots of photos.

2. Manuals for 51J-2, 51J-4 4th edition, Signal Corps R-388/URR, Navy AN/URR-23A, 9th edition 51J-4 manual (1961), Beckman/Berkeley Frequency Measuring System manual(s),

3. Bill Orr's R-388/70E-15 PTO article from December 1969 CQ magazine. PDF available at

4. "A Guide to the Evolution of HF General Coverage Receivers at Collins Radio" - CCA Signal, Q1 - 2014

5. Thanks to Jan Wrangel SM5MRQ for his procedure and photos for accessing the 51J-4 Crystal Filter.

6. Thanks to WB2GCR "Bear" for providing the four photos of four different light-gray panel 51J-4 receivers including a photo of the Beckman/Berkeley Rack Mount Frequency Meter System at KPH.

7. Thanks to W3TA Tom for providing the photo of Beckman/Berkeley greenish-gray wrinkle 51J-4 panel plus the Beckman Frequency Measuring System manuals.

8. Thanks to VE7CA Markus for the write-up on his light-gray panel 51J-4 and the photo of the Collins RC-coupler.

9. KPH photos of Fred Baxter and the Beckman Frequency Measuring Equipment. 

10. Thanks to all of the 51J Series enthusiasts for conversations, information and anecdotes that have been exchanged "on the air," online and in person over the years.

Henry Rogers, Radio Boulevard, WHRM December 2013-2023,.......edits as follows,....Nov 2018, added extra info on 51J-3,...May 2020, edited and corrected various sections,...Oct 2022, added light-gray panel J-4 photos,...Jan 2023, added info on SN:4723 J-4, info on RACAL RA-17,...Jan 2023, major write-up revision and expansion with lots of new information and new photographs in all sections,...added 51S-1 addendum Jan 2023,...

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