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Western Historic Radio Museum

Rebuilding the Collins 51J Series Receivers

(includes 51J-1, 2, 3, 4 & R-388/URR)

Brief History - Circuit and Design Description

General Information on the 51J Series

Rebuilding the 51J-2, Rebuilding the R-388/URR

Servicing and Alignment of the 51J-4 Receiver

by: Henry Rogers WA7YBS/WHRM

photo above: Artwork drawing of the 51J-4 from the manual

Many Collins Radio enthusiasts believe the 51J Series comprises the best looking receivers that the company produced - but, what about 51J performance? Like many pieces of collectible vintage electronic gear, the 51J receivers are now over half-a-century old. Many users are operating their 51J receivers in "as-found" condition. Are they experiencing all that the 51J is capable of? The following article details how to get the most out of your 51J receiver. Also, 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 rebuilding or repairing. As some of the best-looking receivers that Collins produced you definitely want your 51J to perform like that company's best equipment should.   H. Rogers, Dec. 2013

photo above: 1950 advertisement for the Collins 51J-1 Receiver

Brief History of the 51J Receivers

Introduced in 1949, the Collins 51J receiver was intended for the military or commercial 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 reproduction of AM or CW signals. The 51J was also the perfect receiver for the 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 band only 75A-1 receiver two years earlier, but their 51J-1 was for commercial/military users and was Collins' first receiver to provide general coverage reception from .5mc up to 30.5mc with better than 1kc dial accuracy anywhere within that tuning range. The tuning system used thirty, 1.0mc wide tuning ranges and the receiver itself featured performance that was literally years ahead of the competition. 

During the post-WWII era, the U.S. Army Signal Corps was supposedly 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 to be inadequate. These receivers have been reported to be the BC-348 but the BC-342 seems a more likely choice. The story goes that Collins sent an early 51J receiver to the Signal Corps to try out and the Signal Corps' response was certainly positive since the receiver was extremely accurate in its frequency readout and had essentially "no drift." The SC "had to have 'em" and ultimately the R-388/URR was the result. It seems far more likely that Collins was working with the Signal Corps from the beginning to provide them with a thoroughly modern receiver that had the specifications the military sought. From the start, the military bought 51J-1 and 51J-2 receivers designated as R-381/URR receivers (non-A and A versions respectively) but the ultimate design came in late-1950 with the R-388/URR receiver. Many thousands of R-388 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 communications huts. The 51J Series wasn't entirely a military receiver however. Other commercial users found a need for the precision tuning and frequency stability featured in the 51J Series. The 51J-4 version, with its stability and dial accuracy along with its mechanical filters, was found in coastal stations like KPH and KMI, in overseas embassies, in commercial laboratories like Beckman, at universities like Stanford, along with some military uses and even some wealthy SWL (supposedly Jackie Gleason owned a 51J-4) and 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 making the 51J Series one of Collins' most produced receiver.


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 IFs as required in combination with the Crystal Oscillator and PTO to provide the correct tuning range. 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, AVC 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.

Early versions of the 51J receiver have a metal dial bezel, the 70E-7 PTO (with the 6SJ7 metal octal tube located under a screw-mounted shield cap,) the Collins' "winged emblem," no grab handles and an illuminated S-meter. Audio response is restricted at 200 to 2500Hz and is definitely not high fidelity, usually sounding somewhat "muffled" 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 meter that was actuated by a toggle switch next to the meter. Another difference is the nomenclature for the 100KC Crystal Calibrator, was designated as "100KC CRYSTAL" on the 51J-1 and merely as "CALIBRATE" on the 51J-2. 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 post-sale retrofit or a Collins engineering-production upgrade is unknown at this time.  >>>

photo above: The Collins 51J-2 from 1950 in the standard Collins cabinet

>>>  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. Both the 70E-15 PTO and the standard megacycle scale drum are on the standard production of the 51J-2's successors, the 51J-3 and R-388/URR, along with several other changes in the receiver circuit and in its appearance. The 51J-1 was built in a very small quantity in 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 replaced it.



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 ohm Z dipoles or 50 Z ohm (or less) vertical whip antennas. The new upgraded receiver was designated as the R-388/URR 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 special cabinet (along with a manual) that was utilized by the Navy for some of it's R-388 installations. Most of the early U.S. Navy R-388/URR receivers were designated as AN/URR-23A and this set-up included the R-388 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 receivers that were designated AF-30 and were used for backscatter ionospheric purposes. These R-388s had blue panel and lacked grab handles. The largest contract quantities for R-388 receivers are from 1951, 1952 and 1953 with these receivers being the most commonly encountered today.

With the R-388 and 51J-3, 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.) A "Break In" function was added that utilized around +12 vdc to actuate a relay to put the receiver in stand-by. 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.   >>>

photo above: 1951 R-388/URR with field replacement Marion Electric carrier level meter

photo above: 1952 contract R-388 with standard Burlington carrier level meter

 >>>   Late versions of the R-388, usually the 1953 and later contracts, have a switch to turn the "Break-in" on or off. This switch replaced the SPEAKER jack and mounted in that location of the panel. The receiver 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. The metal bezel used on the J-1 and J-2 was replaced with a black bakelite bezel. There are many other minor changes in component types and placement throughout the R-388 receiver. Most production quantity totals are for the R-388 receiver with very few actual 51J-3 receivers produced. The limited 51J-3 production ended with the introduction of the 51J-4 version in 1955.

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. The Marion Electric type of carrier level meter is shown in the 1951 contract R-388 in the photo to the left. Many times MWO (Military Work Order) will have been incorporated to add the Break-in Switch or other changes. The 1953 R-388 shown below has the MWOs listed on the front panel.

photo above: 1953 contract R-388 with BREAK-IN switch  (owner: KF7VLA - Mike Tara)



 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. Collins had recently developed the mechanical filter that provided an IF passband that was defined with steep skirts and a flat top that resulted 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 level 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. 

U.S. Navy catalogs show that the 51J-4 was designated as the R-388A/URR or the R-388B/URR but it is doubtful that any receivers were actually identified as such. At this time, 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 "tagged" as "51J-4."

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

51J Series Summary - Dial accuracy and stability - both necessary for RTTY work - were the 51J selling points to the Signal Corps and still are the 51J's primary attributes today. Although the stock audio is somewhat restricted, the 51J receivers are great performers providing they have been thoroughly serviced or rebuilt and are 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 and dial accuracy (for an analog readout) can't be beat. 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 variable propagation affecting reception, the 51J's audio limitations really aren't noticeable. 

General Information on the Various Types of 51J Receivers

Standard Versions

51J-1 - 1949 - The first 51J model was 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, 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, 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 may be found with the 70E-15 PTO. It's also possible that the green highlighted amateur bands were removed from the megacycle drum dial sometime during the 51J-2 production.

R-388/URR - 1950 to 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" not used anymore on the receivers, the megacycle dial drum no longer had the green highlighted amateur bands (this may have been taken off during the 51J-2 production,) the carrier level meter was now a non-illuminated, sealed unit made by Burlington Company (replacement meters made by Marion Electric and other companies are sometimes found installed,) audio outputs on the front panel allowed for phone or speaker 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 irradite finish (gold color,) the side panels are made of steel and finished with gold color irradite, the schematic was usually applied to the inside of the top cover, most (all?) receivers were MFP coated, remote standby now required +12vdc to be applied to a relay via real terminals for "break in" operation. 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, USN, USCG at various times from 1950 up into the early 1960s.

51J-3 - 1951 to 1955 - Same as R-388/URR but ID tag reflects that the receiver is a 51J-3 rather than R-388/URR. The civilian version of the R-388 is seldom encountered as most so-called 51J-3 receivers actually are R-388/URR receivers with the military tag removed.

51J-4 - 1955 to 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 which is located behind the BFO knob. The MF assembly adds an additional IF amplifier bringing the tube total to 19. Late version panels (made in 1964) may be painted light gray with black nomenclature. Some late versions have been found with "S-line" knobs installed. 51J-4 receivers are usually not MFP coated. AVC and Diode Load pin jacks on rear chassis. "FCC Part 15" information is silk-screened on the rear of the chassis on most of production.

Special Versions

R-388A/URR and R-388B/URR - US Navy version of 51J-4 - it is unknown if this model was actually produced (or tagged 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 Navy designations for the 51J-2 and R-388 installed in a Collins cabinet designated as CY-1235/U with Collins speaker designated as LS-199/U

R-381/URR and R-381A/URR - These are Signal Corps designations for the 51J-1 and 51J-2.

AF-60 - Special USAF version of the R-388/URR with blue panel and no grab handles. Very small contact of only seven receivers. Used for ionospheric backscatter research.

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 top lid, although, with the standard 75A/32V cabinet the 51J top can't be removed without taking the receiver out of the 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-274D (military SX-73.) 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 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 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.

photo above:  1952 R-388 receiver installed in the Hallicrafters CY-1260/G military cabinet. These types of cabinets are built of heavy-gauge steel and their weight is formidable.

photo right: The data plate on the Hallicrafters CY-1260/G cabinet

Speakers - All of the advertising seems to never specify a particular matching speaker for any of the 51J series of receivers. This seems to imply that the standard 8" speaker used in the 270G-2 cabinet was used with the early 51J receivers and the 270G-3 10" speaker was used with the 51J-4. 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 USN version of the R-388 referred to as the AN/URR-23-A that came in a standard Collins cabinet, CY-1235/U and included the 270G-3 speaker (LS-199/U.) The 51J-2 was also supplied in a similar fashion and designated as the AN/URR-23.

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 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, J-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 difficult. To take on a poor condition 51J receiver, you should be experienced in complete disassembly and reassembly of 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 fixed. Once all of the circuits has been rebuilt and the mechanical issues addressed, a full alignment is necessary. Though not essential, the original alignment tools will make many of the adjustments easier. 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.

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 - but especially in the receiver alignment section, circuit description section, the component designations and component identification layouts. How this manual got through the proof-readers is a mystery. The 51J-1 manual is probably just as bad. The later R-388 and 51J-4 manuals are much better 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 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 seem to be aggravated by poor storage conditions. However, there are some component quality issues with some types of capacitors that will 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 encountered will be mechanical in nature and brought on by poor storage, rough handling or "hamster" modifications.

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 but this is rare. 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 power 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. The second is the standard diode detector used in the stock receiver which many users want to replace with a Product Detector. 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 adequately 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 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 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" weak CW signals. Maximum sensitivity in CW was always achieved by "riding" the RF Gain control with the receiver out of AVC. 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 can be relatively easy to reverse and put the receiver back to stock. 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 a very good job with no damage to the circuit components that remained. This allowed an easy removal and rebuild to stock for the BFO and AVC circuits. As an original 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 is spectacular with plenty of sensitivity and with mechanical filters QRM is not an issue. After a complete IF/RF alignment it has become one of my favorite receivers.

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 will work fine for SSB or CW and the AVC mods allow for better SSB response but operating the stock receiver in the manner in which it was designed will also give you great performance in all modes of reception.

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 .001uf cap to increase the audio frequency response and still protect the primary from transients. With the .001uf 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.
Excessive End-Point Error in the 70E-15 PTO - The 70E-15 PTO end-points on nearly every R-388 PTO are excessive and beyond the range of the adjustment capacitor. Usually, if the EPE is more than 6.0kc, it will be out of the range of the adjustment capacitor. All 70E-15 PTO end-point errors (EPE) are similar in that the tuned range (1.0mc in ten turns) of the PTO has increased. I've never encountered a 70E-15 PTO where the EPE actually has a decreased 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 more a combination of the ferrite quality combined with continuous operation with the resulting heat. This opinion is based on the acquisition of an 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 is common to find the 51J-4 receivers with virtually no EPE in the PTO and, of course, 51J-4 receivers post-date 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 and a while 51J-4s will be found with excessive EPE but these were probably in an environment that required continuous operation. If your 51J-4 or R-388 has excessive EPE, correction will require internal PTO compensation coil trimming. This requires disassembly of the PTO. When trimming the compensation 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 maybe remove another half of a turn - but no more. Excessive turn removal will have the opposite effect and make the EPE totally non-adjustable. 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.
The PTO Test Jig for the 70E-15 - The earlier PTO, the 70E-7A, has the end-point adjustment capacitor 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 capacitor is behind a hex-nut that has to be removed and then there is a locking nut that also has to be loosened before the adjustment capacitor can be moved. There are some restorers that have built special right-angle tools for accessing the locking nut and the adjustment capacitor but these do require a lot patience to use. Generally, the PTO has to be removed from the receiver anyway for trimming the compensation coil for end-point correction. I find it easier to just remove the PTO, do the rework and the entire adjustment on the bench with a good jig. >>> >>> The test jig can be easy to make. You just need to reproduce the accuracy of the kilocycle dial to determine a 1000kc change in frequency for exactly ten revolutions of the PTO shaft. A fixed reference line and a very narrow pointer is really all that is necessary. You shouldn't try to use an old 51J kilocycle dial because it will block easy access to the adjustment capacitor. A very narrow pointer and an index scale that has a "zero" line and several other lines on each side of "zero" will 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 during the adjustments will determine your overall accuracy when the PTO is installed back in the receiver. Additionally, a really nice test jig might also have a built-in power supply providing the 6.3vac and a regulated +150vdc necessary to power-up the PTO. Binding posts or a cable with alligator clips could provide the hook-up to the PTO. In this way, the PTO can be completely removed from the receiver. It makes any rework (like compensation coil trimming) much easier since you don't have to have the harness connected to the receiver. However, unless you're going into the business of 70E-15 readjustments, just leave the PTO connected to the receiver and provide enough bench space to rework the PTO with the receiver also on the bench.

Rebuilding the Collins 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 Eisenbath (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.

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 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 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 very nice as I did replace the .01uf across the audio output transformer with a .001uf to increase the high frequency audio response. I am running the receiver into a 12" 4 Z ohm speaker in a large cabinet and the response in very nice with some bass on SW BC stations and powerful ham AM stations along with enough highs to significantly reduce the normally "muffled" audio quality.

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 some spurious oscillations that will be tuned at various frequencies throughout the tuning range. 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 some of the 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. Many thousands 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. The following "basket case" is not typical of that status since most of the parts were present. Just the receiver was mostly disassembled.

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 metes 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 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.

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. 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.) As to why the TM directs you to "load down" the IF transformers with an RC load consisting of a .01uf capacitor and a 47K resistor in series, I believe that was to assure that the alignment tech wasn't going to overload the IF with too much signal and therefore slightly misalign the IF because the input signal was so broad and strong. Keep the IF input signal level low and do the IF alignment with the Crystal Filter on and in position 4 and your alignment will be accurate. You can install the load if you want too but I don't think it's necessary if you watch the level of the signal generator carefully. Though the special alignment tools (that are always missing) will help with the alignment they are not strictly necessary. Reproduction tools are available so you can obtain a set and try them out. I think the original alignment tools make the trimmer caps easier to adjust but not the slugs. The IF is easier adjust with the proper alignment tool, also.

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 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 use the peak of the receiver output at each calibration frequency rather than the actual dial readout. For example, if you have to align to 7.40kc but the actual peak output is on 7.38kc, then use 7.38kc readout on the receiver and 7.40kc input on the RF generator and then adjust the proper trimmers or slugs for peak output. 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.

R-388/URR Performance - 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 a bit wider. However, the IF passband curve is more bell-shaped rather than the steep slopes 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 can usually 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.

Stability is solid and drift-free. Sensitivity is competitive. As a comparison, one might find that the R-390A is a bit better at digging out signals especially if the R-390A has been rebuilt and aligned. Still, with a good antenna, the R-388 will hear just about any signal on the band.

Selectivity is entirely controlled 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. 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.

Set-up is easy for station receiver operation but the Break-in function does require a separate 12vdc power supply that is switched with the Antenna Relay auxiliary contacts. Better audio is obtained from the 4.0 Z ohm output with a matching Z speaker. The 600 Z ohm output was for driving data devices and requires a matching transformer to operate a standard Z speaker.

Though the R-388 will "pick-up" signals with a minimal antenna best performance is with a full-size, impedance-matched (tuned) antenna. With a first-class antenna, the R-388 will hear everything that is on the air with conditions being the only limitation.

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 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 the "Warning" tag that mounts next to the data plate.


Aligning the 51J-4 Receiver - Performance Evaluation

IF Alignment with Mechanical Filters - Since the 51J-4 uses mechanical filters to define the IF passband, it is essential that the IF be centered for the mechanical filters and the crystal filter. Usually an accurate setting of 500kc on the signal generator is okay but it's better to use the crystal filter crystal frequency to set up the IF on the signal generator. With the Crystal Filter on and set to position 3 or 4 and mechanical filter 3.1kc selected, sweep through the IF and note where the crystal "peaks." Usually it will be very close to 500kc - within 0.01 or 0.02kc of 500kc is normal. Note on the digital frequency counter the exact frequency and then make sure that when performing the IF adjustments that the signal generator has not drifted or changed frequency. Again, you must keep the signal generator frequency at the crystal frequency exactly! It's a good idea to do the adjustments to the IF transformers with the Crystal Filter turned on and set to position 3 or 4 and with the Phasing set to the narrowest response. Monitor the Diode Load output with an analog VTVM and keep the signal generator output at a level that produces about -7.0vdc 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 and a 47K resistor needs to be connected to the IF transformers as they are adjusted but I don't bother since we are carefully and accurately controlling both the frequency input and the signal level into the IF section of the receiver. Peak all of the IF transformers top and bottom and 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 filter selected. Carefully listen to the signal as you tune from one end to the other. You should hear the AVC attack at the low end of the signal's passband, then the signal will clear up and as you tune through about 6kc or so, the signal should stay constant and clean. You will hear the AM signal narrow in audio bandwidth as you approach the center of the 6.0kc filter but the signal level should remain constant with no distortion. As you tune out of the high end of the signal's passband the AVC will react to the noise as the signal is reduced by the steep slope of the mechanical filter. This should be repeated for the 3.1kc filter and the 1.4kc filter also.

Other than the importance of the centering of the IF for the mechanical filters, the 51J-4 aligns just like the R-388. The procedure in the 51J-4 manual is somewhat different but the receiver is exactly the same as the R-388 with the exception of the mechanical filters so the R-388 procedure can be used , if necessary.


photo above: The top of the chassis on a 51J-4 receiver. Note the mechanical filter assembly to the right and partially above the PTO. The three mechanical filters are the plug-in rectangular units.

photo above: Under the 51J-4 receiver

51J-4 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. Sensitivity is very good or at least competitive. Where the 51J-4 has problems is in its audio reproduction. To say the receiver sounds awful on AM wouldn't be too much of an exaggeration for an audio purist. I've owned two 51J-4 receivers and both of them sounded pretty much the same on AM. The AM signals are usually 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 for the 51J-4. In fact, Collins recommended that when tuning AM to use one sideband or the other depending on receiving conditions. On SSB, the 51J-4 works quite well if it is 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 the 3.1 kc filter in AM 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. For SW BC the 51J-4 is okay but still the limits on the audio reproduction have most of the SW BC sounding pretty bad. Remember, the 51J-4 was primarily for communications and the basic 51J design was for data reception, not voice. The 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 was 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 selectivity.

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 but, lucky for those amateurs and collectors who want the best, the most common version really has those features that work for those users. Of course, that receiver version is the R-388/URR. Thousands built in the early fifties - a time when vacuum tube design was peaking and the military demanded the very best radio gear. 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.   References:

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

2. Original Manuals for 51J-2, 51J-4, R-388/URR

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

Henry Rogers, Radio Boulevard, WHRM © December 2013


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