Radio Boulevard
Western Historic Radio Museum

Rebuilding the Collins 51J Series Receivers

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

Part 1

Brief History - Circuit and Design Description

General Information on the 51J Series

Part 2

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 (also designated as 51J-3) 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. 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 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 post-sale retrofit or a Collins-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 (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 special cabinet (along with a manual) that was utilized by the Navy for some of it's R-388 installations. Some of the U.S. Navy R-388/URR receivers were designated as AN/URR-23-A (but possibly tagged as 51J-3) 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 receivers that were designated AF-30 and were used for backscatter ionospheric purposes. These USAF receivers had blue panel and lacked grab handles. The largest contract quantities for R-388/URR 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.) 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 +12vdc to actuate an internal relay that disconnected the antenna and removed the IF plate voltage upon actuation. 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.  >>>

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

 >>>    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. The metal bezel used on the J-1 and J-2 was replaced with a black bakelite 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.

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 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. 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 51J-3.)

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.  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 


The 51J-3 is virtually identical to the R-388/URR receiver. 51J-3 receivers will have a Collins data plate with "51J-3" stamped on the plate with an appropriate serial number. Besides the obvious data plate difference, R-388 receivers will have military inspection stamps from the Signal Corps which the 51J-3 should not have. Whether civilian 51J-3 receivers are MFP treated is unknown.

Just to confuse the issue, some "R-388" receivers that were supplied to the USN were officially designated as the AN/URR-23A but the data plate may actually have had "51J-3" as the type designation and the tag will be a Collins Radio Company tag (not U.S. Army Signal Corps.) It's speculation but it's based on the later USN designation of the R-388A receiver and those receivers carrying a "51J-4" data plate. It's probable that the AN/URR-23-A also carried a Collins 51J-3 data plate. Whether all R-388/51J-3 receivers supplied to the Navy were identified with a Collins 51J-3 tag is unknown. But, ALL R-388 receivers supplied to the Signal Corps carried a U.S. Army Signal Corps R-388 data plate.   >>>

>>>   The identification problem comes mainly from receivers that are missing their data plate. I know of two "tagless" R-388 receivers that were sold as 51J-3 receivers. One actually had a Collins' Winged Emblem mounted in place of the R-388 tag. To further confuse this problem there are also reproduction 51J-3 tags out there.

12,000+ Serial Numbers on 51J-3 tags - 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 51J3/4 serial number data plates. To assure Collins collectors (or 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.

Of course, this information puts into question the identification of any supposed 51J-3 that has one of these repro tags. 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. A receiver with USN stamps may be authentic and tagged as a 51J-3 but it's actually an AN/URR-23A.

Always consider that the civilian 51J-3 receivers are 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. 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 that noticeable. The 51J audio isn't for audiophiles,'s for reliable communications. 

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 (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 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 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 at various times from 1950 up into the early 1960s.

51J-3 - 1951 to 1955 - The 51J-3 is virtually identical to the R-388/URR but the ID tag will show that the receiver is a 51J-3 rather than R-388/URR. It's possible that when the USN ordered the AN/URR-23-A the receiver shipped was actually identified on its ID plate as "51J-3." The minor differences between this USN version and the Signal Corps R-388/URR are possibly that under the top cover there is no schematic of the receiver (some R-388/URR receiver top-covers also don't have the schematic, but most do.) There are no Signal Corps acceptance stamps applied to the chassis or front panel. The Collins' build serial number is ink-stamped on the front panel under the serial number tag (this same number is stamped into the rear of the chassis.) Some R-388/URR receivers may have the build SN under the tag but many have acceptance stamps under the tag or nothing at all under the tag. To add to the confusion, sometimes R-388/URR receivers will have Signal Corps stamps AND Navy "Anchor" stamps on the front panel of the same receiver. The actual "civilian" 51J-3 is rare. It's probable that the civilian receivers are exactly the same as the R-388/URR but should be lacking the MFP coating, shouldn't have any military inspection stamps and the tag will identify the receiver as "51J-3." Also, 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 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 and some USN 51J-3 receivers might have "51J-3" on their ID tag while ALL Signal Corps R-388/URR receivers will have "R-388/URR" as the receiver identification.

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. When supplied to the USN as the R-388A/URR or R-388B/URR, it's likely that the ID plate has "51J-4" stamped on it.

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 51J-3 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-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 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 .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.

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 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 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 is 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. If your 51J-4 or R-388 has excessive 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 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 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 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 the entire EPE adjustment on the bench with a simple test jig.    >>> >>> The test jig should be simple and 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. The PTO tunes exactly 3.000mc to 2.000mc in exactly ten turns of the PTO shaft. 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 and a fixed, small, transparent plastic index scale that has a scribed "zero" line is mounted to the PTO case. 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. 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.

If all you need to do is to adjust the EPE, then 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. Simple and relatively fast if all you need to do is an EPE adjustment.



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