Diversity Models - AR-88 Serial Number Analysis & Log
Sweep IF Alignment of the AR-88 Series - RF Tracking Alignment
Operating Multiple AR-88s in Diversity - Comparisons to Contemporaries
AR-88 Performance Today
- Using the CR-91 on Longwave
History of the AR-88 Series
RCA's greatest communications receiver creation was the AR-88, a receiver that achieved its renown by providing top performance and high reliability in service as a surveillance and intercept receiver during WWII and later as a "workhorse" for the RCA and Radiomarine Corporation of America coastal stations, usually in triple diversity receivers that provided world-wide ship-to-shore message handling. RCA's AR-88 planning may have chronologically followed their AR-77 ham receiver but the AR-88 owes much of its design concept as a replacement for RCA's aging commercial-military receiver, the AR-60. The AR-60 had been introduced in 1935 and was still being built as late as 1940. RCA had to update their "cost no object" military/commercial product and the AR-88 was the result. Design stages probably date from as early as 1939 and the demands of WWII in Europe pushed RCA into having the AR-88 ready by early 1941. The finalized AR-88 was a 14 tube superheterodyne that covered .54 to 32MC in six tuning ranges, featuring incredible sensitivity (even up to 10 meters), excellent stability and high fidelity audio. The electronic design was the work of Lester T. Fowler while George Blaker handled the mechanical design. The actual production during WWII was handled by RCA's Export Sales under Charles Roberts in Camden, New Jersey. Additional receivers were produced at RCA facilities in Bloomington, Indiana and Montreal, Quebec, Canada. Use of Majestic Radio and Television Company as a contractor during WWII production is possible, although this company is usually mistakenly identified as the Grigsby-Grunow Company (bankrupt in 1934.)
Outside the USA - Most of the early AR-88 production was sent to Great Britain or Russia (and to a lesser extent China and France) during WWII through Lend-Lease and this accounts for the scarcity of the early versions of the receiver in the USA. The Lend-Lease Act of October 1941, allowed the USA to supply materiel to our Allies in exchange for permission to build and operate bases in the allied countries or territories. The AR-88 was used extensively in Great Britain during WWII for varied purposes. Many of the Allies required coverage of the LF and MF parts of the spectrum and the AR-88LF was created for that service, providing coverage from 70kc to 550kc continuous and 1.5mc to 30mc continuous. Building of the AR-88LF receivers was handled by the RCA plant in Montreal. By the end of WWII, it certainly seemed like tens of thousands of AR-88 receivers had been shipped overseas to our allies. However, careful examination of serial numbers indicate a production level that was far less than the customary published estimates. While it may have seemed like "AR-88s were everywhere" the actual production numbers did not exceed 25,000 units (total WWII production of AR-88D, AR-88F, AR-88LF and CR-91 receivers.)
AR-88s survive in Russia because a large quantity were sent over as part of Lend-Lease in the later part of WWII (after the USSR became an Ally.) The receivers were used for both surveillance and communications during the war. After the war ended, it is assumed that none were returned and it's unlikely that very many were destroyed. The USSR continued to utilize the AR-88 after WWII as it had during the war, that is for military and surveillance purposes. By the late 1960s, the AR-88 was showing its age and the receivers must have become available to ham club stations as it was very common to QSO Russians on 20M CW who were using an AR-88 for the station receiver. During that time, most Russian hams operated from club stations.>>>
>>>Inside the USA - After WWII, RCA and Radiomarine Corporation of America (a division of RCA that handled all of RCA's maritime radio business and operations) continued to use the AR-88 and its variants in their own installations for various purposes. Most were in large coastal stations that provided worldwide ship-to-shore message handling via RCA Radiograms. Although single receiver operation was common, RCA/RMCA also utilized the AR-88 at installations in a triple diversity receiver designated the DR-89. In 1945, RCA replaced the AR-88 with the CR-88 which placed the Crystal Phasing control on the front panel and reduced the size of the RF Gain and AF Gain control knobs so all three controls would fit just below the tuning dial. The CR-88A replaced the AR-88F in the diversity receivers.
Even in the mid-1970s, these incredible diversity receivers were still being used in RCA/RMCA stations. Parts for maintaining the aging receivers were scrounged from the WWII repair depots that had been set-up in Tangiers and San Juan, Puerto Rico during WWII. Unfortunately, most (probably all) of these incredible Triple Diversity Receivers were scrapped out with usually only the receivers themselves surviving to be sold as surplus.
Even the US Military used some of the later AR-88 variations in their installations that required a high performance, highly reliable receiver. During the latter part of WWII the Navy used a Triple diversity receiver that was essentially the RCA DR-89 but was given the Navy designation of RDM. This diversity receiver was mainly used for data transmission in the form of CW, High-Speed CW and RTTY. Voice could be used but the RDM was primarily for reliable data reception. After WWII, the Navy continued to use the RDMs up to the 1970s. By 1949, the U.S. Army Signal Corps wanted their own version of the DR-89 for the same use as the Navy. RCA supplied a slightly updated version of the DR-89 that was designated as OA-58A/FRC. Not very many were produced with estimates being less than 100 OA-58s made. The receiver used was the SC-88 of which about 300 were produced.
Some AR-88s found their way into monitoring positions in several Shortwave BC stations around the world. By the early 1950s, the RCA '88 receiver was still one of the best for stability, sensitivity and high fidelity reproduction available.
With the modernized CR-88B, RCA began producing the last AR-88 version in 1951. The CR-88B, is the only variant to actually dramatically change the receiver, both in appearance and design. The CR-88B increased the tube count to fifteen, adding Push-Pull audio output. Also added was a 100kc Crystal Calibrator. Changes included a two-position Tone control, a three-position Selectivity switch and a different chassis layout that moved the power transformer forward behind the front panel. The CR-88B was in limited production until 1953 and it is the rarest of the entire series.
In the mid-1950s, the Chinese built very close copies of the CR-88 receiver, the WS-430. The front panel nomenclature is entirely in Chinese as is the data plate attached to the receiver's chassis. Russian "octal" tubes are used in the earlier versions but later receivers were equipped with some miniature tubes. Photo below in "Collector's Gallery of AR-88 Series Receivers."
Production level of the AR-88 series was rather high during WWII with approximately 25,000 total receivers built. After WWII, the demand was greatly reduced since the only users were commercial users and the military. The AR-88 series was never offered to the ham market and was generally not available as a new product to the average consumer. The serial numbers seem to indicate that post-WWII production was less than 10,000 total receivers and probably closer to about 5,000. This estimate brings the total AR-88 series production to around 30,000 receivers - far less than the normally quoted 100,000 plus receiver production.
So, what was the selling price of the AR-88? It seems to be a mystery lost in the bureaucracy of the Lend-Lease Act and later RCA commercial advertising. By comparing the AR-88 receiver construction to the RCA RBB and RBC series of Navy receivers one can get a general idea. These Navy 19 tube superhet receivers were built during WWII at a "cost no object" directive. At $2400 a piece in WWII dollars for the RBB or RBC, one can form the impression that RCA equipment wasn't cheap. I would estimate that the AR-88 probably cost at least 25% to 33% of an RBB receiver, about $600 to $800 in WWII dollars. Of course, this is just a guess. If anyone does know a specific price assigned to any of the AR-88 versions, please e-mail me and I will add that information to this article.
Today, the AR-88 and its variants can be found in ham shacks and at amateur SWL set-ups around the world. Its world-wide fame was earned with hard work and service. This hard work has resulted in many AR-88 survivors being found in rough condition, missing parts and almost certainly, non-functional. Fortunately, there are still enthusiasts that scavenge parts in order to perform operational restorations of these incredibly stout receivers. With fans around the world, the AR-88 and its variants are assured of continuing survival.
The Circuit and Construction Details - The AR-88 tunes from .54mc up to 32mc in six bands. It uses 14 tubes in a double preselection superheterodyne circuit. The HF front end coils are wound on polystyrene forms that have extremely low losses allowing the receiver to maintain high sensitivity up to 30mc. The gear reduction tuning is substantial and was often referred to as "continuous bandspread." A logging system using two separate dials provided accurate resetability.
The AR-88 provides five steps of selectivity with position 1 and 2 being rather broad for good fidelity while positions 3,4 and 5 use the crystal filter for increasingly narrow bandwidth. The receivers use three stages of 455kc IF amplification with stagger-tuned IF transformers. Two under-coupled IF transformers and two over-coupled IF transformers are utilized when the receiver is operated in the "BROAD" POS. 1 selectivity position. To assure that the passband is symmetrical usually requires a sweep generator and oscilloscope for proper alignment (a detailed procedure is provided in later version manuals) However, if fidelity is not an issue, there is a procedure to align the IF section using just a VTVM but the results are usually not as good as the sweep method.
AR-88 receivers also have a clipper-type Noise Limiter and a High Frequency (limiter) Tone control. The audio output is from a single 6K6 providing about 2.5 watts of power to a 2.5 ohm Z output transformer (600 ohm Z and Hi-Z phones outputs are also provided.) The various models sometimes had a Carrier Level meter incorporated into the circuit however most receivers didn't, initially due to a shortage of meters that occurred during WWII. The wiring for the meter was sometimes included in the harness for future installation of a Carrier Level meter, if they became available. Generally, the wires for the CL meter connection are bolted to the lamp bracket behind the receiver's illuminated ID window. Diversity model receivers did not incorporate a CL meter because output meters were included in the diversity rack.
The receiver power supply uses a potted power transformer and two potted filter chokes. The filtering is provided by an oil-filled, triple unit utilizing paper capacitors. Only the AR-88LF and the CR-91 receivers have AC line fuses that are chassis mounted. All other versions required the user to provide a fused AC line. A VT-150 is used to provide a regulated +150vdc for the LO and BFO plates and for the RF/IF screens to improve stability and reduce drift.
The AR-88LF and CR-91 were versions with LF and MF coverage in place of the AM-BC band using a 735kc IF to allow continuous coverage in two tuning ranges from 70kc up to 550kc in the LF and MF part of the spectrum and continuous coverage from 1.5mc up to 30mc on the remaining four tuning ranges. The CR-91 uses a 6V6 in the audio output. Early versions of the AR-88LF use a different power transformer with a two position AC primary voltage selector switch and a different audio output transformer that has a single tapped winding providing 2.5 ohm Z and 20 ohm Z outputs.
Mechanically, the receivers were stoutly built. Heavy steel chassis and an almost quarter of an inch thick, copper-plated steel front panel were the foundation for component assembly mountings that are entirely put together with screws, lock washers and nuts. This was to allow extensive disassembly to be easily and quickly done, the repairs performed, followed by easy and quick reassembly. The only rivets used in the receiver are for the clips that mount the adjustment tools. Early receivers had the chassis side panels bolted in place but late receivers will have the chassis side panels spot-welded to the chassis. The ultra-heavy duty construction made for a stable receiver but also added to the weight. The AR-88 weighs in at just over 100 lbs. when installed in its cabinet.
The Diversity Receivers - Many of the AR-88 receivers were used in Triple Diversity Receivers like the DR-89 - a seven foot tall rack loaded with three AR-88F receivers and all of the auxiliary equipment necessary for professional diversity reception. The DR-89 was initially produced during the latter part of WWII. As with the single AR-88 receivers, DR-89s were sent to our Allies during WWII as part of the Lend-Lease Act of 1941. The US Navy also used the diversity receivers with the designation of RDM. The receivers used in the DR-89/RDM are slightly different from the standard "stand alone" receiver. For instance, even if meters had been available, the Diversity AR-88 receivers would not have Carrier Level meters installed because the Diode Load from each receiver was connected to the Tone Keyer of the DR-89/RDM rack where the signal was routed to the Monitoring Unit which contained three Output Level Current Meters, one for each receiver. All AR-88 receivers (and their variations, e.g., CR-88A) that were intended for use in the RCA Triple Diversity Receivers will have a "DIVERSITY IF GAIN" control on the front panel for balancing the three receivers in the rack This provided a method of adjustment for equal diversity effect (referencing the desired signal) even if the receivers and antennas were not exactly identical in their performance. There are some resistor changes in the IF section to enhance diversity and AVC characteristics. The U.S. Army Signal Corps had their versions of the Triple Diversity DR-89 with the Signal Corps ID of OA-58A/FRC. These diversity set-ups used a different, upgraded receiver, the SC-88.
Not all Diversity Receivers were used exclusively in the Triple Diversity Receivers, however. Shown in the photo above is the RCAF listening station #5 Radio Unit at Whitehorse, YK, Canada from the 1950s showing the CR-88A "diversity" version receiver used along with the Hammarlund SP-600 JX receiver. Note that the CR-88 has the "Diversity IF Gain" control which identifies the receiver as a Diversity model (no CL meter either.) When operated as individual receivers, a diversity receiver's performance will be identical to standard non-diversity receivers. It seems likely that RCA supplied Diversity models for many applications other than solely for the Triple Diversity Receivers. More Details on the "Triple Diversity Receivers" below.
Details of the Individual Receiver Versions by Model Designation
The earliest stand-alone receiver version which may or may not have a
cabinet depending on installation. Produced early part of WWII,
earliest versions have solid light-yellow dial (not alternating black and
yellow scales) and possibly the front panel nomenclature might be
CR-91A - Post-WWII upgraded version with front panel Crystal Filter
Phasing, gray panel, other post-WWII upgrades. Successor to AR-88LF with
all (?) versions built in Montreal, Quebec, Canada.
AR-88 Series Chassis
Shown in the photos to the right is the chassis of a CR-91 receiver. All versions except the CR-88B receiver, are virtually identical to each other. The RF cover is removed in the nearest photo to reveal the tuning condenser cover. Normally, both filter chokes are identical however the round choke is a replacement. The middle photo show the underside with the shield removed from the RF and Mixer coil box. If the receiver was rack mounted then a bottom cover was installed. The photo far right show a close-up of the polystyrene coil forms used in the high frequency coils.
General Information About the AR-88 Receivers
|Manuals and Documentation - Like most RCA products of the forties and fifties, documentation is very good on the AR-88 series. There are minor errors in some of the early manuals - like two R50 resistors shown in the under-chassis component view - but, overall the documentation is easy to find and accurate. Original RCA manuals are high-quality publications with heavy covers. The figure drawings are very clear. There are some British Military manuals available that give excellent information and also a slightly different approach to alignment. Almost all of the documentation is available online at the BAMA mirror website. The exception is the R-320/FRC (SC-88) manual, TM 11-899. Fortunately, this is available from Fair Radio Sales. None of the early AR-88 series manuals provide detailed wiring diagrams which are different from schematics and are helpful in locating specific component placement or wire routing. The CR-88 (and later) manuals do provide some wiring diagrams. On early receivers, one has to use the schematic and component location drawings to figure out the correct wire routing and correct component locations. This is only a problem in receivers that have been modified or compromised by poor repair techniques over the years and need to be returned to their original configuration.||
Colors and Cabinets - The first AR-88 receivers had black
wrinkle finish panels. The nomenclature was normally silk screened in white on
most receivers but not all. Many engraved nomenclature panels were made
and supplied with the early AR-88 receivers that were exported. It
doesn't appear that all export AR-88 panels were engraved but it
certainly is fairly common to find engraved panels on early WWII
smooth black finish panels with engraved nomenclature show up in later
WWII production. Not enough engraved nomenclature panels have been
reported to see if they were a product of a specific plant or contractor,
or a specific time period. At the moment, all we can say is that
engraved panels will be encountered. All
post-WWII AR-88 panels are silk-screened nomenclature. Some of the
receivers that were used in the Navy RDM receivers had smooth gloss
finish black panels but many were also black wrinkle. After WWII, gray
panels started to be produced. The plastic panel that covers the dials
was changed to dark gray when installed on the gray panel receivers and
remained black when installed on black panel receivers. Most of the CR-88 versions have
smooth finish gray
panels but there are some exceptions. The RCA "umber" finishes actually have quite a bit
of brown mixed in giving the panel a distinctive brownish-gray look (see
CR-88A photo below in "AR-88 Performance Today.")
However, traditional gray is also seen on some of the CR-88, CR-91A and
later AR-88LF receivers. RCA also produced a medium darker brown wrinkle finish version of the
CR-88 that was probably for their own broadcast installations. The
Royal Canadian Air Force (and possibly the RAF) had some of their AR-88 receiver panels finished in
smooth gloss finish dark blue. There are many other colors that show up
from time to time, especially cream and very light gray both with black
nomenclature, but without a physical inspection it is impossible to tell
if those colors are original from the factory. Silk screened
nomenclature would generally confirm originality of the panel paint. There are also reports
of a black bakelite panel used on an AR-88, though this would have
presented several electro-mechanical issues to deal with. More than
likely the report of a bakelite panel stemmed from an erroneous
identification of the gloss black panels used on some RDM component
receivers. Factory original panels seem to all be
various color paint over copper-plated steel with
silk-screened nomenclature (with the exception of some WWII production
that had engraved nomenclature rather than silk-screened.)
Original cabinets are rare and are usually black wrinkle finish however gray and "RCA umber" cabinets do turn up and the CR-88 that was original medium brown wrinkle also had a matching brown wrinkle finish cabinet. Since the AR-88 chassis is 17" deep very few cabinets other than originals will work. The only fairly "easy to find" cabinet is the type used on the WWII Scott Radio Laboratories SLR receivers. A little cutting in the rear is required for the Scott SLR cabinet to work but it is deep enough (the cutting is for clearance that is needed for the AR-88 AC Voltage selector switch.)
|Antenna Input Connections - The AR-88 receivers that were intended for surveillance or intercept work are connected up and operate much like any other vintage communications receiver. The Antenna Input Z is 200 ohms but the receivers have an Antenna Trim control that allows a wide variety of antennas to perform well with the receiver. Band 1 (AM BC) was designed to work well with an end- fed wire antenna but on the higher frequencies, a matched antenna will give the best results and use of a separate antenna coupler will provide the receiver with the best match to any antenna. The AR-88LF and CR-91 receivers were designed for a capacitive antenna on Bands 1 and 2, which cover the LF and MF frequencies. Anywhere from 500pf to 700pf works fine and can be accomplished with an end-fed antenna around 200 feet in length. Shorter antennas will detune the 1RF amplifier unless capacitively loaded..||Tuning and Logging Dials - The very first AR-88 tuning dials were solid light-yellow with black nomenclature. These dials were used probably up to about serial number 3000. By late-WWII production, the tuning dials became alternating black and light-yellow backgrounds with the nomenclature oppositely alternating accordingly. After that, all versions used the black and yellow "striped" dial until the dial mask was fitted to the SC-88 and the CR-88B. These receivers went back to the solid light-yellow dial with black nomenclature. The dial color we see today on the AR-88 receivers is much darker than original. The darkening is especially intense where the dial was exposed to sunlight. This usually resulted in portions of the dial appearing very dark amber and other sections appearing orange or yellow. Original color was very light yellow.||Power Supply - The power transformer is robust and a potted unit that seldom has any problems. The primary has five taps that are routed to a rear panel switch that selects the proper AC operating voltage. The AC line is not fused on any of the receiver versions except the AR-88F and the CR-91. Two filter chokes are used. Some British magazine articles suggest that very early AR-88s had two different types of chokes. It's reported that the choke's dc resistance was 400 ohms for L-49 and 800 ohms for L-50. However, every manual and parts list I've seen indicates that the two chokes are identical with 400 ohms dc resistance. Actual measurements of the chokes in various AR-88s show the same result - that both chokes are identical. It's normal for L-50 to run slightly warmer than L-49, especially if the receiver has all original capacitors. The filter capacitors change over the years with early receivers using an oil filled, three capacitor pack. Later versions have three individual oil filled capacitors with two capacitors on top of the chassis and one capacitor underneath.|
|Tuning Dial Resolution - The AR-88 receivers have what was called "continuous bandspread" action, which was a large gear reduction in the tuning that resulted in an almost vernier effect for tuning in stations. Since the AR-88 covers large slices of the spectrum in each tuning range, the tuning dial resolution is necessarily vague. When the AR-88s were in active use and the received frequency had to be accurately known (or set,) it was measured with a heterodyne frequency meter - all stations had them when frequency accuracy was mandatory. The Logging Dial then provided a very accurate way to reset to exact frequencies once they were known.||Tuning and Logging Dial Positions - If the AR-88 front panel is removed, it will be noted that the two dials over-lap each other and it's possible to have either the logging dial in front of the main dial or behind it. If the receiver is the older style with the metal frame holder for the dial index then normally the logging dial should be placed behind the main dial. This assures that there is ample clearance for both dials to not interfere with each other and to also assure that the logging dial index doesn't rub against the logging dial hub. Later receivers with the plastic index usually have the logging dial in front of the main dial since the logging dial can be placed very close to the plastic index without rubbing. This latter positioning is also required on the receivers with the dial mask installation.||CRV-46246A/B (RDM) Receiver 2.5 Ohm Z Output - These component receivers for the RDM were designed to work within the Triple Diversity Receiver rack. Specifically to connect the 2.5 ohm audio output terminals to the Tone Keyer input. Terminal 2 on the receiver is not grounded but achieves its ground inside the Tone Keyer. This was then switch routed to the Speaker Panel where the operator could select individual receiver outputs or the diversity output. Probably most CRV-46246A/B receivers have long since been modified to internally have Terminal 2 connected to the receiver chassis to allow the receiver to operate as a "stand-alone" receiver. However, it's possible a few receivers are still out there with the Terminal 2 ungrounded at the receiver. The ungrounded Terminal 2 may also apply to the commercial version AR-88F receiver. Later diversity receivers, CR-88A and SC-88, have Terminal 2 grounded at the receiver.|
|2.5 Z Ohm Output and 600 Z Ohm Output - The required speaker impedance for the AR-88 is rather low at 2.5 ohms to 3.2 ohms. Today it is pretty hard to find a 2.5 ohm speaker and the original AR-88 table speakers are quite rare. The 3.2 ohm Z speakers are fairly easy to find and work quite well with the AR-88. I have also found that the standard 4 ohm speakers work fine and allow a lot more choices for quality speaker selection. Using a 12" speaker in a floor cabinet with a bass reflex port will provide you with the excellent audio reproduction that the AR-88 is capable of. Lots of bass and, if aligned correctly, plenty of highs in the BROAD position. Using an 8 ohm Z (or higher) speaker will require the AF gain to be advanced quite a bit further than with the lower Z speakers. The lower Z speakers work best. The 600 ohm impedance output was provided to drive various devices or monitor functions via earphones and is not really a high quality audio source. I have always found it to sound a little "fuzzy" when compared to the 2.5 Z ohm output.||
|Carrier Level Meter
- Very few AR-88 receivers had an original carrier level meter installed. At
first it was the WWII meter shortage that was responsible. Then, after
WWII, most of the RCA production of AR-88 series receivers were for
diversity receivers where the carrier level meter was not required. As a
result, only a handful of meters were ever installed in AR-88 receivers
by RCA. The meter, when an original RCA installation, is a right-hand
mechanical zero with full scale
deflection to -6DB requiring 5mA*. The meter scale is -6 to 0 to +100 db and also
on the scale is marked "DB ABOVE 1 MICROVOLT" The meter is illuminated and the scale
is light yellow-amber. As mentioned elsewhere, the carrier level meter
wiring is included in the early production receivers with the two wires
having their terminals bolted to the rear of the lamp bracket for the
receiver ID plate. See schematic right for CL meter connections and
resistor changes necessary. R21 is mounted on the rear chassis apron.
* One can't help noticing that the AR-88 CL meter has the same specifications as the Hallicrafters SX-28 S-meter - hmmm.
photo left: AR-88 CL Meter from EB5AVG's AR-88D receiver. Photo by: EB5AVG
|Crystal Filter Crystals - For some reason many AR-88s will have had the crystal filter crystal removed. On the last models this might be understandable since the crystal plugs into a socket on top of the chassis. But the early versions have the crystal hard-wired underneath the chassis and, still, sometimes they will be missing. Fortunately, the 455.00 Khz crystal is available from International Crystal Manufacturing in OKC, OK. The 455.00 Khz crystal is a stock item and the cost is $25.00. If you need the 735 Khz crystal for the AR-88LF or CR-91, that one has to be special ordered from ICM. They will make one for you but as custom work, they are not cheap - $109.00. Remember, they have to grind the crystal and the low frequency crystals do require a different cut from the quartz, thus the expense.||Diversity Receivers versus Standard Receivers - The AR-88F, CR-88A and the SC-88 receivers are slightly different receivers from the standard AR-88D, AR-88LF and CR-91 and other non-diversity receivers. Several resistor values are changed and some extra components added to enhance diversity operation. As individual receivers both types perform almost identically. All diversity receivers were normally rack mounted although there were certainly some exceptions, especially after WWII. Though the Standard Receivers do provide the AVC line output on a terminal marked "AVC" and this can be coupled to another standard receiver's AVC line, this only provides a crude form of diversity effect. In this type of diversity reception, the receiver with the strongest signal controls the AVC line and the gain of all of the receivers. This does work to control fading to a certain limit determined by the AVC delay and antenna types used. True diversity reception requires the detectors of each receiver to be tied together as the Diode Load and the AVC lines tied together for diversity system stability. When working on either a standard receiver or a diversity version of the AR-88 series be sure to use the correct schematic since there are many subtle differences between the receivers.|
|The Diversity IF Gain Control - This control is normally only on the component receivers used in the RCA Triple Diversity Receivers - the DR-89, the Navy RDM and the Signal Corps OA-58A/FRC. This control adjusts the gain through the IF section of the receivers by using a variable resistance in the cathode circuit of the 1st and 2nd IF amplifiers. The Diversity IF Gain control allowed the operator to "balance" the three receivers in the rack using a test signal (usually the incoming signal.) Once adjusted, each of the three receivers responding with the same gain to the same signal which further enhanced the diversity effect. When the receivers are operated individually, the Diversity IF Gain can be advanced fully. Apparently, later in production the Diversity receivers were sometimes sold as individual receiver to certain customers. Consequently, some Diversity receivers might be found mounted in the proper RCA AR-88 cabinet as an original combination.||Diversity, Monitor, Diode Load and Diode Return Connections - These connections on the rear terminal strip are only on the component receivers of the RCA Triple Diversity Receivers. When operating in diversity, it is necessary to have all of the second detectors outputs tied together which is done through the "Diode Load" and "Diode Return" connections. Additionally, the AVC lines from each receiver are tied together via the "Diversity" terminal. These receivers can be operated as a stand-alone receiver but it will be necessary to jumper the "Diode Load" and "Diode Return" terminals together. No connections to the "Diversity" terminal or the "Monitor" output are necessary for individual operation.|
|Tuning Gear Box Variations
- There are two types of gear boxes found on the AR-88 series of
receivers. The most commonly found gear box is "Version One" (V-1) that,
fortunately, is the most robust and problem-free type. The V-1 gear box
is found on all of the AR-88 series receivers except for the CR-91A. V-1
gear boxes will have a 360º split-gear that is spring-coupled and
mounted to the hub of the tuning condenser drive gear. This split-gear
the main tuning dial as the tuning condenser drive gear rotates. The fact that the
split-gear is a full 360º piece adds to its overall strength. The
V-1 gear box seldom has any problems other than the normal dirt and dust
accumulation and occasionally an over-tight bearing adjustment.
"Version Two" (V-2) gear boxes are only found on the CR-91A receivers, all of which were built in Montreal. The V-2 gear box has a 270º split-gear that is spring-coupled to drive the main tuning dial from the rotation of the tuning condenser drive gear. Since the V-2 split-gear is not a complete "circle" (not 360º) it's not nearly as strong as the full 360º split-gear in the V-1 gear boxes. The V-2 split-gear is prone to breaking teeth off of the gear that will result in erratic tuning dial operation. Of two V-2 gear boxes examined, both had broken teeth on the split-gear and neither gear box operated the main tuning dial smoothly. Since only two CR-91A gear boxes have been examined, it isn't known if all CR-91A receivers built had the V-2 gear box installed. It's likely that the V-2 gear box was considered a design "improvement" and may only be found on the later versions of the CR-91A.At present, I have examined an AR-88, CR-88A, CR-91 (Camden version) and an SC-88. All of these receivers have the V-1 gear box installed. Note that the SC-88 is from 1950 and is one of the last versions of the AR-88 receiver built in the USA and it's using a V-1 gear box. The two CR-91A gear boxes were obtained from VE8NSD and were "pulled parts" from Canadian government surplus purchased CR-91A receivers. The conclusion is that the V-2 gear box will only be found on some versions of the CR-91A receiver.
photo left: "Version One" (V-1) of the gear box showing the full 360º split-gear that drives the main tuning dial. Note that the split-gear is mounted to the hub of the tuning condenser drive gear which appears to take up most of the lower part of the photo and is fairly reflective. The split-gear engages a gear that is mounted on the end of the shaft that drives the main tuning dial. Also, note that the anti-backlash spring is a loop type coupling the two split-gears.
photo right: "Version Two" (V-2) of the gear box showing the 270º split gear that is not as robust and prone to failure. Note that these split gears are coupled using a coil spring type of anti-backlash arrangement. It can be seen that the split-gear is not as closely coupled and this may account for its tendency to break some of the gear teeth.
|Operating with Missing Shields - Many AR-88s are nowadays missing the bottom cover, the RF cover and some are even missing the tuning condenser cover. If the receiver was installed in a cabinet, such as the AR-88D or CR-91 were, then the bottom cover was not usually installed since the cabinet acted as the bottom shield. Additionally, the LO and Mixer/RF coil boxes are fully shielded, so the bottom cover is more for component protection than anything else. The receiver seems to operate fine with the RF cover off or missing, too. Its main purpose was to protect the alignment adjustments. The condenser cover should be in place for protection of the tuning condenser but a great many are missing and the receiver's performance doesn't seem to suffer with it off. Originally, the extensive use of shielding was to prevent LO radiation which then allowed the receiver to operate in the present of other receivers and transmitters without causing interference or being interfered with. Also, if excessive LO radiation made it to the antenna, it was possible for enemy Direction Finding equipment to pinpoint the receiver's location. Today, many AR-88s are operated in the ham shack with some of the shields missing with no serious issues. Since the AR-88 series probably should be operated with an electronic T-R switch, in the Voice mode, audio feedback is usually encountered in "transmit." Normally, when using the AR-88 in the ham shack, it is necessary to reduce the RF and AF gain controls during "transmit" to prevent feedback even if all of the shields are installed.||"TRANS" Relay Control in NON-Diversity Receivers
- This function operates in conjunction with terminals 3 and 4 on the
rear of the chassis that are marked "TRANS RELAY." RCA, during
the design of the AR-88, decided that the receiver should be controlling
the transmitter and so the "TRANS" position of the function switch
shorts terminals 3 and 4 when selected. Unfortunately, receiver control
of transmission is the opposite of today's normal transmitter control of ham station's "Send-Receive" operations.
Though it is possible to have an AR-88 actuate the remote Push-to-Talk
line, allowing the AR-88 to control the "Send-Receive" operation, many transmitters don't
have the PTT line brought out as a "remote" function. The
easiest solution is to operate the station using an electronic TR switch
such as the type made by E.F. Johnson. This allows the receiver to be
left on while transmitting and the electronic TR switch will protect the
receiver input. But, if you can't find or build an electronic TR
switch, many operators just resign themselves to individually
switching both the transmitter and receiver for the "Send-Receive"
operation. See section "Operational and Modification Caveats" below for possible problems
that can develop when the receiver is in the "TRANS" position.
Additional Note: On some versions of the Diversity receivers, the two wires from the FUNCTION switch that provide a closure when selecting "TRANS" are included in the wiring harness and are both soldered to Terminal 2. In early Diversity receivers, the 2.5 ohm Z audio ground was not provided by Terminal 2 but was achieved via the Tone Keyer. RCA decided to attach the "TRANS" wires from the harness to this terminal since it was going to be grounded in normal rack wiring and operation.
The Triple Diversity Receivers
|A Brief History
of Diversity Reception - In the mid-1920s, two RCA
engineers, Harold Beverage and H. O. Peterson, noticed that when they
listened to Peterson's radio receiver, located in his home, over a telephone line
to the RCA station in New York and simultaneously listened to the same
station on RCA receivers, the
two separated station receivers showed different fading characteristics. The two
stations were about a half of a mile apart. Beverage and Peterson
continued with several experiments and over the next few years developed
"Diversity Reception" - a method of reducing or eliminating fading radio
Just what causes radio signals to fade? When a received radio signal originates from beyond the horizon it must either have traveled along the curvature of the earth as longwave signals do or it must have been refracted through the ionosphere as shortwave signals do. The ionosphere is made up of several layers that refract shortwave radio signals from different distances above the earth depending on the time of day, the time of year and where in the sunspot cycle we happen to be. Many times, radio waves are refracted from several layers simultaneously. The radio wave is then returned to earth having traveled different paths and different distances and consequently arriving at the receiver's antenna at slightly different times. The radio signal then has several components that will set-up phase differences along a single antenna and when these phases conflict, signal cancellation occurs. When the phases add, the signal increases. Since the ionosphere is in a constant flux, likewise the radio signal refraction varies and we see the result as fading radio signals.
In addition to multipath-distance refraction causing phase conflicts on a single antenna resulting in fading, other signal anomalies occur. Wave rotation through the ionosphere will cause polarization changes that can affect the signal versus antenna relationship. Selective fading is where certain frequency components of a radio signal are refracted differently through the ionosphere resulting in a portion of the signal fading while other components of the radio wave are not affected.
Beverage and Peterson analyzed strip chart recordings of several receivers with multiple antennas and noticed that larger arrays only made the fading worse. The problem was the phase conflicts on a single antenna - no matter how large that single antenna was. To eliminate phase conflicts required widely separated antennas that would see different phases of the received signals and a way to combine these antennas into a single signal with no fading. >>>
| >>> Using multiple antennas and receivers was how Beverage and Peterson solved the
fading problem. By allowing the Antenna to Receiver phase difference to
remain separated until each receiver's detector circuit the individual
fading response was maintained. At the detector, the signal is converted
to an audio frequency and the different phases become irrelevant.
Combining the multiple detector outputs was simple since each was basically a
diode and their outputs could be tied together. Now, whichever signal was
strongest, dominated the multiple detector output and fading was greatly reduced.
It was noted that a gain control of the entire system was lacking and many times some of the fading characteristics were passed through the receivers due to additive phase combining on one antenna. Using the AVC circuits as a feedback gain control allowed all of the receivers to share the same AVC line. This then allowed the receiver with the highest level response to the desired signal to control the AVC line and therefore the overall receiver gain. Fading was under control and practically eliminated as each receiver seamlessly controlled the AVC control based on which one was receiving the signal best.
Beverage and Peterson eventually decided that three receivers were the ideal combination. Dual Diversity would reduce fading by about 90% while Triple Diversity reduced fading by 99%, or almost total elimination of fading. Antenna separation was specified at 1000 feet for commercial and some military installations. However, usable Diversity Effect can be experienced with as little as one wavelength of antenna separation. This type of diversity reception was generally called "Space Diversity" since it depended on antenna separation for diversity effect. By 1930, RCA had 26 Triple Diversity Receivers at Rivershead. NY servicing over 40 different countries around the world with Radiogram communications. The received diversity signals were multiplexed and sent down the telephone lines to the RCA offices in New York City where they were copied and then sent on to their destination.
Diversity reception was the standard for world-wide reliable communications from 1930 up into the 1970s, both in military applications and in commercial use. By the late-1970s, telephone/fax technology advanced to the point where telegrams were no longer the only cheap method of communications. At that time, large scale diversity installations with multiple receivers became a financial burden since the world-wide radio-telegram communications systems that had paid for their necessary existence had become obsolete.
DR-89, RDM and OA-58A/FRC
photo above: The receiving station at Vilnius, Lithuania, ca.1960s. The two center racks are RCA DR-89 Triple Diversity Receivers. The end racks carry Russian-made equipment (R-250 M2.) The receivers were used to monitor and select SW stations to be "jammed." Photo from: www.radiojamming.info
>>> The Tone Keyer uses the signal amplitude variations of a CW signal received without BFO action and uses that amplitude variation to "key" a circuit whose output is a pure tone representative of the incoming CW signal. No background noise or random minor interferences make it through the Tone Keyer. These devices were mainly used for machine copy of CW (or high-speed mechanically sent CW or RTTY) and only occasionally for human copy of CW.
Also included in the rack was a Monitoring Unit. This device picked the signal up from between the 1st and 2nd IF amplifiers ("MONITOR" output on rear of chassis) of each AR-88F and routed it through its own IF section with detector-AVC , BFO and Audio Output. This allowed the operator to monitor the signals without disturbing the diversity set-up of the receivers. The three meters monitored the Diode Load current that was routed to the Tone Keyer.
Before actual reception of signals in diversity, the DR-89 had to be "balanced." This required the operator to accurately tune in the desired station on all three receivers. Then, using the "DIVERSITY IF GAIN" control on each receiver, the operator would adjust the output of each receiver to be equal as measured on the Meters of the Monitoring Unit (although the meters are connected through the Tone Keyer.) This compensated for differences in individual receiver/antenna overall gain and then allowed of equal diversity action of the three combined receivers.
At the bottom of the rack is the speaker panel which allowed monitoring of individual receivers or one selected receiver as the diversity audio output of all three receivers. Also, at the very bottom was a power supply to operate the Monitoring Unit and at the very top of the rack was a patch panel for various antenna inputs.
Other Triple Diversity Receivers
RDM - U.S. Navy designation for the DR-89. The Navy version is identical to the RCA DR-89. Mostly used during WWII. The receivers are identified as CRV-46246B and are either AR-88F or later CR-88A models. The photos show the RDM. Documentation became "Unclassified" in June 1947.
OA-58A/FRC - Signal Corps Triple Diversity set-up using three SC-88 (Signal Corps designation is R-320/FRC) receivers from 1949-1950. The auxiliary components are slightly changed in appearance but have same design and function as in the DR-89 and the RDM. The R-320/FRC is an updated version of the AR-88F. A photo of the OA-58A/FRC is shown below.
Diversity Receiver Components
Shown in the photo right is the Loudspeaker Assembly. This unit provided the operator with the ability to monitor the audio from the three receivers operating in combined diversity or to monitor each receiver individually.
|Shown in the photo left is the Monitoring Unit. This device had its own IF section, Detector, BFO and Audio Output. The Monitoring Unit was connected to the "Monitor" output on each receiver which was a connection between the 1st IF amplifier and the 2nd IF amplifier. This allowed the operator to monitor each receiver individually without upsetting the diversity reception. Separate power supply at bottom of rack.|
SC-88 - SIGNAL CORPS - R-320/FRC
Shown in the photo to the right is the SC-88 receiver. It's actual designation is R-320/FRC and it was used in the Signal Corps OA-58A/FRC triple diversity receiver. The SC-88 is one of the last of the AR-88 versions produced with most receivers built between 1949 and 1950. The SC-88 receiver has the later feature of the Crystal Phasing control on the front panel with smaller knobs for the RF and AF Gain. The SC-88 was the first version to have the "Band-in-Use" Dial Mask. The mask has the frequency ranges printed on it so the frequency ranges were eliminated from the front panel RANGE switch nomenclature. The receivers were MFP'd and the chassis appears gold colored because of it. Note the small knob on the lower right of the panel, between and slightly lower than the SELECTIVITY and AVC-MAN controls. This is the "DIVERSITY IF GAIN" control found on ALL receivers that were used in the RCA Triple Diversity Receivers. All of the SC-88 receivers were originally rack mounted configuration. Additionally, all known SC-88 examples have (or had) the Signal Corps R-320/FRC metal tag on the front panel, top-center.
One obvious change from the typical AR-88 can be seen in the photo below-left showing the SC-88 chassis. There are two individual filter capacitors on top of the chassis and one filter capacitor under the chassis. These were to replace the earlier filter capacitor unit that had all three filter capacitors inside the same can. This may have been more of an economics issue besides reliability. With the failure of one filter capacitor the entire unit had to be replaced in the earlier receivers. Later receivers, like the SC-88, only require that the defective capacitor be replaced. Another change is the BFO can which doesn't have the removable cap on the top of the can. Also, the BFO is now coupled to the third IF amplifier grid with a 10pf capacitor (rather than using electrostatic coupling.) Additionally, the mounting of the main dial lamp is now on a bracket rather than clipped to the front plate of the gear box. Located just behind the 1st IF transformer is the plug-in, HC-6 type, 455kc crystal for the crystal filter. In the earlier models of the AR-88 series, the crystal is mounted under the chassis. It is placed in a clip but is hard-wired into the circuit. The alignment adjustments for the 1st RF stage are now mounted on top of the chassis (under the RF cover) rather than having to access them through holes in the rear of the chassis. The "plunger type" trimmer capacitors are replaced with standard trimmer capacitors. "Cool Ohm" resistors are used in place of "Ohmite" resistors. Since a dial mask is used, the main dial is not alternating black and yellow band scales like the standard AR-88. Instead, the dial is entirely yellow. The front panel is painted ultra-fine wrinkle finish black.
The total production of SC-88 receivers was quite small with estimates usually being around 300 receivers built. The U.S. Army Signal Corps manual for the R-320/FRC is TM11-899. This SC-88 is serial number 214. It was missing the 455kc crystal when I got it in 1985 but it has since been replaced. The receiver is all original (except the crystal) and I've used it over the years for SWLing, AMBC listening and even occasionally as an AM Ham receiver on the air. >>>
>>> I first saw SC-88 serial number 214 on a shelf above the workbench in the workshop of my old radio collector friend, Fred Winkler, back in 1975. Fred had purchased the RCA SC-88 "surplus" several years before and used it for listening to KVLV from Fallon, Nevada. Over the years, I'd always see the old SC-88 sitting there accumulating more and more dust but still playing strong. Fred had a house fire in 1984 that destroyed most of his radio collection. His workshop was undamaged, though. After Fred rebuilt his house, for some reason he asked me if I wanted his old SC-88 and, of course, I was thrilled to obtain the receiver that I had admired over the past decade. Nowadays, I've had SC-88 serial number 214 for nearly 25 years - it still performs great and often reminds me of old Fred who became an SK in 2002.
The photos below show the top of the SC-88 chassis and the underneath of the chassis. All shields are shown installed except for the bottom cover which was removed for the photograph. The shields kept the LO radiation at an extremely low level so the receivers could operate in the presence of multiple receiver installations with no interference generated or received.
AR-88 Series - Serial Numbers and Serial Number Log
|RCA serial numbering contains more information than just an identification of a particular receiver. Through the work of Alan Ford, VK2DRR, who has collected a large sample of AR-88 serial numbers from all over the world, a theory of the methodology of the RCA numbering system has come forth. Much of what is presented here is from Alan Ford's analysis.|
|The typical AR-88 serial number contains six digits that
include leading zeros. However, the six digits are not a numerical
representation of when in the production cycle the receiver was
manufactured. It appears that the most significant digit of the serial
number (sixth digit from the right) represents which RCA plant built the
receiver. This seems to be true because only two digits are used in the
most significant place, "0" or "1." This could represent the plants at
Camden and Bloomington. Canadian AR-88LFs built in Montreal are all serialized with a "C"
in the most significant place (as a prefix.) However, many AR-88 receivers are
serialized with only five digits. Could the "blank" in the sixth
position signify another assembly plant? Several sources indicate that
the Grigsby-Grunow Company was used as a contractor for AR-88 assembly
but this company was bankrupt and in receivership in December 1934. It
might be possible that some confusion exists between the two "Majestic"
companies. Grigsby-Grunow used "Majestic" as their trade name from the
late twenties up to 1934. In 1937, "Majestic Radio and Television
Company" was formed. It may be possible that the wartime contacts were
partially built by this later "Majestic" company and it was mistakenly
identified as the bankrupt Grigsby-Grunow "Majestic" company. For now,
we'll assume that MR&T Co. was used as a contractor. This then
accounts for the four possibilities used as the most significant place,
"C," "blank," "0" or "1."
When looking at the fifth digit from the right, the numeral is never more than 1 on the AR-88 types. With the AR-88D and F versions, it appears that the fifth digit is merely used when the production quantity is 10,000 and higher. However, if considering other models, such as the early CR-91 with the serial number 050068, could "5" in the fifth digit represent all early CR-91 models? It is impossible that 50,000 CR-91 receivers were built and therefore the "5" must represent something else. Since the serial number sequence for the CR-91 is quite different from the AR-88s, it can't be part of the AR-88 serial numbering roster. Perhaps it's a different sequence or roster just for the CR-91. At present more samples of the early CR-91 receiver serial numbers are needed to show whether the "5" in our one serial number example is unique or is always present.
Here are some observations by Alan Ford, VK2DRR, regarding AR-88D and AR-88F models...
1. The 6th digit from the right (where it exists) is never more than
The serial numbers reported so far suggests that at least 14,215 receivers were made. Although the 5 digit system allows for 99,999 receivers, so far there are none at all from the 20,000 upwards (you'd expect even this small sample to be fairly random) so I think the theory that 100,000 receivers were made might not be correct.
Alan has concentrated on the AR-88D, AR-88F and AR-88LF models of the receiver and their serial number assignments. Not long after WWII ended, RCA made some upgrades to the AR-88 series of receivers. The new receivers have different designations as CR-88, CR-88A, CR-88B, CR91A and SC-88. >>>
|>>> The ultimate use and the demand
for a high quality communications receiver had certainly changed and production quantity was much less than
with the WWII
versions of the receiver. The serial number sequence changed with the
introduction of these updated receivers.
The new serial number sequence begins again at some low number. The serial numbers still use six digits incorporating leading zeros as the earlier receivers did.. It's likely that the same manufacturing plant designator is assigned to the most significant place (the sixth from the right) on the early receivers made just after WWII. A CR-88A receiver has been reported that has only four digits in its serial number, implying two blanks ahead of the number. At this time, not enough serial numbers have been gathered to infer whether the two blanks indicate assembly plant and model or whether RCA just dropped the encoding altogether. Perhaps, late in production since demand was obviously much less than during WWII, only one USA assembly plant was used and therefore it wasn't necessary to identify it in the serial number.
It appears that all CR-91A receivers were built in Montreal and serialized (for awhile) like the AR-88LFs produced there, that is with a metal tag mounted to the front panel of the receiver and a serial number with "C" as a prefix. Later production CR-91A receivers dropped the "C" prefix. This was probably following what RCA in the USA was doing with the CR-88 receivers serial numbers and was due to the reduced demand for the receivers.
Another exception to the "six digit stamped serial number on the chassis" is the 1949-1950 manufacture SC-88 receiver which carries its serial number on the metal tag attached to the front panel (there is no number stamped on the chassis.) All SC-88 receivers are designated R-320/FRC by the U.S. Army Signal Corps and were used exclusively in the OA-58A/FRC Triple Diversity Receivers. So far, all SC-88 receivers have three digit serial numbers.
At this time, most of the serial number assignment process is mostly conjecture based on the serial numbers reported so far. You may notice that this section will change as more information is gleaned from the serial numbers reported. It is important to gather as many serial numbers from all of the different versions of the AR-88 series. As more and more serial numbers are reported, a clearer picture of the assignment process will reveal itself. In the future, it may become possible to date a particular AR-88 series receiver by its serial number and it may also be possible to identify which plant the receiver was built at by its serial number.
Please report your AR-88D, AR-88F, AR-88LF, CR-88, CR-88A, CR-88B, CR-91, CR-91A or SC-88 serial numbers and we will add them to the log. Be sure to specify which model receiver your serial number(s) is(are) assigned to. I will in turn report your AR-88D, F & LF serial numbers to Alan Ford for his log.
Send your serial numbers to: WHRM AR-88 SERIAL NUMBER LOG
These serial number logs have worked very well for the Hallicrafters SX-28 and the Pre-war Hammarlund Super-Pro receivers to determine production quantity and to provide an accurate method to estimate a date of manufacture on an individual receiver. Thanks for sending in your information.
Ford VK2DRR - AR-88D, AR-88F
AR-88 Series Serial Number Log
For both lists: Various countries shown after the serial number indicate where the receiver is now located.
* All AR-88LF were built in Montreal and have "C" prefix
Operational and Modification Caveats
|Here are some things to watch out for when operating an AR-88 receiver (or variant) for extended periods. Many of these reports come from British and European amateur users who have had more experience and longer exposure to operating the AR-88s for extended periods in the amateur environment. Also, some of the commonly seen and unnecessary modifications that might be encountered.|
1. High B+ on filter capacitors and audio output transformer when the receiver is in TRANS position - This is because the FUNCTION switch removes the B+ at the output of the triple filter of the power supply. The audio transformer is connected at the second filter capacitor and thus remains connected. Reducing the load on the B+ while in "Stand-By" causes the voltage to soar. Although there are bleeder resistors in the circuit their load is slight and many of these 560K A-B resistors have drifted quite high in value so they no longer perform their function as designed. Some overseas users report that the B+ excursion is high enough to cause problems in the filter capacitor unit. I tested my original CR-91 receiver and found that the B+ at the first filter capacitor was +385vdc in REC MOD and measured +450vdc in TRANS (our AC line is 115vac here.) This is about 100vdc of "head room" and should not cause any problems - the filter capacitor is rated at over 550wvdc. It might be possible that the 240vac primary tap, the highest available, doesn't provide enough "head room" when operating the AR-88 on 240vac 50~ mains. This may account for higher than normal B+ in some overseas locations where the house voltage is more than 240vac and could approach 250vac. RCA rated the 240vac primary to be able to go up to 260vac but, naturally, the secondary voltages will also be higher, resulting in higher "no load" B+ voltages. Normal setting in the USA would be on the 125vac tap which provides ample "head room" for most locations. The normal encountered AC voltage in the USA is 120vac. All of that being said, it's probably a good idea to not use the "TRANS" position as the SEND/REC function. Safe operation of the AR-88 in the station receiver position can be accomplished by utilizing an electronic T/R switch. This would have the receiver remaining in the REC MOD or REC CW position all the time with the T/R switch protecting the receiver input during transmit.
2. Audio Output Transformer failure when using speakers other than the original 2.5 Z ohm speaker - This is nonsense. An important part of the impedance calculation involves frequency. So what frequency is standard for speaker calculations? Well, actually the specification of a nominal impedance for a typical speaker is the "lowest impedance that the speaker load would present in its normal operation over its typical frequency range." Pretty vague? Suffice it to say that speaker impedance is "nominal impedance" and not a fixed value of load to the circuit. All of that being said, the manual specifies 2.5 to 3.2 Z ohms as the desired speaker nominal impedance. Note that when operating on Hi-Z earphones the phone jack connects a 5 ohm 4 watt wire wound resistor as a load across the 2.5 ohm winding of the audio transformer. Apparently RCA thought that 5 DC ohms was a close enough match to protect the AF transformer. Using a 4.0 Z ohm speaker is perfectly safe. If you can find a 3.2 Z ohm speaker (fairly easy) you will find that the AF gain control doesn't have to be advanced quite as much as with higher Z speakers. Although I have operated AR-88 receivers with 8 ohm Z speakers and haven't had any trouble, I think the 3.2 ohm Z speakers work very well with the receivers and are worth looking for. One thing that is important relates to receiver operation exclusively on Hi-Z headphones (with no loudspeaker connected,) in which case the operator must be sure that the 'phones are plugged into the phone jack "all the way" so that it operates just on headphone winding of the audio transformer. This is a two-position jack and when the 'phone plug is "all the way" in, then the 5 ohm resistive load is across the 2.5 Z ohm output and the phones operate on their winding. Half-way in was to allow the Hi-Z 'phones to operate in parallel with the 2.5 Z ohm speaker winding to allow both speaker and 'phone operation simultaneously. NOTE: The first 3000 or so AR-88LF receivers have a different audio output transformer that has a single tapped secondary with 2.5 Z and 20 Z ohm outputs.
3. Audio Output Fixed Bias Operation - The cathode of the 6K6 AF Output tube is tied directly to chassis and the grid is biased negatively by a voltage divider operating from the negative voltage developed by elevating the CT of the B+ winding of the power transformer with resistors. Fixed bias allows the AR-88's 6K6 to develop more power with better audio characteristics with less distortion. The other option would have been self-biasing which uses a cathode resistor to elevate the cathode above chassis and therefore have the grid operate negative when compared to the cathode. This was the economical way to bias an audio amplifier and the tube will develop a little less power than it would with fixed bias. However, there are dangers when operating fixed bias. If, for some reason, the bias voltage is removed, the grid will float positive and the tube will conduct heavily. This can rapidly cause damage to the tube and sometimes might damage the audio output transformer. Some AR-88 users advocate adding a 100 ohm resistor to the cathode circuit to protect the tube and transformer in case of bias voltage failure. Theoretically, bias failure is possible, BUT, the resistors used in the bias voltage dividers are high rated, wire wound resistors (except for the 15 ohm resistor that carries very little current.) Failure of this circuit is rare. However, if you are a "worrier" then add the cathode resistor. Most of the time, these types of failures came about in a military or commercial use where the receiver was left operating continuously and may not have been "looked at" or "listened to" for long periods of time. If a problem developed it might be several hours before anyone noticed and the ensuing damage could be extensive. In amateur use today, a failure would be detected almost immediately and therefore serious damage unlikely.
4. Insufficient BFO Injection - Most versions of the AR-88 rely on electrostatic coupling of the BFO into the grid circuit of the 3rd IF amplifier tube. This adds some gain to the BFO at the detector stage and greatly reduces weak signal "masking" or "pulling" in the CW mode. But, along came SSB which required a higher ratio of BFO injection to the signal level to allow the BFO to substitute for the signal's missing carrier and achieve decent audio reproduction. The normal ham modification approach added a capacitor (or increased the electrostatic coupling with wire stubs) to have more BFO injection which seemed to give better results on SSB. However, it's much easier to just turn off the AVC by switching to MAN, increase the AF gain to maximum and the reduce the RF Gain to a level where the signal to BFO ratio is correct for good audio reproduction in SSB. This then allows the receiver to still be used for weak CW reception. However, listening to a SSB net can be a real pain because of the different signal levels require readjustment of the RF gain for each net participant. If you don't do CW, then maybe increasing the BFO injection makes sense. Incidentally, the final versions of the AR-88 series, the CR-88 uses a fixed 2pf coupling capacitor and the SC-88 uses a fixed 10pf coupling capacitor rather than the electrostatic coupling for the BFO.
5. Remove Negative Feedback for "Audio Improvement" - This all depends on what your definition of "audio improvement" is. This originally was done by hams who wanted louder audio with more of a "communications edge" to it. Today, most vintage receiver users, especially AM hams, would find this type of audio to sound harsh and lacking in quality. The audio response was carefully designed in the AR-88 to provide relatively low distortion, reduced harmonic content and wide range reproduction for many varied applications. Using a close match to the 2.5 ohm Z audio output probably does more to improve the audio than removing the negative feedback. I have a feeling that this modification originated around the time that the receivers had aged enough when capacitor problems and resistor drift were affecting the audio output and the receiver would have benefited more from a rebuild and alignment rather than removal of the negative feedback. When the AR-88 receivers are rebuilt and correctly aligned, their audio level is very responsive and VERY LOUD. Normal position of the AF GAIN control is about 20% advanced with a 4 ohm Z speaker.
6. Reduce Value of RF Gain Control Divider to Improve Sensitivity - The RF Gain control consists of a 66K ohm potentiometer connected to the negative bias divider to provide about -27 volts to the high end of the RF Gain pot. The low end of the RF Gain pot is connected to chassis through a resistor that can be from 5.6K to 6.8K in value depending on the particular model of AR-88 receiver. The value of this resistor determined the how close to zero volts the bias could be reduced by the RF Gain control and therefore controlled the maximum sensitivity of the receiver. This resistor value was usually set to allow about -2 volts bias at the maximum setting of the RF Gain control (minimum R of the pot.) Some users feel that a bit more sensitivity can be had by lowering the value of the series R to ground and thereby reducing the minimum bias even further. Generally, most users feel that around 2.5K is a good value. This will reduce the minimum bias to about -1.8vdc which will result in a slight increase in the receiver sensitivity maximum. Before changing the value of the resistor be sure to check its actual value as it is quite possible that the resistor has drifted to some value that is much higher than the 5.6K to 6.8K called out by the receiver design. If the resistor value is within the proper range and you want to somewhat increase the maximum sensitivity, then reduce the value of the resistor to around 2.5K This is an easy mod that can be accomplished by adding a 5000 ohm resistor in parallel to reduce the total value of the resistance down to about 2.6K. The results are somewhat subjective but its an easy mod to remove if the results are undesirable for your receiver operation and set-up.
7. Tube Substitutions - There is a subtle difference between the metal 6SA7 (used as the mixer in the AR-88) and the glass 6SA7GT tubes. Some users believe that the glass tube produces less noise. This is easy to try since it is just a "plug-in and try" experiment. I have only found this substitution in one receiver so it's not too well known, or else it doesn't really do much. Also, substitution of 6SG7W tubes for standard 6SG7 tubes was supposed to improve signal to noise ratio. Expect results from subtle to no apparent improvement at all. What does seem to be important is the quality of tubes used in the receiver. Many users report better performance with all NOS RCA tubes installed. Certainly, a complete set of NOS JAN tubes will significantly enhance the receiver's performance.
8. Miniature Tube Modifications - CQ magazine, notorious for publishing articles that advocated the wholesale destruction of radio equipment, did it again when they published the article titled "Worthwhile Improvements for that Old Receiver" by Paul H. Lee, February 1957. The article details modifications done to a CR-88A receiver that involved the replacement of most of the metal octal tubes with seven and nine pin miniatures. Most of the article details replacing the 6SG7 1st RF amplifier tube with a 6BK7A dual triode tube, claiming a reduction in front-end noise (a common claim with miniature tube substitutions.) Also, the article continues with the BFO change to a mixer tube, 6BE6, acting somewhat like a product detector. Replacement of the Mixer tube with a 6BA7 nine-pin tube (it was the Mixer tube used in the Collins 75A-4 so it had to be good.) Additionally, all of the remaining 6SG7 tubes were replaced with 6BA6 tubes. The article concludes with the author bragging how Venezuela was now installing CR-88As that incorporated his RF modification. The article is reminiscent of post-L.E.Geisler's Hammarlund Super-Pro "modification-destruction" articles that were also published in CQ. Nowadays, we all know that ham modifications seldom live-up to their hype. Extensive modification of vintage ham gear, in order to make its performance more "modern," seems to contradict the whole idea of collecting, restoring and operating vintage radio equipment in the first place.
Restoration Suggestions for the AR-88 Receivers
Front Panels - The front panel is a little under .250" thick copper-plated steel. The plating was to reduce the corrosion under the paint. Unless the receiver was abused, the panel usually holds up quite well. However, most AR-88s were abused and the panels aren't in good condition. Unfortunately, most AR-88 panel nomenclature is silk-screened which will make a complete re-paint impractical. Thorough cleaning with Glass Plus and a moderately stiff brush will let you know where the dirt leaves off and the paint begins. After a good cleaning, the panel can be touched up using matched Artist's Acrylic paint. Let the touch-up paint dry over-night. A final wipe down with Armor-All or Boiled Linseed Oil (not both) will make the panel look almost like new.
There are some engraved AR-88 panels that turn up from time to time. I haven't seen one up close, so I'm not sure of the quality or even the origin of the engraved panels. It appears from what has been reported so far that the early production AR-88 for export only had engraved nomenclature.
On the rear of the panel are eight 1/4x20 studs that are spot-welded to the panel. These are what mount the front panel to the side panels. Sometimes, with really rough treatment, one or more of these studs might break at the spot-weld. Since the panel normally can't be re-painted, the broken stud can't be spot-welded back in place. It is possible to re-mount the stud using J-B Weld. The tensile strength of cured J-B Weld is over 3000 lbs. With careful preparation of the surfaces and by drilling a small hole in the back of the stud for the J-B Weld to bond to, it is possible to end up with a bond that is nearly as strong as the original spot weld was. When tightening the nut back onto the stud be sure to only tighten enough to compress the lock washer. Though the pull of the nut doesn't work against the J-B Weld joint the torque or twisting action does. Compressing the lock washer provides ample tightness.
RCA "Meatball" Emblem - These metal emblems seem to really react badly to moderately harsh environments. While the receiver can look pretty good, the RCA emblem will be rusty and pitted. A lot of restorers think the emblem is chrome plated but it isn't - it's the same material all the way through. Therefore, it's pretty easy to correct the damage that the rust has done. First, you should remove the front panel. This allows easy access to the emblem when the logging dial index is removed. To remove the emblem from the plastic dial cover panel note that there are two thin metal tangs are bent over to secure the emblem to the dial cover. Unbend these carefully just far enough to remove the emblem - you don't want to break off these tangs. Next, use a pair of vise grips to gently hold onto the emblem by gripping it by the tangs. It isn't necessary to grip the tangs hard - just enough to hold the emblem with something other than your fingers. Now, using a small brass brush remove as much rust as you can. Next, using a small flat Swiss "Jeweler's" file go over all of the rim area to remove any pitting from the rust. You might also have to file the RCA letters depending on how serious the corrosion is. The thing to remember is that the emblem is the same material all the way through so filing to remove the pitting won't damage the base material. Once the pitting is removed (or mostly removed in serious cases,) you can start to polish using 600 grit aluminum oxide paper. Use the 600 grit paper until the emblem is smooth and fairly polished. Next, use Semichrome or Wenol's to polish the emblem to a high shine. Remove from the vise-grips and the emblem will look like new. Normally, the red background paint comes through this process unscathed. However, if the paint is poor it can easily be removed with paint remover. >>>
>>> Then repaint in the background with gloss red paint. Wipe off the excess and you don't have to be very careful here. After the paint has dried, polish the front of the emblem using a cloth-covered flat surface and Semichrome or Wenol's. This will remove the remaining excess paint perfectly to expose the RCA logo just right. The whole emblem will look like new. Remount to the plastic dial cover panel and carefully bend the tangs to secure. Remount the logging dial index and then remount the front panel.
Plastic Panel Dial Cover - The plastic panel dial cover is actually a series of paint layers applied from the back via a silk-screening process. Black is applied first, then white and finally the yellowish-greenish protective layer. The black is well protected but the white sometimes seems to have damage occur to it. Usually, the white can be reapplied as Artist's Acrylic paint matched to the white/gray/beige color and thinned appropriately. Mask off the area if the white is damaged around the band indicators. This has to be very straight and masking is the only way to have it look correct. Be sure the paint is really thin so that you won't see an edge on the band indicators. You will have to apply about three coats allowing for drying time between coats. The decorative white lines around the logging scale window and the ID plate window can be applied by hand. Finish by over-coating the touch-up with a yellowish-green mix of Artist's Acrylic to protect the white. The plastic panel dial cover should end up looking absolutely original from the front and, depending on your abilities, stock from the back.
Dials - The main tuning dial, the logging dial and the ID plate on nearly all AR-88 receivers are all made out of the old type of yellowish plastic that is very photo-sensitive. The dials and ID plate will darken considerably with long term exposure to bright light (very common with the rack mount versions that are no longer in a rack.) You can clean the dials but don't use any harsh chemicals like Windex or any kind of solvents. I use foaming plastic cleaner that is designed specifically for cleaning plastic. Wet a cotton cloth with the foam cleaner - don't use paper towels. Make sure the cloth is damp but not dripping wet. Rub the dial gently with the cloth. Only the dirt and oxidation will come off. Be gentle - don't scrub. Always watch the lettering for any indication of thinning - the AR-88 dials are pretty tough but be careful anyway. With foaming plastic cleaner and a cotton cloth that is "just damp" no damage to the dial lettering will occur. Clean the back of the dials to enhance their illumination. That's about all you can do to the dials without damaging them. It is impossible to remove the photo-sensitive discoloration because it is deep into the plastic.
Knobs - Knobs can be cleaned by soaking in a dish washing soap and hot water solution for 30 minutes. Then scrub the knobs with a tooth brush to clean. Sometimes the knobs are scarred from rough handling with deep scratches or small chips. The knobs are made of solid bakelite and can be reconditioned by installing the knob onto a .25" diameter shaft about 3" long. This can be chucked-up in a drill press or lathe. Turn the knob and by using a file remove material where the deep scratches are. This is followed by 600 grid aluminum oxide paper or 0000 steel wool. This can only be done on the front curved surface of the knob - not on the edge or flutes. Now, remove the knob from the drill press or lathe and hand polish the knob using Semichrome or Wenol's. Because of the index pointer you can't "turn polish" the entire knob - it must be done by hand. The knob index line can be filled if necessary with Artist's Acrylic paint mixed to a "manila beige" color. Plain white will look way too bright.
|If your AR-88 Series receiver doesn't seem to have superior sensitivity and stability (far above the typical 1940s design) then it probably needs to be rebuilt. If your power supply filter chokes run hot then you have some bypass capacitors that are "leaky." Most lack-luster performance reviews of the AR-88 series receiver can be traced to operation of the receiver with 60+ year old capacitors and resistors along with no recent check on the IF/RF alignment. A full electronic rebuild followed by a full IF/RF alignment is necessary for the AR-88 series receivers to function at their design level of performance.|
Capacitors - All of the filter and bypass capacitors in
the AR-88 receivers are extremely high quality, oil filled units that
rarely fail - this includes the filter capacitors. All of the metal tub
capacitors are triple-capacitor units that are bolted to the inside
walls of the chassis. It is very easy to check these capacitors for
leakage current. Most, if not all, will be found to be useable but if
you feel uncomfortable operating equipment on 60 year old components -
and you probably should - then rebuild the tubs. Top performance
will require that the majority of capacitors be rebuilt or replaced.
Additionally, sometimes another type of leakage is encountered - actual
weeping of the oil from inside the tub. This usually is very minor
leakage but besides the mess there are other issues with the tub
One should remember when rebuilding the tub capacitors that up until around 1977, all oil used in oil-filled capacitors was made from Polychlorinated Biphenyl, usually referred to as PCB. These oils were complex chemical structures that had multitudes of variations, some of which were extremely toxic. Large scale, commercial chemical dumping over years created many environmental problems that ultimately were harmful to wildlife and to a certain extent, humans. Long-term exposure to PCBs can cause skin and liver problems since the skin readily absorbs PCB oils. Common latex gloves do not provide a barrier against PCBs and protection is only afforded by a few types of rubber gloves, e.g., Butyl, Nitrile or Neoprene.
Rebuilding the capacitor tubs is a messy, smelly, laborious process. Unless you are a zealot for originality it might be better to just replace the tub capacitors with individual replacement capacitors mounted on tie strips. However, here's the process I use for rebuilding tubs - remember, you're dealing with PCB oil, so proceed at your own risk.
The tub capacitors need to have the bottom plate removed so that new capacitors can be installed into the can. Removing the bottom plate requires an old-time large soldering iron with a one inch tip. These old behemoth soldering irons provide tremendous amounts of heat to melt the solder and remove the bottom plates. This work must be done outside with lots of ventilation as the residual oil vaporizes with the heat. I also advise wearing nitrile or neoprene gloves when disassembling these types of oil-filled capacitors. You should assume that the oil contains PCBs. You must drill a couple of holes in the bottom plate before hand and let the oil drain out. The holes will also allow a way to grab the bottom plate for removal. The tub should be held in a vise and then the iron applied to the bottom and to one side. Let the iron continue to heat up the entire bottom although the solder will only become molten on one side near the iron. After about a half-minute insert a screwdriver into one of the holes and pry the bottom cover up on that side. Then move the iron to the other side of the bottom plate to keep it hot. Use a pair of heavy-duty needle nose pliers and "roll up" the bottom plate and it should come right out of the tub. Once the bottom is off, you'll have to remove the old capacitors and then wash the entire inside and outside of the can with denatured alcohol to remove the oil residue. Install the new replacement capacitors inside the tub - it isn't necessary to re-install the bottom plate (besides, it's all bent up from removal) - then re-install the rebuilt tub into the receiver chassis.
The same re-stuffing can be done on the filter capacitor but it will have to be cut apart, capacitors replaced and then either soldered together or put together with epoxy and then repainted and re-installed. Be aware that the filter capacitors have to be rated for over 550 working vdc. Since the original capacitors were 4uf paper caps, use two 10uf 450wvdc electrolytic capacitors in series. This will result in a 5uf 900wvdc capacitor. To equalize the drop across the capacitors, a 1 meg ohm resistor can be connected across each 10uf electrolytic.
All of the Micamold capacitors must be replaced since there really isn't anyway practical way to restuff them. Use SBE orange drops as replacement capacitors. The Micamolds might seem like they were high-quality capacitors but they are not "micas." They are typical paper-wax caps in a molded case - just like early versions of Sprague's "Black Beauties" with the same kind of leakage problems. >>>
>>> COMPONENT QUALITY ISSUE - IMPORTANT NOTE: Micamold capacitors are not "mica caps." They are molded paper-wax caps - an earlier version of the infamous Sprague "Black Beauty" molded capacitors. Micamolds have the same problems (maybe worse) as the typical Black Beauties with excessive leakage current. If you examine the Micamold capacitors carefully you will almost always see that the center of the capacitor body is bulging due to excessive leakage current causing heat buildup and the resulting swelling or bulge. All the Micamolds must be replaced for reliability and top performance.
When all of the capacitors are rebuilt or replaced, you should note that the power transformer and the two filter chokes run significantly cooler. This is due to a reduced load that was caused by current leakage on some (most) of the bypass condensers. Additionally, alignments proceed as described in the manuals and the overall performance of the receiver is significantly improved, especially in the audio section.
Checking the Resistors - All resistors in the circuit should be checked carefully. Some of the Allen-Bradley type resistors are prone to drift high and it isn't uncommon to find these resistors over 100% out of tolerance. It really isn't predictable which resistors will drift because sometimes you'll find the 2.2 meg resistors are fine but the 33K resistors will have drifted. Normally, higher value resistors will drift higher and low values remain stable but this doesn't seem to be the case with A-B resistors. For some reason, you'll find A-B resistors mixed with 1/2 W JAN types in many AR-88s. The JAN resistors seem to hold their value quite well. Top performance of your AR-88 series receiver will require careful checking and replacement of all out of tolerance resistors.
COMPONENT QUALITY ISSUE - IMPORTANT NOTE: Allen-Bradley resistors that were manufactured during WWII (and somewhat later) seem to be plagued with a "carbon drift" problem. Almost all A-B resistors of that period will not be at their original manufactured resistance. It isn't predictable which values will drift or by how much the resistance will change. The only solution is to physically check each resistor for its present resistance value and replace any resistors that are more than 20% out of tolerance. This A-B resistance drift problem is found in every type of WWII electronic gear that used A-B resistors, not just the RCA gear.
Cleaning and Lubricating the Gear Box - The main tuning gear box is easy to remove for a total emersion cleaning. It is mounted to the chassis with four nuts and lock washers. It is also bolted to the tuning condenser front frame with four screws and lock washers. You will also have to loosen the coupler to the tuning condenser and slide it back onto the tuning condenser shaft. You will have to dismount the front bearing support which is held to the chassis with two screws and lock washers. Then the gear box should be tipped slightly forward and removed pulling it upwards off of the chassis.
I use an ultra-sonic cleaner and use a non-flammable de-greaser to clean the gear box. The driveway degreasers work pretty well in the ultra-sonic and are easy to find anywhere. 30 minutes in the ultra-sonic and the gear box comes out looking like it is new. I then rinse the gear box in very hot water to remove stubborn grease and to rinse away the cleaning solution. Then I put the gear box under a 100W lamp for about 15 minutes to dry it out. If you don't have an ultra-sonic cleaner then you can use WD-40 in a flooding-spray along with a small long handle paint brush to clean and degrease the gears. Use lots of WD-40 (it's cheap) and the gear box should look really clean afterwards. No need to rinse in water if you use WD-40 as the cleaning solution.
It's probably a good idea to check the settings on the single ball bearings. I have found that many times these bearings are way too tight. Do one bearing at a time. Loosen the bearing lock nut and back the screw out slightly. Retighten until you just feel the bearing point contacting the ball, then tighten a bit more and then back the screw out slightly. You want just a slight pressure that holds the gear in alignment but doesn't hinder its rotation. These single ball bearings should have a drop or two of 10W oil as a lubricant.
The three ball bearings cups will need to be repacked with high quality bearing grease. You can work the grease into the bearing with your fingers and with a plastic screwdriver acting like a spatula. The bearing cup is full when you see grease emerging out the other side around the shaft. On the bearing that is for the tuning condenser shaft you will need to work the grease in from the shaft side. This has to be done with your fingers surrounding the shaft to push the grease into the bearing cup. It is more time consuming to do this bearing but it does need to be repacked for noiseless and smooth operation. Re-install the gear box and adjust the tuning shaft bearing that is in front of the flywheel weight for best action. When finished the tuning will be very smooth and light.
|RCA Cabinet - Rack
Mount Slot Covers - If you happen to run across
an original RCA AR-88 cabinet, it might be missing these two pieces.
They are the Rack Mount Slot Covers and their purpose is to trim the
edges of the front panel. If these pieces are missing they are easy to
are made out of thin steel and bent on two sides only. Be aware
there is a right side and a left side. Two holes
allow for 12x24 round head machine screws to be used to mount the
trim pieces and the receiver panel
to the cabinet. Trim pieces are painted the same color as
the cabinet. Outside dimensions* are:
Length* = 10 17/32" Width* = 25/32" Height* = 9/32"
Nearest Hole Center to Bent Side, distance* = 1 1/2"
Farthest Hole Center to Bent Side, distance* = 9"
Hole Diameter = 9/32" Material = 20 ga. steel (.037")
* "Outside Dimension" includes the thickness of the bent side
photo above: The Rack Mount Slot Covers for the original RCA AR-88 Series cabinet shown with the 12x24 round head machine screws for mounting
photo right: The right side slot cover showing how it is mounted
Sweep IF Alignment of the AR-88 Receivers
|Why bother doing a sweep alignment? A lot of users don't have the proper equipment. It's time consuming. Why bother? Well, if you are going to use your AR-88 series receiver for CW or SSB only, then maybe a standard alignment is all that you'll need. However, if you want a very smooth response while tuning through those broad AM signals with no changes in the amplitude as you tune from one side of the signal to the other, in other words - no "peaks and dips" - then you are going to have to perform a sweep alignment on the IF section of your AR-88 series receiver. Here's how it's done...|
Preliminary Alignment - The AR-88 uses a stagger-tuned IF system when in Selectivity POS. 1 and POS. 2 with two under-coupled IF transformers and two over-coupled IF transformers. This requires a sweep generator for proper symmetrical alignment. A complete description of the procedure is below, including actual photographs of the oscilloscope patterns taken during an actual sweep alignment of an AR-88D. However, before you do any sweep aligning, you need to center the entire IF section at the proper frequency, which is dependent on the crystal filter's crystal frequency. Before doing any adjustments on the IF or the RF, it might be a good idea, especially if the receiver is one that was stored in less than ideal conditions, to put a drop of thin oil on all of the threads on all of the adjustment shafts. Just a drop will do and let it soak over night before proceeding with the alignment. Input an IF signal to the Mixer grid and peak all of the IF transformers - remember you're going to sweep align this later so just peak them for now. Proceed on to the RF tracking adjustment per the appropriate manual. When this is completed, you can go on the the sweep alignment of the IF. The manual's instructions for sweep alignment are vague and assume you've performed sweep alignments many times before. Only drawings of the oscilloscope patterns are provided with some very basic directions (and these are only in the later manuals.) The following is a more detailed set of instructions that assume you've done alignments before but maybe not sweep alignments. I also assume that you are familiar with the operation of the test equipment described. Be sure to thoroughly read the later RCA sweep alignment instructions and use them along with my instructions. The RCA instructions specifically call out the component designations for adjustment for positive identification where I just refer to the function of the connection or adjustment, e.g., 4th IF transformer or 3rd IF amplifier grid, etc.
Sweep Alignment - Sweep Alignments of a receiver's IF section will require a signal generator that is capable of a frequency "sweep" function. Back in the forties, it was common to find a "Wobulator" or mechanical FM signal generator. These devices used a motor to turn a variable capacitor that was in the signal generator's oscillator circuit. The effect was to rapidly change the set frequency at a rate determined by the speed of the motor. The frequency deviation, or by how much the frequency changed from a "set frequency," was based on the variable capacitor's maximum and minimum capacitance. Most of the motors ran at 1728 RPM which is about 28 Hz and this is an ideal sweep rate for alignments. Today, we generally use a modern Function Generator that has an electronic frequency sweep function built-in. There are many Function Generators available that were at one time "laboratory equipment" that are now on the "used market" and very reasonably priced. These will work fine for sweep alignments. Be sure that the sweep rate is adjustable and that a "ramp output" (or "modulation out" that is specific to the sweep rate) is provided. Also, be sure that the Function Generator will produce sine waveforms up to at least 1.0MC - most do. I use a Hewlett-Packard 3312A Function Generator for my sweep alignments. The oscilloscope is a Tektronix 475.
The easiest type of Oscilloscope to use in a sweep alignment is a two-channel 'scope that can be set-up to an "X-Y" configuration. Most modern 'scopes have this function. Look at the Time Base control and usually just past the slowest sweep is "X-Y." This allows you to look at the receiver's IF output on the basis of just one sweep at a time determined by the "sweep ramp output" of the generator. It results in a very stable pattern that shows the IF passband characteristics. When the old "Wobulators" were used with 'scopes the sweep ramp from the Wobulator had to be fed directly into the Horizontal input of the 'scope to generate the "sweep" pattern. Stability was always an issue due to the mechanical nature of the signal. Modern gear is much better.
The "Y" input to the 'scope is connected to the output of the second detector at terminal C of the fourth IF transformer and is left there - you don't have to move it. The "X" input to the 'scope is connected to the Ramp (Mod.) Output from the Sweep Generator - again, it stays put. This provides a "one sweep" look at the IF passband that is monitored by the "Y" input but since the sweep generator provides a continuous ramping sweep of about 25hz, the IF pattern appears to be stable and continuous. Your signal input to the receiver comes from the signal generator's P-P Amplitude Output which is the waveform that is amplitude adjustable and has its frequency sweeping. You may find that you have to "invert" the input of the Y channel because the output of the AR-88 detector is negative. You will have an "upside-down" or "U" shaped pattern unless your 'scope channels have the ability to invert the signal input. Always use real oscilloscope probes since they don't load down the circuit. The connection from the Function Generator Ramp Output to the "X" channel of the 'scope can be a coaxial cable with BNC fittings. Any loading or radiation pickup on unshielded cables will affect the 'scope patterns.
The alignment process depends upon always to be looking with the 'scope at the 4th IF transformer (detector output,) terminal C and then work your way forward through the IF section by first injecting a sweep signal at the grids of the 3rd IF amp, then on to the 2nd IF amp, then on to the 1st IF amp, until you are finally injecting the sweep signal at the grid of the Mixer stage. You will then be viewing the sweep signal passing through the entire IF section of the receiver. This allows you to see the actual characteristics of the IF passband, including its shape and bandwidth. >>>
>>> Unfortunately, there is no specific set up for amplitude, frequency deviation, X-Y gain settings or generator starting frequency that is "correct" since every receiver is somewhat different and many of the settings will interact. Some of the settings will be individual preference. So the best thing to do is to connect up the test equipment as described and experiment with what gives the best image. Be sure to keep the sweep rate low at around 25Hz or less - this is important for good stability and an accurate image of the IF passband. Also, keep the generator amplitude as low as possible that still gives a good representative image of the IF passband. Once you have experimented around with the set-up, you'll notice that the start frequency is not very critical as long as the frequency deviation is adequate to cover the IF passband. Usually 20Khz to 25Khz deviation gives a good image. The IF for the '88 series is 455kc while the '91s and AR-88LF use 735kc for the IF. You will set the sweep generator starting frequency near these frequencies depending on the receiver type.
Be sure to couple the sweep generator signal to the various grid inputs of the receiver through a .01uf capacitor. Most modern generators are very low Z, usually around 50 ohms, so the capacitor will help prevent loading of the grid. Now, we are assuming that you have already aligned the receiver to its center IF with reference to the crystal filter's crystal frequency. Connect the 'scope "Y" input to terminal "C" on the fourth IF transformer. Now, start with the 4th IF transformer and inject the sweep generator signal to the grid input of the 3rd IF amplifier tube. Adjust the 4th IF transformer for a symmetrical pattern but be sure that the amplitude of that pattern remains at the same level. This IF transformer is not switched so the position of the SELECTIVITY control has no effect. See photos and drawings below.
Next, leaving the "Y" input connected to terminal C, inject a sweep signal to the grid of the 2nd IF amplifier tube. This brings in the 3rd IF transformers which are over-coupled in "BROAD" POS. 1 and normally coupled in POS.2. You have to switch between POS. 1 and 2 and adjust the IF transformers for the very slightly "dipped" pattern for 1 and a normal slightly "rounded" peak for 2. Symmetry is most important here. In performing all of these adjustments be sure to try to keep the IF amplitude unchanged, otherwise you are detuning the IF. Only very small changes are required to achieve the correct patterns. NOTE: The British AR-88 manuals specify only adjusting the bottom IF adjustments. The RCA manuals aren't specific. I have found that you have to adjust both the top and bottom slugs of each IF transformer to achieve the best patterns.
Next, inject the sweep signal to the grid of the 1st IF amplifier tube. Now, you'll be seeing the combined 2nd, 3rd and 4th IF transformers. You may have to adjust the sweep generator amplitude a bit for a good 'scope pattern. Adjust the 2nd IF transformers for a significantly "dipped" pattern in POS. 1 and a narrow-flat pattern in POS. 2. Again, try to keep the patterns symmetrical and the amplitude unchanged. Move the sweep signal to the Mixer grid. At this point you will have to increase the sweep generator amplitude for a good pattern on the 'scope. Then adjust the first IF transformer for a broad flat pattern in POS.1 and and narrower but somewhat flat pattern in POS.2. See photos and drawings below.
Only one IF transformer is used for the 4th and the 1st IF transformers. Two IF transformers are used for both the 3rd and 2nd IF transformers. While doing all of the manipulations to the pattern, be sure to try and keep the amplitude of the waveform unchanged. Detuning just to achieve the pattern will detune the IF resulting in reduced gain. Try to keep the patterns as symmetrical as possible. This is a time consuming process and only minute adjustments are required.
IMPORTANT NOTE: Don't expect the patterns that you see on the 'scope to look exactly like the drawings. See the photographs below of the actual patterns from an AR-88D alignment for an idea of what you might really see. You won't be using the same type of equipment that RCA used back in the 1940s when they made the drawings. Also, maybe when the receivers were new it was possible to get the exact patterns but, after 60+ years, the components and the IF transformers have aged enough that just getting close is about the best that can be achieved. Try to adjust for the closest appearance to the drawings with good symmetry, fairly flat tops and no loss or very little loss in amplitude. Also, remember - this is a time consuming process that requires only minute adjustments of the IF transformers. Don't expect the adjustments to easily "fall into place" - they don't. Take your time and only make small adjustments.
After the 1st IF transformer is adjusted all that remains is to adjust the crystal filter section. Selecting POS 3, 4 or 5 will operate the Crystal Filter. The object of this alignment is to maintain a symmetrical pattern while progressively narrowing the bandwidth. This is very easy to accomplish with the RCA instructions and adjusting the Crystal Load L-34 and the trimmer capacitors provided.
4th IF Transformer Only
The oscilloscope pattern to the right shows the actual pattern on a rebuilt AR-88D receiver. Compare this to the drawing from the manual shown and you'll note that the actual pattern is very similar although a bit sharper at the peak. This scope pattern could be broadened out with the oscilloscope controls but it is only necessary to obtain a symmetrical pattern at this point.
HP - 3312A Function Generator
Tektronix - 475 Oscilloscope
3rd & 4th IF Transformers
Actual patterns shown to the right with POS 2 on the left and POS 1 on the right. The 'scope patterns are similar to the drawing although the patterns are overlaid in the drawings and we are showing individual photos. The 'scope patterns are a little more rounded on top. Note the width difference. We are trying for the best symmetry of both patterns without loosing gain.
2nd, 3rd & 4th IF Transformers
Actual 'scope patterns shown to the right with POS 2 on the left and POS 1 on the right. The patterns are similar to the drawings although the dip in the POS 1 pattern is not centered and is a little bit sharper. POS 2 is a nice flat pattern, though rising slightly to the left. Note the width difference between POS 1 and POS 2.
All IF Transformers
Actual patterns shown to the right with POS 2 on the left and POS 1 on the right. The patterns are still similar to the drawings although POS 2 has a bit of a rise to the top of the pattern increasing to the left. POS 1 looks symmetrical and should provide good fidelity. Note the width different of the patterns. This AR-88D sounds great and one hears no "gain peaks" while tuning through a broad AM signal.
Alignment - This alignment will require some special
tools. RCA did supply some with the receiver and they were usually
installed in the mounting clips on the tuning condenser cover. Nearly
all of the original tools are always missing today but you can utilize
standard tools instead.
Note in the photo to the right that the capacitive adjustments are long "plunger" shafts with a hole at the top. Also, note that there is a "locking" collar at the bottom of the capacitive shaft. These "locking" collars require a 5/16" wrench to loosen. I find that a very deep 5/16" socket (1/4" drive is what I use) works pretty well but a 5/16" open-end wrench would probably also do the job. These capacitive shafts can slide up and down once the collar is loosened and this is how the capacitance is adjusted. It's probably a good idea to spray a small amount of De-Oxit on the shafts before beginning the alignment. This will help with the common sticking that's encountered and will provide a good chassis contact for the "plunger" shaft. The hole in the top of the "plunger" shaft allowed a tool to be inserted that provided an easy way to move the shaft up or down. I just use an Allen wrench that fits the hole and use that to move the shaft to the correct setting. After the proper capacitance is set then the "locking" collar can be tightened. Monitor your output and be sure that tightening the locking collar doesn't change the adjustment.
The inductance adjustments are much easier to deal with only requiring a small blade screwdriver for setting. One thing that is awkward is the inductance adjustments for the 1st RF amplifiers which requires accessing the adjustments through holes on the rear chassis apron. The SC-88 is different and has all adjustments on top of the chassis.
A 200pf capacitive load on the BC band and a 200 non-inductive resistance ohm load on the other bands will be required for signal input loads. I use a Weston Audio Output Meter connected to the 600 ohm output as an indicator for the receiver's maximum output. I have the RF signal generator in the modulated mode with a 400 hz tone and the receiver in MVC. Always keep the RF signal generator at the lowest levels of output that provides enough signal for the Weston to show an indication using its lowest scale. You can also adjust the receiver output level with the RF Gain control and the AF Gain control. The idea is to use the lowest input signal that gives a good response in the receiver and a good indication on the lowest scale of the Weston Audio Output meter.
With the SC-88, the capacitive trimmers are changed to a more standard trimmer type of adjustment that is adjusted using a small blade screwdriver.
A Typical AR-88 Rebuild
|The AR-88 receiver shown to the right was a donation from a
fellow ham. He had sent the receiver off to be rebuilt by an
out-of-state "restorer" but, after a couple of years, the AR-88 was
returned minus several critical parts. Frustrated with the whole
project, the ham gave me the incomplete receiver to see if something
could be done to keep the AR-88 from becoming just another "parts set."
Inspection - Obviously missing was the main tuning gear box. Without this crucial part, the receiver was not going to do much of anything. Along with the gear box, the logging dial was missing. All of the knobs were also missing. On the underside, the AC voltage selector switch was smashed. Several wires had been added and several other wires removed. Of course, the shields were gone too. The rectangular plastic dial cover had some wear on the inside that affected the band indicators. On the plus side, this receiver had an incredibly nice front panel - it was just beautiful. Also, just a little corrosion was evident on the chassis. These great "pluses" were really the inspiration for trying to put this AR-88 back together.
Parts Procurement - Luckily, I made contact with VE8NSD in the Northwest Territories of Canada who is an avid AR-88 collector. He buys them at Canadian government auctions where the receivers are notorious for poor condition and incompleteness. Garth was able to supply nearly all of the missing parts for this AR-88. Shields were one exception. The sheet metal parts seem to vanish never to be found again. Also, if one is looking for a Carrier Level meter, it is another part that is beyond difficult to find. This was because only one in a thousand AR-88s was ever equipped with a CL meter.
More Issues - After a thorough inspection I discovered that this receiver chassis was actually an AR-88F "diversity receiver" that had been partially modified into an AR-88D by removing the DIVERSITY IF GAIN control potentiometer and wiring. Then a very nice panel from an AR-88D was added to the package. It was decided that since this AR-88 was incomplete and wasn't ever going to be a great original example I might as well do a full rebuild on the receiver to see what kind of improvement that would result in when compared to the three nice original receivers I have (CR-91, CR-88A and SC-88.)
What was apparent when looking underneath the chassis was that the former rework was poorly done. It appeared that new wiring was put in place for the rectifier socket replacement. The rework was very sloppy and had to be removed. There were some other problems caused by careless rework, mostly in the form of damaged wire insulation. The AF Gain control had been replaced with a short shaft potentiometer. In order to have the correct long shaft, a 1/4x20 threaded shaft was soldered to the end of the short shaft - ultra crude, to say the least.
|Required Component Replacement
- A closer check of the resistors
showed that many of the Allen-Bradley type resistors were out of
tolerance by a significant amount, usually in excess of 100%. For the
resistors that needed replacement I used standard JAN type 1/2W carbon
resistors. In looking at the original A-B resistors, which look like 1W
units, one would think the 1/2W resistors would be too small but the
later versions of the AR-88 use standard JAN 1/2W resistors throughout
the receiver. I looked through several boxes of carbon resistors and found
NOS replacements for all the needed resistors.
COMPONENT QUALITY - IMPORTANT NOTE: Allen-Bradley resistors that were manufactured during WWII (and somewhat later) seem to be plagued with a "carbon drift" problem. Almost all A-B resistors of that period will not be at their original manufactured resistance. It isn't predictable which values will drift or by how much the resistance will change. The only solution is to physically check each resistor for its present resistance value and replace any resistors that are more than 20% out of tolerance. This A-B resistance drift problem is found in every type of WWII electronic gear that used A-B resistors, not just the RCA gear.
I decided to replace all of the Micamold and metal tub capacitors. I chose SBE orange drops to replace the .0047uf, .0033uf and the .0022uf Micamold capacitors (these are listed in the parts list in picofarads, so 4700pf = .0047uf.) Most of the Micamold capacitors were "bulging" at the center - that couldn't be good. The bulging center indicates excessive heat generated by high leakage current. It's possible that the Micamold's with excessive leakage may have caused the A-B resistor drift depending on the particular use of the capacitor-resistor connections in the circuit.
COMPONENT QUALITY - IMPORTANT NOTE: Micamold capacitors are not "mica caps." They are molded paper-wax caps - an earlier version of the infamous Sprague "Black Beauty" molded capacitors. Micamolds have the same problems (maybe worse) as the typical Black Beauties with excessive leakage current. If you examine the Micamold capacitors carefully you will almost always see that the center of the capacitor body is bulging due to excessive leakage current causing heat buildup and the resulting swelling or bulge. All the Micamolds must be replaced for reliability and top performance.
Rebuilding - The metal tub capacitors all had red
silicone rubber applied around the terminals. This was probably to deter
oil seepage although there was no evidence of any leakage. I removed one
of the tubs to see if they had actually been rebuilt but the bottoms
were still in place indicating they had not. For the tub capacitors, I
chose the "yellow jacket" polyester type capacitors since they fit into
the metal tubs better.
There are six triple capacitor metal tub mount units used in the AR-88. These were removed and using a large old-style soldering iron, the bottoms removed - details (and a PCB oil warning) about the procedure is above in Restoration Suggestions. I chose to install the polyester film type of capacitors because they fit into the metal tubs and are excellent replacement capacitors. I don't put the bottom back on the tub for two reasons - first, it's not necessary since the tub is mounted up against the chassis and second, removal of the bottom ends up destroying the bottom cover anyway.
|Wiring and Rectifier Socket - Another problem was the
replacement tube socket for the rectifier tube. Although someone in the
past had installed a ceramic socket, it was made by National and
the mounting holes didn't line up. Fortunately, no extra
holes were drilled but the socket was just mounted with one screw instead. Finding the
correct ceramic socket might have been difficult except that I
remembered I had a parts
chassis from the IF/AF section of an old RBB receiver. These WWII
receivers were built by RCA and they used the exact type of ceramic
socket needed for the AR-88.
The sloppy wiring that had been installed in the receiver ended up removing much of the cable harness lacing. I replaced most these wires because they were too short for proper routing and then I had to replace the missing cable lacing. I used waxed string that I laced around the wiring harness with a "needle" made out of 20 gauge wire. The needle allows easy threading around the harness without too much lifting of the harness. After all of the work was completed to the wiring, amber shellac was applied to the harness to hold everything together better and to impart an aged look to the wiring.
AC Power Switch, and the RCA "Meatball" - Since the AC voltage selector switch was broken and a good condition replacement couldn't be located, I soldered the "arm" wire to the 125vac terminal on the switch body which accomplishes what the switch would do if set on 125vac. A vintage two-conductor power cord was installed with vintage plug. Since I always operate these receivers in the ham station, the chassis is always grounded via the station ground. Therefore a non-original three-conductor power cord with grounded plug becomes redundant.
I installed a modern replacement 2 meg potentiometer (linear) for the AF gain control. The replacement was a high quality unit but the shaft was very short. I used a metal .25" sleeve coupler to add the proper shaft length.
The RCA "meatball" emblem was pretty rusty. It was restored as described above in "Cosmetic Restoration" with excellent results.
Plastic Panel Problems - The plastic panel that covers the dials looked to be in excellent condition but someone in the past decided the back was too dirty. Unfortunately, the dirt was lacquer based and apparently the panel was scrubbed intensely with Windex or some other kind of harsh cleaner. Of course the lacquer based dirt wasn't removed but some of the white paint transfer parts of the dial cover were affected. Restoration required first to not worry about the dirt on the back of the panel. Then, luckily only the white was affected by the solvent and the black transfers were still in fine shape. This only required that the white be reapplied. I mixed a matching color with Artist's Acrylic, masked off the area so I would get a very straight edge. I thinned the paint quite a bit so several coats were required with time between coats for drying. >>>
Power up - Problems are always to be expected when a project of this complexity is
involved - that is, major parts missing, non-original wiring and with some disassembly of
the unit. So, we weren't too surprised when, upon power-up, all
this AR-88 did was to produce a low frequency "growl." Pulling a couple
of tubes isolated the problem to the 1st AF amplifier section and the
problem was caused by a misplacement of the 1 meg resistor in the grid bias
divider. This resistor is located on the component board and all that
was necessary was to move one end of the resistor to the adjacent
terminal to have the proper connection. This stopped the low frequency
oscillation but still no signals. A quick check of the IF section
revealed that there was no screen voltage on the 3rd IF amplifier tube.
This was traced to a mis-wire that involved the screen bypass capacitor,
a multiple cap tub mounted on the chassis inside wall, not having the
screen voltage wired to the correct capacitor terminal and therefore not connecting to the
1K screen load resistor. The correction required swapping the
connections to the outside terminals on the three terminal tub. This
receiver sprang to life upon power up. A quick check showed that all bands were functional and
all controls seemed to function correctly. I performed a "quickie" IF
alignment (not a sweep alignment) to see how far off the IF was. It was
very close. A run through the AM BC band showed that the audio was
everything I was expecting. On shortwave, there were some 40M and 20M hams
operating on SSB and CW that were received. Also, Radio Venezuela was
tuned in and luckily they were broadcasting music - incredible audio. I
let the AR-88 operate for about five hours and checked the power
transformer and the chokes for temperature - all three were just barely
The "power up" test resulted in everything electronic functioning correctly on the AR-88D. When the alignment of the receiver was started it became apparent that we weren't going to be able to complete the RF Tracking part of the alignment due to gear box problems that prevented accurate dial readout.
|Gear Box Problems
- The gearbox was obtained from VE8NSD from one of his parts sets. I
cleaned and lubricated this box as described in the Electro-mechanical
Restoration section above. The gear box looked great but a problem
surfaced when the gearbox was mounted to the chassis, coupled to the
tuning condenser and the main tuning dial mounted to the shaft. Though
the tuning condenser rotation was fine the main tuning dial would move,
then stop, then jump ahead. The problem was caused by severe wear in the
dual spring-loaded 270º gear that drives the main tuning dial from a
gear mounted on
the tuning condenser drive gear. In fact about half of the gear teeth
were broken off the brass gear that drives the dial. This problem was
mostly likely the result of one of the original military operators
"spinning" the dial until it "slammed" against the mechanical stop which
happens to be on the split-gear that drives the dial/condenser gear. The
mechanical stop-tab was severely bent showing that the gear had "slammed
into" the stop several times with considerable force. I exchanged the first gear box for another one from
Garth. This one was in excellent overall condition but when installed,
it also "jumped" at the top of its range. This time the problem was due
to only one tooth being missing from the dual 270º split-gear.
It seemed likely that since all of Garth's CR-91A "parts sets" had been acquired from surplus sources they probably all had similar mechanical issues and that was why they were sold off by the Canadian government as surplus in the first place. All of the other CR-91A parts purchased from Garth were in excellent condition. Garth provides a great service for the AR-88 series restorers. It just seemed that the receiver gear boxes had suffered at the hands of the former military users. See section above "Tuning Gear Box Variations" for more details on the gear boxes.
A Third Gear Box is Found
- In August 2011, I received a telephone call from Don Trueman VE4AY in Winnepeg,
Manitoba. Don said that he had an "unused" (maybe "NOS")
AR-88 gear box that he would donate to this restoration project. He
indicated that the AR-88 Gear Box had been setting on his parts shelf
for the past 15 years and that he was glad his AR-88 "spares" was
finally going to be installed into a receiver. When the gear box arrived
I examined it carefully and was surprised to find that the dial drive
split-gear was quite different from gears in the surplus CR-91A gear
boxes. A quick examination of my AR-88 series receivers showed that
they all had split-gears just like the one in the gear box I had just
received from Don. My conclusion is that probably only CR-91A receivers
have this 270º split-gear that has been such a problem. So, not only
did I get a nice condition, working gear box from VE4AY but I also
learned why the former replacements had been so problematic.
I cleaned the new AR-88 gear box with a WD-40 flush and brush. I then repacked the shaft bearings using the old style "stringy" sodium-based grease. This grease is similar to the original grease used in the 1940s and has a higher viscosity than modern "red" grease. After mounting and connecting the coupler, the new gear box performed perfectly.
Another Part Surfaces - I was able to trade some medium size RCA knobs for a nice condition tuning condenser cover. Though the RF cover is still missing, having the tuning condenser cover is important as it provides protection to the tuning condenser and slightly alters the RF tracking, though not enough to affect alignment significantly.
Sweep IF Alignment - The IF alignment was performed as described in the section "Sweep IF Alignment of AR-88 Series Receivers" above. No problems were encountered. In fact, the photographs that illustrate the various IF patterns as seen on an oscilloscope were taken while sweep aligning this AR-88 receiver. What is noted with a sweep IF alignment is the absence of "peaks and dips" in the signal level when tuning through very broad AM signals.
RF Tracking - Fortunately, the arrival of the NOS gearbox and the tuning condenser cover all happened before I actually performed the RF Tracking alignment. This of course meant that I only had to do the RF Tracking alignment one time. I performed the alignment as described in the above section: "RF Tracking Alignment."
Tube Problem - Although all of the tubes were tested and replaced as necessary, tube testers do not find all tube problems. A microphonic tube is one type of problem that many tube testers can't find. That was the case with the 6SJ7 First Audio Amplifier tube. Unusual noises, especially "ringing" when tuning or operating the Audio Gain were the symptoms. Tapping on the side of the tubes when the receiver was operating narrowed the suspects to the 6SJ7. A known good 6SJ7 from another operating '88 receiver confirmed that the suspect 6SJ7 was microphonic. Another 6SJ7 was installed and that cleared up the problem and allowed the receiver to operate correctly with great audio and plenty of gain.
Is it worth all of the work to fully rebuild an AR-88? If you want an
example of the AR-88 series that is functioning like it did when it was
new, that you can leave turned on for hours and that sounds incredible, then the answer is,
yes! What is apparent right off is the
audio level available. Many users operating original component AR-88s will
comment on the fact that the Audio Gain has to be advanced over fifty
percent for average output levels. When rebuilt, the Audio Gain is quite
loud at about 20% advanced. Not much difference is noticeable when
switching between a 3.2 ohm Z speaker and an easier to find 4 ohm Z
speaker. Both only require about 20% advancing of the Audio Gain for
comfortable volume. Bass response is greatly improved also.
With a rebuild and a true sweep IF alignment, AM BC stations and SW BC stations sound incredible and when tuning through the really broad ones, the output remains constant with no "peaks or dips." One of the things that always bothered me was tuning through an AM signal and finding two peaks or one peak that wasn't centered in the passband of the receiver. When performing the sweep alignment you are adjusting each IF for a specific shape of its passband with the object of achieving an overall symmetrical pattern. The result is an even response as you tune through any AM signal.
What is noticed after a complete rebuild is just how cool the power supply filter chokes operate. The hot chokes were certainly indicating that many of the original bypass capacitors were leaky. This AR-88D is now a receiver that can be left on for hours and provide excellent sensitivity and super audio reproduction. Currently (11-2011) this AR-88D is paired up with an ART-13 transmitter in our vintage military amateur radio station.
Collector's Gallery of the AR-88 Series
WS-430 & WS-430II - Chinese Version of CR-88A/SC-88
In the mid-1950s, the Chinese started building copies of the RCA CR-88A (possibly the SC-88.) These receivers were designated as WS-430 and while they are very close copies, they are not what one could call replicas. Instead the Chinese used Russian tubes, slightly changed the locations of some of the components and ultimately replaced some of the tubes with miniature versions. These later copies were designated as the WS-430II. These receivers were used in airports and other communications locations in China. Note in the photograph that this receiver is equipped with a Diversity IF Gain control which implies that this receiver may have been used in a diversity system. With the top of the chassis mounted crystal for the Crystal Filter and the Diversity IF Gain control it is possible that the SC-88 was copied.
Today, the WS-430 series doesn't seem to be very popular with Chinese hams or collectors although this one was pictured on a website of Chinese equipment. Apparently, most Chinese hams prefer new, modern equipment and are not fans of receivers that were copies of USA designs.
Thanks to WA6OPE and WB6NVH for the WS-430 info.
ZS6JPS - CR-88A
This nice condition CR-88A is located in South Africa and is owned by Jacques Scholtz, ZS6JPS. The receiver's serial number is 001700. It has an original, RCA umber '88 series cabinet although the two screw covers are missing. CR-88A receivers were typically for triple diversity receivers but many were sold as "stand alone" receivers by RCA. The CR-88A is the post-WWII replacement for the AR-88F.
Operating Multiple AR-88 Receivers in Diversity
|Successful diversity operation requires multiple spaced
antennas and multiple receivers - not something that every operator has
Here's how it's done if you have the physical space and the receivers.
Let's take antennas first. The DR-89/RDM manuals state that the required antenna spacing is 1000 feet oriented in a triangle. However, this was military/commercial set-ups and provided absolutely the best diversity effect for nearly all frequencies in the MF and HF part of the spectrum. Usable diversity effect can be achieved with antenna spacing at around one wavelength at the received frequency, sometimes even less. If space is really tight, one antenna can be a vertical and the other a dipole, in other words, one vertically polarized and one horizontally polarized antenna - assuming dual diversity. For most frequencies in the HF region, 100 feet of separation will give usable diversity effect most of the time. It's a little close for 80 meters so if you can get greater spacing then try for at least one wavelength.
Next are the particular AR-88 receivers that you are going to use. Your AR-88 receivers should be the actual diversity models that were used in the DR-89, RDM or OA-58A/FRC receivers. That way you'll have the separated Diode Load and Diode Return lines and you'll have the AVC line available. Additionally, only the diversity receivers have several resistor changes in the IF section for better diversity effect. Although all AR-88 versions provide a terminal to access the AVC line, only the true diversity versions will provide terminals to allow "diode-OR'ing" of the second detectors of each receiver to achieve true diversity operation.
When receivers designed for diversity operation are used, you'll find that the second detector output is brought out to terminals on the rear of the receiver marked "Diode Load" and Diode Return." When operating two or more receivers in diversity, each of the receiver's detector outputs, the Diode Load, are connected together, in other words, "diode-OR'd." This then allows the receiver with the strongest signal to dominate the other receivers. As the signals fade because of multipath phase shifting on any of the antennas, whichever receiver has the strongest signal will dominate the Diode Load line.
In order to keep the diversity system stable it is necessary to have feedback for control and this is accomplished with the AVC line. All AR-88 versions of the receivers will provide an output for the AVC line at the rear of the receiver. For diversity operation, all of the receiver's AVC line terminals need to be connected together. >>>
>>> In theory, you would only need one receiver (the "master") out of the two or three used in Diversity to provide the audio output. The Diode Load would be connected to the Diode Return on that receiver only. However, this makes tuning rather difficult and it's really necessary to be able to activate the other receiver's audio in order to hear or measure their response to the desired signal. The easiest way to accomplish this is to connect the Diode Load to the Diode Return on each receiver (don't connect the Diode Return lines of each of the receivers together, though.) Then, when tuning in the desired signal each of the receivers can be providing their own audio output for ease of tuning. You can use the headphone jack for individual receiver monitoring. Just be sure to provide a load resistor across the audio output terminals on the second or third receivers. Once balanced, only the "master" receiver is necessary for audio output to drive a speaker and the other receivers ("slaves") will have load resistors installed on their audio outputs (when using 'phones, since a load is already on the audio output, only plug the 'phones into the jack half-way.)
Tuning requires that each of the receivers are first tuned to the desired signal. Next, each of the receivers have to be balanced to respond equally to that signal using their DIVERSITY IF GAIN controls. Listen in turn to each receiver and adjust the Diversity IF Gain control so that each receiver-antenna combination is responding to the desired signal equally. You'll have to judge the signal strength between the fading. Once the receivers are balanced, the audio from the "master" receiver can drive the loud speaker and fading should be practically non-existent.
Now in the original DR-89 diversity rack, the operator had the Monitoring Unit where each receiver's Diode Load current could be actually measured. However, most amateurs don't have all the pieces to the diversity rack. An elaborate set-up could use three current meters (0-300uA) in the Diode Load to Diode Return line to act as a more accurate reference for balancing. In the professional set-ups, much of the copy was done by machines that required absolutely perfect signals but since our use is only for listening pleasure, balancing by ear is certainly close enough. As the receivers become equal in response (balanced,) the diversity signal will become more stable and fading will disappear.
Dual diversity with properly spaced antennas will reduce fading about 90% while triple diversity with properly spaced antennas will reduce fading by 99%.
Comparisons with the AR-88's Contemporaries
|I really can't title this section "'The Competition" since the AR-88 really didn't have any - given its unique history. However, for comparison I've selected three of the best performing receivers of the 1940s that, in addition to having a history of military use in WWII, also were popular ham receivers - both before the war and after.|
|Hammarlund SP-200 Series 'Super-Pro' - The SP-200 was a popular commercial receiver before WWII but was generally too expensive for many hams to purchase at that time. By the 1950s, the SP-200 military receivers, BC-779, BC-1004 and BC-794 were becoming available through the surplus market and for many hams this was their first exposure to this great receiver. Certainly, performance and quality build reputation was what motivated the Signal Corps to have thousands of SP-200 receivers built for their use during WWII. Many of the surviving surplus examples are victims of years of use/abuse and now require extensive restoration to achieve their true performance capabilities. The SP-200 has double-preselection on all bands, an accurate 0.5% dial readout, a unique bandswitching system that uses a cam-operated plunging "knife-type" switch contacts and probably its most famous attribute - its variable-coupled IF selectivity. Also famous - the 'Super-Pro' 14 watt P-P audio. As for advantages over the AR-88, the Super-Pro has the following - a higher power and certainly to some ears better quality audio reproduction, bandspread tuning, infinitely adjustable selectivity, an S-meter and an easy to use Send-Receive/Remote Stand-by set-up. Disadvantages are - actual tuning range is rather limited on any individual model, antenna input is not adjustable, requires a separate power supply, the audio output is 600 ohm Z only (other than Hi Z phones) and sensitivity falls off rapidly above 20mc on the SX (BC-794) versions. Also, since the SP-200 was really designed to be on continuously, it takes a very long time to stabilize - drift lasts for at least an hour, sometimes longer. High front-end noise was also a common complaint with the Super-Pro receiver but a matched antenna would generally help out this particular issue. On the weight side of things, the receiver generally weights about 60 lbs in the cabinet and the power supply weighs about 50 lbs depending on whether its the heavy-duty military version (~60 lbs) or the standard Hammarlund type (~45lbs.) Below 20mc, the SP-200 will certainly equal the AR-88 in performance with a positive edge given to the Super-Pro on the audio performance and the infinitely adjustable selectivity.|
|National HRO Senior - As with the SP-200, the HRO was available long before WWII and already had a large group of enthusiastic users - hams. Great Britain was sending over individuals before Lend-Lease to buy HROs to take back to England for intercept work. The HRO has double preselection on all coil sets - that's right! - you have to change coil sets to change the tuning range. Usually, four coils sets gave the user about 1.7mc to 30mc coverage. Most military users got the complete set of nine coils for coverage from 50kc to 400kc and 500kc up to 30mc. Storage of the unused coil sets was a problem. Then there was the micrometer dial - the readout had to be compared to a graph to determine where you were tuned. Also, a separate power supply was required. Why did a receiver that used plug-in coils, indirect readout and had a separate power supply rate so high with military users? Because the HRO had absolutely the quietest front-end with the best sensitivity available. The mechanical tuning was fabulously smooth and accurate (given that charts and graphs were involved.) For finding those weak signals, the HRO couldn't be beat. Audio is from a single-ended 2A5 or 42 depending on the vintage and while not in the Super-Pro league, it is respectable and sounds great with a good matched speaker. The HRO has an easy to set-up Send-Receive/Remote Stand-by function. Disadvantages are: numerous accessories required, stability takes about an hour to achieve, no selectivity control other than the crystal filter and no direct frequency readout. While the chassis is well constructed, the cabinet is only held together with a few sheet-metal screws. Weight-wise there's no comparison - the HRO receiver only weighs around 30 lbs and the power supply about 10 lbs. Additionally, the receiver is very small when compared to most of its contemporaries. Compared to the AR-88, the HRO has superior sensitivity, with the exception of coil set A (10M.) If used in the bandspread mode (only available on the non-WWII versions and only on coils sets A, B, C and D) the tuning resolution is unbeatable.|
|Hallicrafters SX-28 - The SX-28 was also available just before WWII being introduced in August 1940. By early 1942, the SX-28 was becoming very popular with the Signal Corps and, in addition to the standard configuration, the SC also had the heavy-duty version, AN/GRR-2, and the airborne version R-45/ARR-7 built. The SX-28 has double-preselection above 3mc and six positions of selectivity but it's ham band calibrated bandspread is unique to all of the receivers mentioned here. That the military had any use for the ham band calibration is doubtful but it was present on all versions except the R-45/ARR-7. SX-28 advantages are - its famous audio reproduction which used two 6V6 tubes providing 8 watts of P-P audio with great bass response, the Lamb Noise Silencer was a tuned noise blanker that worked great - even on CW, a calibrated and fairly accurate bandspread tuning, an Antenna Trimmer, a built-in power supply and an easy Send-Receive/Remote Stand-by set-up. Disadvantages are few but the audio output only provides 500 and 5000 ohm Z outputs that almost dictates the use of a Hallicrafters speaker, usually the PM-23. Additionally, drift stability takes at least 30 minutes to achieve, usually longer. Also, the build quality is certainly below that of the AR-88 or the Super-Pro with lots of self-tapping sheet metal screws used in the construction. Additionally, like nearly all of 1940s receivers, the SX-28's sensitivity drops off dramatically above 20mc. Weight is considerable because of the built-in power supply and usually runs the scale up to 75 lbs when in the cabinet. The SX-28 is certainly an equal performer to the AR-88 (below 20mc) and, like the AR-88, only a speaker is required as an accessory. The audio is more powerful and, to most ears, more impressive than the AR-88's.|
The AR-88 Performance Today
|This review should be considered for "rebuilt and aligned" AR-88 Series Receivers. Original condition receivers, while they may seem to operate quite well, are never functioning at their design limits. Best performance results will be experienced with fully rebuilt and aligned receivers.|
The AR-88 series receivers are examples of 1940s receiver designs that will perform so well you'll think it was designed in the 1950s or 1960s. They were actively used by RCA and RMCA up into the 1970s when most 1940 radio designs had long since retired. Highly sensitive all the way up to 10 meters with five steps of selectivity to cope with crowded bands, an effective noise limiter and, of course, that fabulous audio that reproduces those low bass tones with no distortion.
The AR-88 high frequency sensitivity is renown and was achieved by using polystyrene coil forms in the higher frequency bands - and this in a day when most people had never heard of polystyrene plastic. Full sensitivity is available all the way up to 10 meters. Hearing stations was really never a problem with the AR-88. Selectivity is a little problematic because the early AR-88 design assumed that all you wanted the Crystal Filter for was to narrow the IF passband. It does that very well but, as many AM ham ops know, a good crystal filter can also be used to eliminate heterodynes. CW ops know that a good Crystal Filter can enhance certain audio tones making CW easy to copy through QRM and QRN. RCA decided to change the AR-88 design after WWII and moved the Phasing Control to the front panel. If you want to have the advantage of a Crystal Filter Phasing control easily accessible on the front panel, you'll have to find the CR-88 version or other late versions of the receiver.
Dial accuracy is where almost all vintage receivers find their detractors and the AR-88 is no exception. The tuning dial is "1940s" accurate but since wide slices of the spectrum are covered in each band, the resolution of the dial is necessarily vague. The ability to have 1.0kc readout (or better) on a receiver has been around since the late 1940s but many of today's hams have never been exposed to how frequency accuracy was achieved without a Collins-type readout (or, a modern digital readout.) Before the digital world, every commercial and military station had heterodyne frequency meters and if it became necessary to know exactly where the receiver was tuned, these instruments allowed the operator to set the receiver frequency to better than 1.0kc accuracy. The logging scale then allowed the operator to return to that exact frequency once it was known. >>>
>>> SSB signals are easy to copy on the AR-88 and you don't need to do any modifications either. Simply turn off the AVC, increase the AF gain to near maximum and then reduce the RF gain by about 25%. Tune in an SSB signal with the receiver in REC CW (BFO on) until the voice sounds normal but overdriven. Then slowly reduce the RF gain until the audio clears up. Usually, the RF gain is about 60% or so on most SSB signals but it depends on signal level and the band in use.
Since a VT-150 tube regulates the +150vdc voltage (which is used for the LO and BFO plates besides the receiver screen supply) the AR-88 is very stable. Drift is practically non-existent after about a 10 minute warm-up. You can listen to the West Coast 40M Swap Net (or your local Amateur HF Swap Net) without getting up to retune every five minutes. For CW, the stability makes this mode easy to enjoy when using the AR-88 as the receiver.
AM is where the AR-88 really becomes a "great" receiver. Audio reproduction is wide range in SELECTIVITY BROAD POS 1 and the receiver will produce undistorted bass notes like no other "communications receiver" can. Vintage AM ham stations sound great but the ham stations that run retired AM Broadcast transmitters sound incredible on the AR-88. If the AM signals are weak, better IF gain is available in POS 2. When QRM becomes a problem, POS 3 and sometimes POS 4 can be used to eliminate the problem, especially if you have the later versions of the receiver with the CRYSTAL PHASING on the front panel.
The antenna connections are originally terminal strips but it is easy to utilize coax as the feed line by connecting the shield to the ground-A2 terminals and the center conductor to A1. Spade lugs can be crimped or soldered to the coax shield and center conductor for easy connection to the terminals. This provides an unbalanced input to the AR-88 which is the normal output from a standard antenna coupler that would be used with a typical ham antenna. The receiver performance will be at its best with a matched antenna.
Using the correct impedance loud speaker will provide very loud audio output from the 2.5 ohm Z terminals. The original speaker is hard to find but any speaker from 3.2 to 4.0 ohms Z will provide very loud audio output. Speakers with 8 ohms nominal impedance require that the AF gain be advanced substantially. Try to use the lower Z speakers for best audio performance. The AR-88 AF gain control is normally advanced about 25% to 40% for normal listening when in the AVC-controlled AM mode and driving the correct speaker impedance. You can also utilize the 600 ohm Z "line" audio output with a matching transformer but I have always found the 600 ohm Z audio to not be quite as clear sounding as the 2.5 ohms Z output. If you do use the 600 ohm line for the audio output be sure to install a 4 ohm 2W resistor on the 2.5 ohm Z output terminals for a load.
Using a good quality, 3.2 ohm Z speaker will provide excellent audio but don't be fooled into thinking that a modern 4 or 8 ohm Z stereo speaker box will sound really great - they usually don't. The reason is they aren't very efficient with only 2.5 watts of drive power available from the AR-88. The old style, fairly stiff suspension, 12" PM 4 ohm Z speaker mounted in a large box with bass reflex port will sound best. If you can find a large speaker with a 3.2 ohm Z voice coil, the performance will be even better since you won't have the AF gain quite so advanced as with the higher Z speakers. The original MI-8303D speaker is only an 8" speaker but it is very good for most communications-type audio. The larger speakers with good enclosures will show-off the AR-88 audio best.
Lastly, there is the weight issue. There's no doubt that the AR-88 is a very heavy receiver. In the cabinet, with all of the shields installed, it will weigh just over 100 pounds. This makes moving the receiver around a strenuous effort. I find that it is helpful to remove the receiver from the cabinet when moving. This will lighten the load by around 10 to 15 pounds. Also, the use of a roll cart is a tremendous help when moving the receiver from the work bench to another room - like the ham shack.
Using the AR-88 as a Ham Station Receiver
The easiest solution is to use an electronic T-R switch to protect the receiver's input. These devices were originally developed to allow "break-in" keying which actually allowed you to keep the receiver operating while you were transmitting CW. You could actually hear some signals in between your sending (unless you sent fast CW in which case you had to pause between sentences for "break-in" stations.) The electronic T-R switch can be used on AM also, although you may have to reduce the AF gain during transmit to prevent feedback. >>>
| >>> The E.F. Johnson Electronic T-R switch is popular although a bit expensive. If you don't want to pay the high
prices that the E. F. Johnson T-R switches are selling for today, T-R
easy to build and the ARRL Handbooks from the sixties and seventies have
good plans. However, sometimes problems are encountered using electronic
T-R switches due to the necessity of mounting the unit very close to the
transmitter output. T-R switches are sometimes very sensitive to the
length of coax used between the transmitter and the T-R switch and
coax lengths to the receiver will
cause signal reduction. Consider your station set-up
carefully before deciding on an electronic T-R switch and if you can't mount the unit very
close to the transmitter you might have to come up with a different solution.
If you don't want to build or buy or can't use an electronic T-R switch, you can use a Dow-Key relay. Most of the Dow-Keys will have an automatic disconnect built into the receiver-side coax connector and auxiliary contacts that switch with the relay action. By wiring the AR-88 antenna lead (using coax) through the auxiliary contacts before connecting to the receiver coax connector of the Dow-Key, one can achieve not only a receiver antenna disconnect but also "short" of the receiver antenna input to ground on transmit. Utilize the coax shield as the ground connections and switch the coax center conductor (Arm contact should go to Antenna terminal on receiver.) Due to the size involved with the Dow-Key contacts, RG-58 coax will have to be utilized for the receiver antenna connections. This setup with the Dow-Key relay will protect the receiver input and allow the AR-88 to remain on, although you will certainly have to reduce the RF and AF gain to prevent audio feedback from the receiver to the transmitter audio input. Feedback, of course, won't be a problem when operating CW.
Using the CR-91 on Longwave
The AR-88LF and the CR-91 receivers will tune from 70kc up to 550kc on Bands 1 and 2 allowing the user to tune in to the thousands of Non-directional Beacons, NDBs, around the world. NDBs transmit in both the LF and MF part of the spectrum and are found from 195kc up to 525kc. Each NDB is assigned a two or three letter identification that is sent in MCW (Modulated CW) every few seconds. The transmitter antenna is usually located very near the airport runway since these were a primary navigation signal at one time. Most marker NDBs run 25 watts while regional beacons may run up to several hundred watts. Transoceanic beacons may run as much as 2000 watts. The modulated signal uses a 400 hertz tone and the CW is sent at a rather slow speed, usually about 10 words per minute (or less.)
During the daylight hours only local marker NDBs will be received. Regional NDBs can sometimes be received as far as several hundred miles during the day, depending on the time of year. Night time listening is by far the most fun as NDBs can be received from thousands of miles away. Even 25 watt marker NDBs can be received at great distances when using good equipment and a proper antenna.
Since the AR-88LF and the CR-91 are designed for a substantial LF antenna of 200 feet or about 500 to 700 pf of capacitive load, performance in the part of the spectrum will suffer with a small antenna. I use my 135' center-fed tuned dipole with the feed line shorted together and fed to the antenna as an unbalanced load. This could be acting as a 43 foot vertical with a large capacity hat, kind of like the old style "T" antenna. Anyway, it seems to work quite well with the CR-91. A tuned loop antenna will work wonders on a superhet receiver providing lower noise reception and directional characteristics. I highly recommend using earphones for NDB reception.
During an evening and a short morning listening sessions this past February (2010) I was able to tune in around 50 or so NDBs with the CR-91. The best DX was YQA 272kc located in Muskoka, ON, Canada, about 2000 miles from Nevada. Best marker beacon DX was SYF 386kc in St. Francis, KS. This was somewhat past the optimum winter conditions of December and January. Next season, I'll be trying the CR-91 on the tunable loop antenna. The advantages of the CR-91 are an accurate tuning dial and plenty of sensitivity. The disadvantages are that all superhets are pretty noisy in the LF/MF part of the spectrum.
I was lucky enough to have purchased CR-91 SN 050068 from a surplus store in Reno, Nevada in 2010. I was told about the receiver by a visitor to our museum who had seen it in the surplus store the day before. To my surprise, in Reno the following day, the CR-91 was still there. After a little haggling, I purchased the receiver for $125. It was installed in a rusty Scott SLR cabinet. Though it needed considerable restoration the photo left is the end result.
Today, the AR-88 series of receivers have world-wide admiration, not only for their historical significance but also for their tremendous performance capabilities and fabulous audio reproduction. AR-88s and their variants can still be found in AM ham stations where they are providing marvelous audio reproduction of quality AM signals transmitted from classic ham AM transmitters and even from the retired AM BC transmitters that several hams operate today. The AR-88's robust construction, which resulted in a receiver that weighed over 100 pounds in the cabinet, has somewhat curtailed the easy exchange of surviving examples due to expensive shipping rates. However, the AR-88's superior audio, great sensitivity and fabulous "looks" has resulted in many devoted fans from around the world that are willing to pay for shipping or are willing to travel great distances to pick-up one of these incredible receivers. The AR-88 was a milestone classic of radio design and the surviving examples are still reliably reproducing great radio signal audio just as they did sixty-plus years ago.
Henry Rogers WA7YBS/WHRM - April, 2010
1. RCA Manuals for NAVY MODEL RDM, AR-88D, CR-91 and
www.radiomarine.org - Header
Photo of Fred Baxter at KPH - Great photos and history of RMCA's KPH
1. Thanks to Alan Ford, VK2DRR for providing his AR-88D, F
and LF serial number logs and his thoughts on the RCA serial number
Henry Rogers WHRM © April 2010, more photos and information added May 2010, more info added August 2010, new info and photos added August 2011
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