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May 24 2019

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Radio Boulevard
Western Historic Radio Museum

RCA’s Amazing AR-88 Receivers
Includes: AR-88D, AR-88F, AR-88LF, CR-88, CR-88A, CR-88B,
CR-91, CR-91A, SC-88, R-320/FRC, DR-89, RDM and OA-58A/FRC

Part 1 – History and Design – The Various Models – General Information

Part 2 – Triple Diversity Models – AR-88 Serial Number Analysis & Log

Operational and Modification Caveats – Restoration Suggestions

Part 3 – Sweep IF Alignment – RF Tracking Alignment

Part 4 – Operating AR-88s in Diversity – Performance Comparisons

AR-88 Performance Today – Easy and Reversible Muting Mod

by: Henry Rogers – WHRM – WA7YBS

KPH operator, Fred Baxter, flanked by two RCA CR-88 receivers and a Collins 51J-4 receiver. The CR-88 receivers were the workhorses for Radiomarine Corp. of America’s radio station KPH during the late forties, through the fifties and even into the sixties. Photo from KPH History website: www.radiomarine.org

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,” highly reliable 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 along with mechanical and electronic reliability that couldn’t be found in any other receivers of the day. 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.

photo left : The AR-88 was used extensively as an intercept receiver during WWII as shown in the 1956 Norwegian film “Konakt!” – photo from ON4ROB

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.)

Many of the British AR-88s were destroyed after WWII ended. This was due to the provisions in the Lend-Lease Act which stated that materiel had to be either returned or destroyed. In just one incident, a load of AR-88 receivers was “dumped” into an abandoned well by USA forces after the war ended. Some sources even indicate that RCA had made it clear they did not want to see the receivers back in the USA for any reason. The AR-88 survivors, along with other surviving materiel, generally were placed in groupings that were “sold back” to Great Britain at discount, which was usually at “ten cents on the dollar.”

AR-88s survived in Canada because the receivers were built in Montreal and during WWII remained in Canada for various needs there. The AR-88LF versions, which were only built in Montreal, found their way to airports, civilian and military, ship-to-shore coastal stations around Canada and for general communications. Although Canada did export AR-88LFs to Great Britain, many remained behind for wartime use. After WWII, commercial users, such as the airports and coastal stations, did continue using their AR-88LFs for sometime. Eventually, most of the receivers have made it to the Canadian government surplus sales and many were available though other Canadian surplus outlets. >>>

photo left: VE1HO Stadacona, NS in Canada using the CR-88 version in their club ham station. photo from: www.jproc.ca/rrp/stadacona

>>> 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 any 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-1950s and early-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. More on UA-UK use of the AR-88 in the section “AR-88 and Russian Hams” below.

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.

The CR-91 version of the AR-88 from around 1945

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.


The CR-88A version of the AR-88 from around 1947

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 to its predecessor, the AR-60, which sold for $475 in the configuration used by the USCG (the CGR-32-1,) one can estimate that the AR-88 cost at least $475 – maybe even slightly more. 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 CR-88B version of the AR-88 from around 1952

This is the last of the AR-88 series, the 1951-53 CR-88B. Note the dial mask, the different placement of controls, different front panel mounting and the shorter chrome strips. Note the three position SELECTIVITY switch, 500kc CAL switch and the two position TONE switch. Inside, the “B” has push-pull audio output. This excellent condition example of this rare receiver is owned by N6YW who provided this photo (also see Collector’s Gallery.)

The first release of surplus AR-88s to the Russian hams was in the late-1950s. The military was stocked with the KV-M receivers that were Russian-built close-copies of the Hammarlund Super Pro BC-779. They also had Krot/M receivers that were Russian-built copies of the Nazi E-52 Koein and the early version of the Russian R-250 called the AS-1 and AS-2. As a result of the quantities of various types of receivers available, the military released the AR-88, DR-89 and original Nazi E-52 receivers to the Ministry of Communications where they ultimately became available to hams and ham clubs. Later, in the sixties, the Soviet military had the R-250, R-250M/KMPU/Kalina/Step and later still, the R-250M2/R-670M/KMPU-M. A rack with two R-250M was designated as KMPU and a rack with two R-250M2 or 670M was designed as KMPU-M. >>>

photo right: UA1OSM Serge with his AR-88F mounted in a cabinet ca: late-sixties

photo left: Ernst Krenkel, famous Russian polar explorer and radioman for many polar expeditions. Krenkel Observatory in Franz Josef Land is named after him. Krenkel wrote a book on radio, was featured in several radio magazines, worked in the radio industry and in the scientific instrument industry. He was awarded several honors from the USSR (Order of Lenin, Hero of the USSR.) Krenkel was a very active radio amateur. His calls were U3AA, UA3AA and the call shown on the photo, RAEM (written as “RaeM” – the photo was sent to LA2JE as a QSL.) The receiver is an AR-88F and the transmitter is a BC-610. Note that Krenkel is using an enclosed key. Photo from Valery Mirgorodsky, Pres. Russian Antique Radio Club


photo right
: UA&#216SQ Vlad with his AR-88 ca: 1970
photo from UA1OSM

Many Russian hams started in amateur radio by becoming an SWL (Short Wave Listener) at a ham club. This usually required the aspiring ham to confirm reception of several ham stations in the CW mode by obtaining QSL cards from the station copied. That was why it was so important for USA-hams to actually respond to the SWL QSL cards they received – it helped the aspiring ham prove that he could copy “on-the-air-sent” CW, issue accurate, dated signal reports in the form of a QSL card and was able to receive a confirmation QSL card “via the Bureau.” Often, once the license was issued, the new ham operated at a club station until he was able to buy or build his own equipment.

Many UA and UK stations worked on 20M CW in the sixties and seventies were club stations. However, many of the AR-88s also were in the hands of ham stations owned by individuals. Nowadays, most of the AR-88s in Russia are either in the hands of collector-hams or in various technical or radio museums.

Thanks to Serge UA1OSM for the details on Russian military receivers and the AR-88 history in Russia.

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

AR-88 – 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 engraved.
AR-88D – The later stand-alone receiver version with table top cabinet, produced during WWII and for a short while after, sometimes with Carrier Level meter, matching speaker – usually black but there are some exceptions. “D” does NOT indicate a Diversity receiver.
AR-88F – Diversity receiver for the RCA Triple Diversity Receivers, has IF Gain Control, no carrier level meter, rack mount only – component receiver of the DR-89 and RDM Triple Diversity Receivers
AR-88LF – Stand-alone receiver early LF/MF version with 70kc to 550kc coverage and 1.5mc to 30mc coverage, earliest versions have two position AC voltage selector (115/230vac,) different (from AR-88) power transformer, different audio transformer, different phone jack operation (no load resistor,) changed after SN 3000 to use same power supply and audio section components as in AR-88D. Possibly all “LF” versions were assembled in Montreal.
CR-88
– Stand-alone receiver, post-WWII Military/Commercial version of AR-88D, Crystal Filter Phasing control on front panel, cabinet mounted, CL meter, smooth finish gray panel and wrinkle finish cabinet (but there are exceptions.)
CR-88A – Diversity receiver with IF Gain control, gray panel, no CL meter, used in later versions of RDM and DR-89
CR-88B – Upgraded stand-alone receiver with many changes including P-P audio, 15 tubes, dial mask, crystal calibrator, 3 position selectivity, 2 position tone switch, gray panel, shorter chrome strips, last version produced 1951 to 1953
CRV-46246A & B – Navy designation for the diversity receivers AR-88F and the later CR-88A – component of the RDM triple diversity set-up – usually this number is on a metal tag mounted to the front panel along with RDM reference >>>

CR-91 – Stand-alone receiver, late WWII version with table top cabinet, circuit similar to later AR-88LF, black wrinkle finish panel and cabinet with “PHONE” and “BFO” on the Function switch rather than “REC MOD” and “REC CW.” The CR-91 recievers were the RCA-Camden, NJ,USA version of the AR-88LF for WWII.

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.
R-320/FRC – Signal Corps designation of the SC-88 diversity receiver version for the Signal Corps OA-58A/FRC triple diversity receiver, with dial mask, black ultra-fine finish wrinkle panel, rack-mount only, 1950 contract
SC-88 – RCA designation of the Signal Corps R-320/FRC

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.

The CR-88B chassis was changed to allow adding the push-pull audio tubes. The power transformer was moved forward to have the receiver weight distributed more towards the center of the chassis. The AR-88s tended to be very heavy towards the rear of the chassis adding to the difficulty of lifting the receiver. Note that standard “can” electrolytic filter capacitors are used rather than the large oil-filled paper filter capacitor assembly. The IF transformers were reduced from eight transformers to six which implies two IF amplifier stages in the CR-88B rather than the three used in the AR-88. Rear chassis connections show a “phono input” available. Also note that “AVC” and “DIODE LOAD-RETURN” terminals are provided which implies that the CR-88B could be used in a diversity set-up. The antenna input was moved to the RF tuning box.

CR-88B photos from: N6YW

General Information About the AR-88 Receivers

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.)

RCA Table Speaker for the AR-88D and CR-91 – When the receiver was intended for table top operation a matching speaker was available from RCA. It was listed as part number MI-8303D . The speaker was an eight inch diameter PM type that had a 2.5 ohm Z voice coil to match the output Z of the AR-88. The speaker cabinet was usually black or RCA umber wrinkle finish and the grille was nickel-plated. There are some photos that show the speaker grille where it appears painted but the RCA manuals always pictured the plated grille. The speaker cabinet has a back that is screw mounted and that is why the speaker cable exits from the side. The table speaker was only intended for use when the receiver was going to be installed where the requirement called for one receiver providing loudspeaker output. The MI-8303D was not used with the Triple Diversity Receivers and was certainly not required where multiple receivers were in use, such as for surveillance. Multiple receiver operations require the use of headphones for the individual operators.

photo left: MI-8303D , the matching speaker for the AR-88D and the CR-91


photo above:
AR-88 CL Meter from EB5AVG’s AR-88D receiver. Photo by: EB5AVG

The Hallicrafters SX-28 receiver uses an S-meter that has similar characteristics as the AR-88 meter. It is a right-hand mechanical zero with full-scale deflection to the left at 5mA. One can utilize the scale shown to the left to create an accurate reproduction of the AR-88 meter scale and then utilize the SX-28 meter. The circuitry shown to the right must be installed. A mechanical mount will have to be made to hold the meter in place.


Connections are:
To the junction of R20 (100 ohms) and C74 (4700pf) connect Meter +, Meter – to chassis. From Meter + connect R21 (100 ohm potentiometer) pin 3, pin 2 (arm) to chassis, pin 1 NC. Change R1 and R6 to 47K and R55 to 5.6K.

“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 270&#186 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 360&#186) it’s not nearly as strong as the full 360&#186 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 360&#186 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 270&#186 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.

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.

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