UPDATED 2/9/17. And Hello to my friend in Zagreb!
A friend recently gave me an RT-654A/TRC-77. Also known as the AN/TRC-77A. The RT-654(*) is the basic component of the “TRC-77″ set.
(“TRC” = Transportable Radio Communications. The designator PRC as used with contemporary field sets indicated Portable Radio Communications. Those PRC sets could be operated while being carried; eg, the PRC-10, 25, 77, 70, 74 etc. The TRC-77 cannot be operated while being carried due to the large HF wire antenna requirements. It was therefore deemed to be “Transportable”, a designation otherwise rarely utilized for a packable set.)
It seems to me that the AN/TRC-77 is noteworthy for being extremely obscure in the historical record.
However, in my view, this is a very capable, simple, rugged field set. But it requires an operator well trained in morse code and the art of long range HF radio. Add this set to your inventory of TTP’s (Tactics, Techniques and Procedures.)
Don’t go on a long range Recon mission without one.
Above: Parts and accessories comprising the complete set.
“You see, wireless telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat.” Albert Einstein.
Above: Sending out the nightly SITREP from the hide site. Night time is the right time……Radio Silence (and light discipline) has been waived……….
Basic Specifications: (Organizational and Maintenance Manual, Radio Set AN/TRC-77 and AN/TRC-77A, TM11-5820-473-12, October 1965)
Its primary capabilities are 10-14 watts* output from 3 to 8 mc. Planning Range is “5 to 7″ miles. It transmits and receives Morse code (CW) but it can also receive on-channel AM (with BFO Off) or SSB (with variable BFO On). It is configured to be operable with an external J-45 CW “knee key” or the GRA-71 code burst keyer. The transmitter and receiver are each crystal controlled on one of 6 independent, panel-selectable channels.
(* The Depot Overhaul Standard is 2.5 watts minimum with a battery voltage down to 10.0 volts)
It is all transistorized except for the transmitter oscillator tube and the PA amplifier tube. The receiver draw is in the order of 15-20 mA, amazingly low for what it is capable of. Transmit draw is in the order of 3.5 amps, key down.
Power was provided by a BB-447A/TRC-77 wet cell NiCad battery pack. The battery box (a rare item) had connectors for an external 12 volt power source in the event the NiCad battery had discharged or otherwise failed. The BB-447A/TRC-77 was made up of ten, 1.2 volt BB-418/U nickel cadmium wet cells in series producing 12 volts nominal. These are the same cells as used in the PRC-74 radios as an option for that set. The TRC-77 systems included an AC and a DC powered battery charging system. The DC powered charging circuit was powered by any expedient 28 VDC source and current-limited by a 1 ohm, 240 watt resistor. That’s it. Simple. An operational set in the field with battery weighs about 25 pounds.
The set was designed to work with the GRA-71 Code Burst Keyer via the inclusion of the Cable Assembly, Special Purpose, CX-11389/TRC-77A as seen in the above cartoon. This plugged into the radio Key jack and one end went into the GRA-71. A third connector plugged into the radio battery to directly power the GRA-71. Encrypt your message and then send it at 300 words per minute, BZZZZZT, drop the antenna and boogie to your next location.
The author of the ARRL’s Low Power Communication chapter regarding military radios commented that the AN/TRC-77 is not really a transceiver; that it is actually a transmitter – receiver since both functions were independent of each other. Hmmmmm. Interesting…Upon looking however, the receiver final audio amplifier stage is shared by the transmitter sidetone function. Otherwise, they are electrically pretty independent of each other..
The Planning Range quoted in the manual is 5-7 miles, which is interesting in itself. The radio was developed specifically to meet an Army requirement for a “long range reconnaissance set” and the TRC-77 was certainly capable of producing “long range” in the hands of a skilled operator with reasonable propagation conditions.
However, absent even “fair” propagation, it could produce a “brute force” ground wave or LOS signal reaching shorter ranges, (like 5-7 miles). The PRC-10 and later PRC-25 FM sets that were then becoming standard could also deliver 5-7 mile ranges – so that tactical requirement was already met. How the TRC-77 became documented as a short range HF CW set for “Planning” purposes is a mystery so far.
Above: A size comparison. The TRC-77 and PRC-25 receiver-transmitters are of comparable size however the TRC-77 has a significantly deeper battery box. Read on.
The development of the basic set started in early 1959 after the Army issued a requirement for a new “long range reconnaissance set”. Note that this was several years before the US Army’s 5th Special Forces Group began operations in their training, advisory and special reconnaissance work in Vietnam. I think that the timing alone says the TRC-77 was not specifically developed for Special Forces in Vietnam as some have surmised. The Russians began construction of the “Berlin Wall” in August 1961.
When the Army issued this new equipment requirement in 1959, the focus was Europe.
So in 1959 the development was started on a “crash basis” at the Army’s Electronic Defense Laboratory which was managed by Sylvania Corporation, an early pioneer in transistor development. This new requirement was released even before the Army adopted the CIA’s RS-1 set design as the basis for the GRC-109.
The requirement was likely initiated by the promise and availability of viable early transistors in the late 1950’s. This held the possibility of smaller, light weight, entirely battery powered HF sets. No heavy, clumsy hand-cranked generator needed for high voltage power amplifier tubes! High current Germanium PNP power transistors like the 2N174 enabled switching oscillators to produce high voltages. No more HV machines necessary.
So production was then started by Sylvania with the first 120 units being produced as the AN/TRC-77 (RT-654) in June 1961. That order was completed in May 1962. A follow on order for 970 sets was placed with Arvin Industries with deliveries starting in Sept 1965 as the RT-654A/TRC-77 with some minor modifications over the initial model. My particular example has 1965 date codes on the components but I don’t know when production ceased.
It appears so far that those 1,090 sets was the total produced. Reference (51). Also note the multi-year gap between procurement orders. The Army was apparently having a hard time deciding between the competing technologies that were emerging – and not measuring up. Robust, high power RF output stage transistors were not quite ready yet. Hence the hybrid TRC-77 and PRC-25 among others.
So there I was……
Cartoon from the Organizational Maintenance Manual TM11-5820-473-12
The AN/TRC-77 was first fielded by Long Range Reconnaissance Patrol (LRRP) units of the US Army Southern European Task Force in the winter of 1961-62 as the “then new” AN/TRC-77. It was utilized by the V and VII Corps and Third Infantry Division LRRP units in Europe. The unit communicators received TRC-77 training from Sylvania Corporation technical representatives as the equipment arrived in theater. Prior to that, all LRRP units in Europe were equipped with the RS-1 / AN/GRC-109 which had replaced the AN/GRC-9 / GRC-87. The AN/GRC-109’s were later replaced in Europe (at least for some applications) by the heavy AN/PRC-74 frequency-agile SSB/CW set which in turn was nominally replaced by the AN/TRC-77 for long range HF CW communications (again in some applications). (Reference 38).
These LRRP units left the AN/TRC-77’s in Germany when they subsequently deployed to Vietnam in mid-1968, just taking their AN/PRC-25 FM sets with them (Reference 41). This 1961-62 time frame is the earliest reference I can find on the operational deployment of the AN/TRC-77(*). (The first production radios did not leave the Sylvania plant until June 1961.)
A modified AN/TRC-77 was then evaluated by DARPA in 1963 on the Maylay peninsula in Thailand in comparison tests with the experimental TRC-88, the Hughes HC-162 and others. That TRC-77 was specially modified to include an AM voice capability so “it” could be compared with the other sets in voice communications over 5, 10 and 22 mile jungle propagation circuits. The experimental TRC-88 was very similar to the TRC-77 however it included an SSB voice capability.
In the Thailand tests, the SSB HC-162 generally produced more reliable communications during these tests (Reference 47) and eventually a modified version was produced after “extensive redesign” was done. It evolved into the AN/PRC-74 which became the Special Forces defacto issued HF SSB voice manpack radio set later on.
Equipment like this was also provided to our NATO allies for their use as well. Below is a photo from PA3ECT’s nice website showing a Netherlands Special Forces Commando operating CW with a TRC-77 from what appears to be a cave, hut or bunker. His arm is resting on what may be a cache burial container. (photo circa 1964)
Hans’ site also has a nice post on the restoration of his TRC-77. His set has a BNC antenna connector and a different type of identification label. Maybe some sets were built there under a licensing agreement of some sort? PA3ECT
Vietnam? Some time later, a PRC-64 / TRC-77 comparison evaluation was performed by Special Forces in Vietnam from October 1964 to January 1965 (Reference 52). That reference described the TRC-77 as “experimental”. Incidentally, the photo in Reference (52) mis-identified the TRC-77 as the PRC-77. Thanks to Bill, W8FIX for pointing that reference out to me; see his comments below. It is possible that the “Thailand DARPA” radio was the same one that was evaluated in Vietnam.
The AN/TRC-77 (unmodified, stock CW set) transceiver was also used by the US Army at the Ft. Clayton Jungle Warfare Training Center in Panama in October 1964 again as a comparison “control” while the AN/PRC-64 was being developed and evaluated to meet Special Forces jungle warfare requirements (Reference 37). Production AN/TRC-77 units were also fielded in Europe with Company C, LRRP units of the 58th Infantry in Germany. It was fielded by them along with the AN/PRC-64 which was reported by them as being “far superior” to the AN/TRC-77 (Reference 38).
From the perspective of “Mudborne” troops, I would assume that a large part of that assessment was based upon the small size and weight of the PRC-64 versus anything else. If you could make your Comm Window CW contact with a TRC-77, you could probably also make it with the PRC-64. The PRC-64 has about 3 db less transmitter power output but that is not much of a difference on the receiving end of the contact. However the difference in weight is substantial while being carried along with all their other gear.
Anecdotal information indicates that AN/TRC-77’s were possibly also used by the Australian Army. That makes sense as Australian Army signals personnel (including Major W.B. Fegan, Royal Australian Signals) were involved in the development of specification requirements for the AN/PRC-64 as a jungle patrol radio which they subsequently used themselves in Vietnam.
During that development, Major Fegan also contributed a critically important, but otherwise easily dismissed observation based upon the Australians earlier jungle warfare experience in Borneo. “The most important component of a man pack radio system is the man.” Indeed. Keep it small, lightweight and simple. Sounds like the PRC-64.
However, the online history of the Royal Australian Corps of Signals makes no mention of their use of the TRC-77 while deployed to Vietnam, but it does detail other tactical equipment that they employed there. That includes the PRC-64, PRC-25 and 77, GRC-106, PRC-47, VRC-12 etc. (Reference 55).
The US Army provided “only a few prototype models of the TRC-77, an experimental high frequency set” to the government of the Republic of Vietnam at the request of President Diem for his 20 nascent Commando units. (Reference 3 states that these few prototype sets were provided in 1960. That date is interesting since the first radios were not even out of production until June 1961. So any “prototypes” provided must have been just that, and not production models sitting unused in a warehouse as some have surmised.)
The AN/TRC-77’s may not have been used operationally by U.S. forces in Vietnam at all, as (so far) I have found no authoritative data to confirm its use by our forces beyond the 1964 comparison evaluation. Anyone?
The AN/PRC-64 DID receive further evaluation by the 5th Special Forces Group in Vietnam in late 1964 and early 1965 but no mention of the AN/TRC-77 being further evaluated alongside the PRC-64 at that time, Incidentally, the 5th SFG evaluators in Vietnam found the AN/PRC-64 “far superior” to the AN/GRC-109 in patrol operations (this authors emphasis). This assessment was made primarily on the merits of its size, weight and ease of setup (their words) as compared with the GRC-109. It was also noted that both sets provided approximately the same level of performance. (References 37 and 40).
Transistor technology for portable military radios was maturing and catching up. However the AN/PRC-64 itself apparently didn’t see much more than evaluations in Vietnam by the US Army. That small size and light weight came at a performance and operational flexibility cost. It required a unique dry cell battery that provided 3 different voltages; a type used in no other radio set. Additionally, its 1.5 watt AM voice transmitter capability was evaluated as being unreliable, especially when used outside the Mekong Delta area with its high ground conductivity and flat terrain. I can also find no authoritative references to the use of the PRC-64 or TRC-77, or intent to be used, by “Military Intelligence” or as a “spy” radio – such an imprecise term. Internet hype notwithstanding. (The PRC-64’s predecessor Delco 5300 apparently WAS used by the CIA however.)
A Military Assistance Command Vietnam Studies and Observations Group (MACV-SOG) Communications Officer report in early 1969 (Reference 1) listed the equipment held in inventory and being used by those Special Forces units in Vietnam. MACV-SOG was the parent Command for joint service unconventional warfare units, including Army Special Forces. They were the primary users of long range portable HF gear in Southeast Asia. No mention of the TRC-77 in that report, however it did note this as a matter of interest:
“The AN/PRC-64 (lightweight CW and Voice Set) for U/W [Unconventional Warfare] is receiving little use now but should remain in inventory for intermediate range (beyond range of PRC-77 but not requiring PRC-74) operations”. This tends to shed light on the thinking regarding portable HF CW radio systems at that time and place by units most likely to use them.
There is a lot of speculation, assumptions, YouTube videos, E Bay sales claims etc. about the use of the TRC-77 in Vietnam. As a Vietnam era radio, those assumptions are easy to understand given all the “circular research” out there on the Web. Significantly, there is no mention of the TRC-77 in the declassified MACV-SOG Communications history although the PRT-4, PRR-9, PRC-25, 64, 70, 74, 77, GRC-109, RS-1, FRC-93 (KWM-2A) etc are all discussed. (References 1 and 7).
Aside from anecdotal reports of “used” TRC-77 radios showing up in Australian surplus shops, most of the ones I have seen, or photos that I have seen, appear to be in pristine, unused – unissued condition. The set I have seems to be an exception; the front panel ID tag carries a different serial number than the number stamped on the chassis. It also shows some internal repair evidence and the usual service wear but most likely not by “hammering” by a previous civilian owner.
The TRC-77 was originally developed to meet a US Army operational requirement. Some time later the AN/PRC-64 was being evaluated and then the AN/PRC-74 and (later on) PRC-70 were appearing in Army supply channels. Coupled with the Army’s dwindling interest in HF, and particularly in HF CW-only radios, conjecture is that was likely the reason the AN/TRC-77 did not see wide acceptance.
Also, sustained operational combat comms on the same frequencies of a non-frequency agile radio is a significant OPSEC problem. However this was somewhat routine by US forces in Vietnam as they often traded security for speed. We were starting to realize the obvious consequences of that. The December 1969 capture of the View Cong’s “A3 Technical Reconnaissance Unit” in South Vietnam (among other events) affirmed their effective COMINT capability against sloppy US communications security on voice and morse code circuits. (Reference 63).
Which equipment appeared in what numbers, in which theater, in which units, for which mission, in which time frame is always a nebulous thing. There are few “absolutes”. Again, more research needed here; a few weeks in the National Archives should sort that out…
Neither the AN/TRC-77 nor the AN/PRC-64 appear in FM 31-20 (December 1965) Special Forces Operational Techniques, although the AN/GRC-109, and particularly its preferred antennas are prominent. It also mentions the use of the AN/GRC-87 (GRC-9) (Reference 42). The AN/TRC-77 does not appear in the US Army Infantry Communications Data catalog dated 1973 although the PRC-25/77, AN/PRC-74 and 70 are listed under Portable / Special Equipment. (Reference 39). The AN/TRC-77 also is not listed in FM24-24, Radio and Radar Reference Data, December 1983 (Reference 36) however the AN/GRC-109, PRC-47, PRC-64A and PRC-74 DO appear.
Aside from References (3 and 52) above, I have yet to find any reference to it in similar books or in US Army documents in my collection or on the Web. That includes other Field Manuals, other Technical Manuals, Training Circulars (TC), Infantry school class notebooks etc. The specific manuals for it, Organizational Maintenance Manual TM11-5820-473-12 is available from Robert Downs as a nice reprint. TM11-5820-473-35 can be found online. Originals of either document seem to be rare.
All this tells me where the TRC-77 probably wasn’t used operationally.
So, absent any new, authoritative reference surfacing (aside from the 1964 comparison experiment noted in Reference 52), I’m calling claims of the use of the AN/TRC-77 by US forces in Vietnam as “Busted”.
Is that a problem? Heck no! I am sure that they served the Army’s Long Range Reconnaissance Patrol units in Europe very well. For use on the Ham bands, the TRC-77 still has way more “street cred” than any plastic ricebox ever made by KenYaeIc.
Back to the radio itself:
Above: The RT-654A built by Arvin Industries. Note the U-228 “new family” panel connector for the H-140/U, U-229 headphone plugs. The RT-654/TRC-77 (unlettered model built by Sylvania) had a standard 1/4 inch earphone jack instead. The “plain” and A Model also have minor internal differences; mainly the power connectors, LC circuit tuning differences, minor component changes and panel machining. Note the shipping protectors on the panel handles. Split PVC tubing pieces. Owners seem to like to keep those on for some reason. Those black speckles on the front panel seem to be present in some of the photos of these radios that I have also seen. Upon close examination they appear to be contamination on the panel under the surface paint. Hmmm, Camo? HiHi
This radio has a very simple “user interface” in today’s parlance. A CW-qualified Ham, or a CW-qualified Army operator could be taught how to operate this set in less than 3 minutes. An additional 5 minutes of on-air tuning up to a wire antenna would complete checkout. Making a few on-air contacts and you are Good to Go. Very simple, soldier-proof.
The battery boxes for these radios seem to be rather hard to find. Boxes that saw service may have been damaged due to spilled potassium hydroxide electrolyte corroding the aluminum box. So absent a battery box I made one from some scrap plywood and a few hardware store latches and corner protectors. This box holds two 8 Amp-Hour Sealed Lead Acid batteries in parallel – providing more capacity than the original 14 AH NiCad wet cell battery. Wiring now includes a 5 amp fuse and a 2 foot jumper for powering from an external battery if necessary. Not pretty, but it does the job:
Above: My DIY system includes alligator clips for an external battery and it uses Anderson Power Pole 30 Amp quick disconnect connectors. The connectors also enable convenient access for my solar panel or an AC charger to charge the batteries. Room for spare fuses, antenna wires, Commo Plan, One Time Pads, schematic set, etc. inside.
Above: The 12 volt gel cells being charged by a small “automobile dashboard” solar panel. These are designed to keep a car battery “trickle charged” while sitting on the dashboard in the sun. This one is capable of pumping 150 milliamps into the paralleled SLA batteries. This is about 10 times the receiver current requirements – so one hour of full sun operates the receiver for about 10 hours. One hour in full sun will also power the transmitter for a couple of minutes of “key down” time. More than enough for many casual contacts. Indefinite Ops with this simple setup (and it fits in my ALICE medium pack). Of course a larger panel like my 20 watt unit will permit a higher duty cycle at the Forward Operating Base.
Above: The DIY battery box clipped to the R/T. It has the same external dimensions as the original BB-447 battery pack.
The set does not have a purpose-made carrying harness or bag. It was probably intended to be lashed to the then-standard plywood pack board. However there is a Bag, Cotton Duck, CW-618/TRC-77 that is provided to carry the associated accessories, see drawing above. The radio was probably out of active service well before the ALICE packs came along (1973) as basic 782 field gear. However the ALICE Medium pack is a good fit for the radio, but when clipped to the battery pack it does not fit in the internal radio compartment which was designed for the PRC-25/77.
Above: The radio, with battery attached fits well inside the main compartment of the trusty Medium ALICE pack. Seen here also rigged with the aluminum pack frame and cargo shelf (not visible) for a more comfortable carry configuration. All the peripheral gear including the LS-454 speaker, the headset, J-45 key, antenna and counterpoise components, message book and government issued Memorandum pad and Skilcraft pen etc all fit in the pockets along with space for other mission-essential items if needed. This is a complete field set, ready to go for a week-long “camp trip with comms”. The heavy speaker is not necessary (nor is it a part of the basic system), but it is handy for monitoring the Alert Net while engaged in other covert activities at your hide site.
Like flipping the steaks…..
Above: The first field tests with the new battery box. I was running a issued-equivalent antenna; on 7050 kc it was a 25 foot slant wire with two 50′ ground radials made from rather springy WD-1/TT infantry field telephone wire.
Above: Forward Operating Base (FOB) “Red Coffee”: The TRC-77 in the Medium ALICE Pack, LS-454 speaker remaining in the center, external pocket. Here feeding a 50 foot “Bush Antenna” with radials – but the chassis was still “hot” with RF on 3560 kc. So I installed a tent-stake ground in addition to the radials. Problem resolved, good signals in and out of this location. On this campout I had solid CW comms with my buddy, day and night on 7050 and 3560 respectively. A 78 mile NVIS shot. I also worked many other CW ops up and down the west coast with this setup.
Observations on the Receiver:
The 11 transistor receiver is amazingly sensitive. With the chassis out of the case, and no antenna connected, I can clearly hear Radio Havana (from California) in the evening on 6.000 kc, one of the “stock” crystal channels originally provided. The Castro boys’ communist propaganda outlet is plenty powerful for sure, but this is pretty impressive receiver sensitivity… Initial bench tests indicate a very usable CW SNR with a 0.3 microvolt input.
The stock BB-447A/TRC-77 rechargeable NiCad battery provides 14 Amp-Hours of capacity at 12 volts. The manual states that the BB-447A/TRC-77 battery will run the 20 mA receiver for 30 days. 24/7. Non-stop. Pretty impressive. The receiver will also work quite well for around 24 hours just using a common 9 volt “transistor radio” battery.
The RT-654A receiver is very simple to tune up on-channel. Just adjust the “RF” (antenna) and “Mixer” (actually the RF amp collector) circuits for maximum on-channel signal to the headphones. Requires a signal source or a generator like a URM-25. Simple. The receiver has an AGC circuit, active in the AM mode, controlling the RF amplifier, mixer and first and second IF amplifiers. It has an RF Gain control for the same circuits (the AGC is disabled for CW operation) and a volume control. It will drive a speaker quite well. The BFO is adjustable. With a 455 Kc IF selectivity quoted as 6 Kc (6 db down) and 11 Kc (60 db down), it sounds very good on AM and very easy to work CW even if something is “off” a little. (The selectivity is not so great on 40 meters during a CW contest! – You are not doing THAT while deployed anyway).
Above: The RT-654A set internals. The transmit crystal sockets are on the left, the receive crystals are along the rear; each set has a spring-loaded cover to keep them properly seated. Note the provided adapter to permit the use of CR-18A/U crystals in the small HC-6/U metal holders to fit in the older FT-243 sized sockets.
The earlier RT-654 (“Non-A”) shown below is similar but with the additional complication of moveable jumpers (called “Tuning Links” in the manual). These jumpers can connect an additional 56 pf fixed capacitor across the variable tuning capacitors in each of the receiver RF and Mixer stage tuned channel circuits. The original RT-654 used 1-90 pf variable capacitance piston capacitors to tune the fixed inductors in these tuned circuits. However, 1 to 90 pf was insufficient to cover the entire tuning range of the radio from 3-8 mc. So the designers added these jumpers to allow the insertion of the additional 56 pf capacitors in parallel to enable the each individual circuit to tune below 3.8 mc.
No big deal, just set them to the necessary positions before you start the alignment.
Above: The RT-654 receiver RF front end circuit board. The receiver crystals seen here at the rear of the chassis are Channels numbered 1-6, left to right. Their associated Channel Tuning Links are reverse-numbered 6-1, left to right. The alignment tool points to the Mixer link for Channel 1. Note the small rectangular 56 pf capacitors above the link sets. The Antenna tuning links are the ones furthest from the crystals at the top of this photo.
In this photo, the mixer and antenna Tuning Links for Channel 1 are moved towards the front panel (top of the photo) to connect in the 56 pf capacitors since my Channel 1 crystal was on 3550 kc at the time. The other links are towards the rear panel for the rest of the channels in this radio because they were all higher than 3.8 mc, Why is it important to note all this detail?
Because the alignment instructions in the AN/TRC-77 and AN/TRC-77A Radio Sets Manual (DS, GS and Depot Maintenance, TM11-5820-473-35, June 1966) are incorrect.
Page 3-12, Paragraphs 3.7 (b)(5) (b/c) says “For channels receiving frequencies between 3 and 3.8 megacycles the RF and mixer links should be positioned closest to the rear of the unit.” [and] “For channels receiving frequencies from 3.8 to 8 megacycles the RF and mixer links should be positioned closest to the front of the unit.”
They got the link position instructions exactly backwards.
The earlier Organizational Maintenance Manual, TM11-5820-473-12, October 1965, has ambiguous Link position instructions: It does not clearly describe what “top” and bottom” means in the placement of these Tuning Links. If you are having problems tuning your RT-654 “Non-A” receiver, this may be your problem. Heads up.
(The RT-654 “Non-A” Transmitter also has a set of Tuning Links in the PA output stage. The manual is correct in setting the position of those Tuning Links.)
The receiver output stage is designed to drive a 600 ohm load, usually the H-140A/GR headphones (RT-654A). The RT-654 has a standard 1/4 inch earphone jack on the front panel, the RT-654A accepts the 5 pin U-229 connector common to the “new family”, VRC-12 audio accessories. The set will drive an LS-454 speaker to room-filling volume. When used with the LS-454, the combination has a nice peaked frequency response of around 800 cps, perfect for CW but it still sounds good on AM.
Observations on the Transmitter:
The transmitter is a MOPA type, Master Oscillator (a 3B4 tube) and a Power Amplifier, (a 2E24 instant heating cathode type). This is the same PA tube as is found in the RT-66, 67 and 68 FM sets. It is essentially a 2E26 (as found in the RS-1, RS-6 and GRC-109 sets) but with a directly-heated “instant on” cathode. The 3B4 is also a directly heated cathode type and it is also found as the PA tube in the PRC-6 FM walkie talkie and as PA drivers in the RT-66. 67 and 68. In this transmitter both tubes are up to temperature in the time it takes to move your hand from the Transmit-Receive switch to the CW key.
There are 4 transistors in the HV power supply; 2 oscillators, 2 in the voltage regulator. There are 2 transistors in the grid-blocked keying circuit plus a mysterious 4-layer Shockley diode sidetone oscillator.
The manual’s Operating Instructions, Chapter 3, Section II, “Operation Under Unusual Conditions” Paragraph 3.5 states to let the transmitter “warm up” for 2 minutes after turning the Function Switch from RCVR to XMIT. WHAT? That is certainly unnecessary, the transmitter is stable and ready to go almost instantly. Two seconds warmup, maybe. How would one operate full Break-In or even in a Net with that restriction? Section III, also titled “Operation Under Unusual Conditions”, Paragraph 3.9, discusses the usual “Operation at Low Temperatures” (no mention of a “warmup” requirement there) and “Operation Under Tropical Conditions” (Standard fare in Army Tech Manuals).
It is possible the intent was to stay in the XMIT mode for 2 minutes at very low temperatures to let a very cold BB-447A NiCad battery warm up a little with “key-up current” and then produce rated voltage. That “warmup time” under normal operating conditions (implied) has to be an error by the manual’s tech writers and the engineers didn’t catch it. That makes more sense, but if so, that precaution is mislabeled, misplaced and badly confusing. Sloppy editing.
Once the Function Switch is in the XMIT position the oscillator runs continuously and the telegraph key keys the PA stage via grid-block keying. Very stable, no chirp. Since the transmitter has an independent Sidetone Generator you cannot hear your own signal (or oscillator Backwave); because the receiver is muted. Sidetone is also necessary when operating on separate transmit and receive frequencies even if the receiver was not muted.
Above: The set is a “hybrid”. That’s the 2E24 power amplifier tube in the black shield on the right. The high voltage power supply generates 400 volts DC for the PA tube, 120 volts DC for the oscillator tube and -175 volts DC for the PA grid blocking and operating bias. The tube filaments are fed with square wave AC from the power oscillator. The HV power supply includes a 2-transistor voltage regulator circuit to keep the outputs within specifications if the primary input voltage went as high as 17 volts from a vehicle battery being charged by a generator. The power supply is reverse-polarity protected by a series diode. The HV power supply uses a conventional 2N174 DC-DC oscillator circuit that flips open exposing some receiver circuitry. The oscillator runs at around 2.2 Kc to keep the efficiency high but you cannot hear it in the receiver audio or in the transmitted signal. Good shielding, filtering and bypassing design here.
When the function switch is in the XMIT position the set draws 2.01 Amps, key up. Key down it draws 3.34 Amps. (both measurements made with 13.00 Volts at the power connector.) The Depot overhaul acceptance criteria state that the set must produce a minimum of 2.5 watts RF output with a battery that is down to 10 volts.
One strange test noted in the manual is to measure the acoustic noise generated by the radio while in the transmit mode, key down. The test is to be performed in a sound-proof booth with a calibrated microphone and recording system. Maximum sound pressure levels are specified for various audio frequency ranges. Presumably so the enemy could not hear the set while in operation – from sounds coming from other than the earphones. I have not seen that test requirement in any other radio. I wonder where that came from? I sure can’t hear anything, even the high voltage power supply oscillator.
This radio has difficult access to most internal components due to its compact construction. The only “modular” component is the HV power supply. But it is beautifully built and well protected environmentally. The aluminum parts are clear-anodize passivated and the glass-epoxy PC boards are protected with a clear epoxy conformal coating. A hefty panel-case gasket and sturdy case snaps along with control/connector gaskets keeps it dry inside.
Tuning the transmitter to the channel frequency is very simple, requiring only a tuning tool and a VOM on 2 test points, which measures the Driver plate current and the PA plate current to indicate resonance. The set incorporates an under-chassis switch for “Tune” and “Operate”. In the Tune position the B+ to the oscillator plate and screen and the screen voltage to the PA plate is reduced for tuning adjustment. When done, just switch back to Operate – if you forget, the switch automatically moves to Operate when the chassis is slid back into the case. Handy, easy on the components, Soldier Proof. This switch is designed to protect the power supply transistors and the 2E24 screen grid from excessive heating (melting) while the transmitter is being tuned (off resonance). Follow the procedure!
When tuning up into an antenna for a transmission, place the radio in Transmit mode, hold down the key and turn the “Ant. Tune” switch to one of the 12 detent positions that produces maximum brilliance (antenna current) from the Antenna Tuning Indicator lamp on the front panel. Simple, no thinking required. You’re good to go. The tuning lamp circuit includes a front panel switch to reduce the brightness when driving very low impedance antennas; the lamp can also be bypassed during light-discipline conditions (and get a little more RF output – not that the guy on the receiving end could possibly even notice).
Above: Key down, Antenna Tune in position 3 delivering power to the antenna. On this first trial I worked KF6GC in Atascadero CA, a 178 mile shot on 7050 Kc. Easy. My second contact was with K3SEW in Howard PA, 2350 miles again on 7050 Kc (using a dipole). Ron is always good for a cross-country radio check on 7050. Five to seven miles indeed. This set is easily capable of cross-continent comms on a fairly routine basis.
Antennas provided: AT-1098/TRC-77, a set of 3 color-coded wires (white, purple, red) of length 57, 40 and 25 feet respectively, depending upon the frequency band in use (see table in manual). This slant wire is a quick, simple, compromise antenna, although not particularly good.
Note that each of those 3 wires is shorter than one quarter wavelength at the lowest frequency in the range specified. This means they will all present a capacitive load to the radio with a fairly low resistive component. The simple output network was designed to drive this capacitive load to avoid large tuning excursions possible on either side of resonance – requiring an additional adjustable reactance.
Sam, a veteran RTO with LRRP’s in Europe and Rangers (see his comments below) offered the following observation on antennas used with the TRC-77. He found that the “issued” slant wire antenna was not very effective so they used dipoles instead. He described the performance difference between the slant wire and dipole as “night and day”. (Reference 58).
If driving a resonant (no reactance) antenna such as a quarter-wave wire or a longer (inductive) antenna, the match circuitry may not be transferring the full power that it is capable of. The set uses a 12-tap toroidal inductor in series in the output network to approximately tune out that nominal capacitive reactance and match to the resistive component of the specified, electrically short antenna. (Simple, but note there is minimal harmonic suppression.)
I have operated it many times on 40 meters into a resonant dipole. The output ANT TUNE indicator gets brighter as you turn the ANT Tune switch all the way down to position 1. (Position 1 removes the toroid inductance completely from the circuit.) But you can’t remove any more inductance. This indicates a likely mismatch but it will be usable anyway. The output components may be seeing higher RF voltages in this configuration but I would assume a conservative component selection was used. These are engineering details, it will work. (Note that the Depot performance acceptance standards are measured while using a 50 ohm resistive dummy load.)
The AT-1098/TRC-77 also includes two 50 foot “counterpoise” wires to be laid out on the ground 90 degrees apart and connected to the Ground terminal. See cartoon above.
How about a resonant dipole antenna connected directly to the antenna terminals – no feedline – as an expedient antenna:
Above: The radio connected directly to the two dipole legs (brown wires, 66 feet each) for 80 meters, no transmission line, the Hasty Dipole. This balanced antenna works very well with the TRC-77. However, running this same antenna setup, now on 40 meters, makes each dipole leg a half wavelength long on 40 meters. That presents a very high impedance to the radio and it will not “load”. No Tuning Lamp glow, no change on the field strength meter at any Ant Tune position. Bad mismatch, little power transfer to the antenna. Not recommended. So use two quarter wave wires on your operating frequency:
Here running a Hasty Dipole on 40 meters with two, quarter wavelength legs connected directly to the radio. Loads properly. Shown above working a guy who was located near Glacier National Park, 450 miles away, from this lake in central Oregon. This antenna is quick and dirty – and works very well with the TRC-77. That little solar panel keeps the battery charged, see below.
I have found that the transmitter will load properly on both 80 and 40 meters – while using a 60 meter half wave, coax fed dipole antenna. Although I currently do not have any crystals for 60 meter operation, that antenna is in my kit and makes for a good compromise in the field. One efficient antenna for both bands. Improvise, adapt, overcome.
I have also tried the set in the field with a single-wire half wave “Bush Antenna” up into a nearby tree as an experiment (chassis grounded to a wet stake) . That set up was actually a quarter wave wire for 80 meters (66′) being used on 40 meters – and hence a half wave on THAT band). This type of antenna also presents a high enough impedance that the antenna current is correspondingly low – and as a result the Antenna Tuning lamp does not glow. At any setting. So you cannot set the Ant Tune switch to produce maximum power without a field strength meter. But it does not “load”.
The design engineers knew all this – hence the AT-1098 lengths are designed to present a much lower impedance than a half wave wire. Also, with the half wave antenna the chassis was HOT with RF despite the ground radials. A tent stake ground took care of that (See above photo) but the end-fed half wave antenna basically does not work with this set, at least of 40 meters. It was worth trying. Incidentally when using a low impedance antenna the tuning lamp is quite bright in a darkened campsite. The light attenuator switch is an important tactical feature.
For more information on the field antennas I use with this set and other portable radios, see Portable Field Antenna Kit
This radio needs a separate crystal for each transmitter and receiver frequency. For a total of 12 crystals. That’s a lot of money to set up one of these radios on 6 different frequencies these days. The manual states that the receiver crystal must be 455 Kc above the desired receiver operating frequency; the transmitter crystal must be on the actual operating frequency.
However there is no reason why the receiver LO crystal could not be below the channel frequency by 455 Kc as well – as long as the LO L/C circuits can be adjusted that low. The tuned IF amplifier just amplifies the difference between the LO and the signal, (or vice-versa), 455 Kc. So I am using a 3530 Kc crystal to put the receiver on 3985 Kc, one of our local military AM radio net frequencies. It works just fine.
Designed for CR-18A/U (HC-6/U holder) types, the set came with adapters to permit the CR-18A/U crystals to be inserted into the larger sockets. The installed sockets are the familiar “dual” A/B bakelite sockets also used in other sets directly for FT-243’s. The presence of these FT-243 compatible sockets may lead one to believe the set will also work with (by design) FT-243 crystals. The manual is silent on the use of FT-243’s versus the specified CR-18A/U crystals. So I now believe that the “design” to use FT-243 crystals is an unfounded assumption. More to follow on that.
The sets came from the manufacturer to operate on the following channel frequencies: 3.0, 4.060. 5.1, 6.0, 7.0 and 8.0 mc. Note that 5.0 was not included, presumably because of the presence of many frequency/time-standard stations operating on that frequency worldwide. Interference avoidance? Also, 4.060 seems to be an odd choice.
Those 6 frequencies are not “military channels” as some have stated. I think those were supplied by the manufacturer, via specific contract requirements, to provide a radio that was tuned up and demonstrably working on those channels. Having some “random” crystals installed at the factory would also permit a Signal Corps QA inspector to perform a full-function acceptance test. Otherwise, no-can-do.
Once the radio got out to a Line unit, the technicians could quickly test it for proper operation, then install the assigned channel frequency crystals required by their CEOI/Comm Plan and discard the ones provided. (No tech worthy of the name would throw them away!)
Having the radio pre-tuned to those six “whole” mc channels would make retuning to a specific, nearby operational channel somewhat simpler for the technicians. Just speculation on my part but that would make sense.
Of course if the Army supply system failed to deliver the required crystals per your Comm Plan frequencies, you still had an operable radio on those “stock” channels. (If you are using this radio set operationally, you are pretty much at the far end of the logistics train – you know, it’s where YOU are located.) Take it up with the theatre Joint Frequency Coordinator and G-4/G-6 Shops.
“Items that must work together cannot be shipped together” Murphy’s Laws of Combat
It is known (Reference 7) that the crystal controlled AN/GRC-109’s employed in Vietnam were operated on separate transmit and receive frequencies (at least by US Navy PTF boats conducting unconventional warfare operations “up north”). I would assume the same procedure would be used with the AN/TRC-77 where ever it may have been deployed at the time. Independently selectable transmit and receive frequencies simplifies operating in that mode. Good, time-tested Fieldcraft for covert or tactical CW communications networks.
Today: With some preliminary checkout I find that the receiver local oscillator works well with the CR-18A/U crystals (HC-6/U package) or FT-243’s. However the transmitter Pierce oscillator seems to be more fussy. Putting a high impedance scope probe on either the grid or plate of the 3B4 oscillator tube kills the oscillation, odd for a low frequency circuit like this. I have tried 8 or 10 original, “known-good” FT-243 crystals (80 and 40 meter frequencies). However the oscillator won’t start up with many of them on 80 meters, and none will work on 40 meters. Hmmmm The circuit seems to be very sensitive to stray capacitances. There is about 5-6 inches of hookup wire between the transmitter crystal sockets and the oscillator tube, via the channel switch, plus more going to the plate tank circuits and switch. No PCB, all hard-wired. Lots of stray L and C around. See the long crystal lead wires all laced tightly together in the chassis photo below. There are additional long wires to the tuning inductors (under the shield) and back to the 3B4 as well. What to do?
I am evaluating modern, parallel resonance crystals in the HC-49 wire-lead package in the transmitter. These crystals are physically pretty small so their heating response in a different keyed oscillator circuit may produce some chirp or longer term drift. That would depend upon the crystal current in a particular circuit with these crystals.
In the TRC-77 the oscillator runs continuously on transmit (only the PA is keyed) so no detectable chirp. However, with some HC-49 crystals the frequency drifts a few tens of cycles when the radio is initially switched from receive to transmit as the crystal current begins to heat it. The filament of the 3B4 oscillator “instant heating” cathode is also coming up to temperature at the same time. By the time I am on the key, it has settled down. Give it a couple of seconds. I currently have the following frequencies installed; subject to change:
Channel 1) 3550 SKCC Night Working Frequency
Channel 2) 3560 (Classified)
Channel 3) 7030 Current QRP Channel
Channel 4) 7040 Original QRP Channel
Channel 5) 7050 West Coast Agent Net
Channel 6) 7055 SKCC Day/Evening Working Frequency
I have a pretty good selection of original FT-243 crystals but the more common ones are allocated to my GRC-109, GRC-9 and RS-6 field set kits. But FT-243’s don’t seem to work well in this transmitter. Even with different, “known good” 3B4 oscillator tubes. Strange. So I ordered some transmit crystals from qrpme.com and expandedspectrumsystems.com. They have a good selection of the popular CW transmit frequencies but they are the miniature HC-49 wire-lead types. Improvise, Adapt, Overcome. They cost less than $3.00 each.
To evaluate them I just tack-soldered them to the underside of the 6 transmit crystal sockets as shown in the photo. In this particular circuit, the oscillation is about 1 Kc below the marked frequency, this may be due to the large series inductance of the associated wiring in this transmitter. The material on the Expanded Spectrum Systems webpage states that the HC-49 crystal specs are for an “18 pf” circuit. The operating test specification for the supplied CR-18A/U crystals is in a 32 pf circuit. That may explain the lower frequency oscillation of the HC-49 crystals in this radio.
I could add a variable trimmer capacitor in series with each crystal but that is a complication and there is little room for that. So I just inserted a single, small ceramic variable capacitor in the common lead feeding the grid (Pin 3) of the oscillator tube. Improvise, adapt, overcome.
When adjusted, this capacitor causes the oscillation of all transmit crystal frequencies to move up to within 100 cps or less of the desired (marked) frequency. Close enough for a ship this size. And for my buddies’ vintage military receivers that that are similarly about 5-6 Kc (-6db) wide.
I bought six CR-18A/U crystals from International Crystal Manufacturing (Catalog number 60850) as my only option for the receiver crystals at this point. At $25.58 each. Ouch. But they work fine.
Above: The HC-49 crystals tack-soldered directly to the undersides of the crystal sockets. Note the long (colorful) crystal leads (two per crystal) to the Channel switch. The common wiper lead from this switch is the green wire going up to the oscillator tube grid. Note the variable trimmer capacitor soldered in series with this lead to the tube socket. Tack-soldered together for evaluation, this arrangement is simple and works well. The transmitter keys cleanly, no chirp, full power out. Time to head for the hills….
Above: The second field trial of the TRC-77, here chatting with Net Control. We were up on a tree-less hilltop on the Camp San Luis Obispo National Guard base during the Military Radio Collectors Group 2015 event. We also had an ARC Type 12 set on 144.450 mc AM and a PRC-25 on 51.00 mc FM, both voice. The TRC-77 was operating on 3550 Kc CW and connected with Net Control at the base in the background of this image. Net Control was operating an MRC-56 communications trailer equipped with an ARC-1 VHF AM set as well as a TCS on CW. Hardly “DX” but good comms all around.
The TRC-77 was driving a 32′ slant wire antenna against a similar wire counterpoise laying on the ground. The antenna end was held up at about 15 feet by a bamboo pole lashed to a “support of opportunity” as seen below. A direct-wave path for sure but reports were that the TRC-77 sounded solid with a clean CW tone. As one would expect.
Above: An antenna with a view. That’s the Pacific Ocean on the far horizon. This low antenna is nearly ideal for regional (0-300 km) NVIS operation on an appropriate frequency at an appropriate time of day. Otherwise it is also good for local “direct” or “ground wave” propagation.
More “off-grid” evaluations: ARRL Field Day 2015
Above: Running both the TRC-77A and GRC-109 sets on both 40 and 80 meters CW using an Inverted “L” antenna and a ground stake with radial wires. Good comms up and down the west coast, including with Andy in Salinas running his PRC-174 for some initial CW contacts with that set. The ME-61/GRC Field Strength Meter came in handy in adjusting the GRC-109 transmitter to that high impedance antenna on 40 meters. Not enough antenna current available to illuminate the “Tuning” lamp. Lots of RF however…
Radioactive cabin camping.
Fair Radio Sales was carrying the TRC-77 in their 1981 Catalog and Supplement for $75. But setting up a TRC-77 today on 6 Ham frequencies could cost a couple of hundred dollars +, just in crystals. I think that, and the lack of a tunable receiver has significantly limited the interest from the Ham community in these sets, not to mention the large number of today’s Hams that just operate FM repeater or maybe SSB voice radios. CW? What’s that?
“Yeah – it weighs 5 times more than my plastic KenYaeIc Ricebox and it doesn’t have dual VFO’s, digital readout, LCD display, DSP, roofing filters, a 15 band voice equalizer, 100 watts, RF Preamp, Bandscope, VHF/UHF, microprocessor controls, RF processor, direct frequency entry and everything. Whatta piece of junk! ”
Or, as my buddy told me, “Nothing says BadAss like a field set with no microphone connector.” Didididit didit !
Cartoon by Andy.