TRC-88 HF CW/SSB Transceiver

UNDER CONSTRUCTION 3/9/18

I am currently working on reviving an RT-665( )/TRC-88 transceiver. This will be a challenge since I cannot find any documentation on the set or even a simple schematic. These radios seem to be few in number. So far, there’s no evidence of a TM manual for it..

(Not to be confused with the Radio Shack TRC-88 CB set!)

Without any documentation and due to its very dense, well shielded packaging, reverse engineering this set will be very difficult.

The US Army Signal Corps went to considerable expense to develop the TRC-88 and its father, the TRC-77. This is a very capable field radio. What happened?

RT-665( )/TRC-88 Radio

Some Basics: This TRC-88’s power output could be about 10 watts CW, the same as the 2E24 PA tube produces in the TRC-77. Same 3-8 mc (probably) frequency coverage, six independently selectable crystal controlled TX/RX channels, 12 VDC power source and simple wire antennas – but not operable while being carried. Hence the TRC (Transportable Radio Communications) designation rather than PRC (Portable Radio Communications), radios that CAN be operated while being carried.

The TRC-88’s SSB (Upper Sideband) voice capability is its defining operational characteristic. SSB voice was likely requested by the Signal Corps but possibly as an experimental or speculative effort to increase its utility beyond its progenitor: the TRC-77 CW set. Both sets can also receive AM voice comms.

As with the TRC-77, this set was likely designed to drive an electrically short, slant wire antenna. The evidence is that it does produce more RF current into a low impedance, capacitive load than with a 50 ohm resistive load.

TRC-88 ID Tag

This radio, like the TRC-77, was built by the Army’s Electronic Defense Laboratory managed by Sylvania Corporation in Mountain View CA. The Order date is 1962. This one carries Serial # 2. I know of only one other set in captivity, that one is Serial # 9. They seem to be quite rare.

Operational use? The only reference I can find of any type regarding the TRC-88 is in Reference (65) series, the Advanced Research Projects Agency (ARPA) Project AGILE work. That study was performed by Stanford Research Institute (SRI Project 4240) as a contractor funded by ARPA.

That study was to evaluate and compare available tactical voice radios over distances of between 5 and 22 miles of jungle. Those 4 reports indicate that a TRC-88 was evaluated alongside a TRC-77*, a Hughes HC-162, a Japanese OKI TRP-4 and GRC-9 voice sets in jungle propagation tests performed in Thailand in 1963.
(*Ten TRC-77 CW sets had been modified to include an AM voice capability.)

The TRC-88 was judged the best of those sets over 5-22 mile “tactical” ranges; simplicity of setup and operation were strong points. However the complex HC-162 carried the day due mostly to its frequency-synthesizer capability to avoid interference. With significant modifications that set eventually morphed into the PRC-74.

In the December 1963 ARPA semi annual (and possibly final) report on Project AGILE communications studies, the author stated that none of the HF radio sets evaluated in Thailand were ‚Äúrecommended for immediate procurement‚ÄĚ. Absent later documentation surfacing, it appears that this field evaluation and resulting recommendation ended the development and procurement of the AM-modified TRC-77 as well as the TRC-88 sets. (Reference 66).

I can find no evidence that the TRC-88 was used operationally by the US in Vietnam or anywhere else. A TRC-77 had undergone a brief 3 month comparison test in 1963 by Army 5th Special Forces Group in Vietnam against the PRC-64. Another brief, and similar comparison test using the TRC-77 was done later at the Ft. Clayton Jungle Warfare Training Center in Panama. However after extensive research I’ve found no evidence that the TRC-77 was ever deployed in Vietnam.

In both cases, the Vietnam era TRC-77 and 88 were either the right radio at the wrong time or the wrong radio at the right time. In any case, probably neither saw widespread use although the TRC-77 was briefly used by US Army Long Range Recon Units units in Europe in the early 1960’s. (Reference 38.)

The TRC-88 is a clear derivative of the original TRC-77 “Non-A” model CW set, also built by EDL/Sylvania. It is also clear from the Order (Contract) dates that the TRC-88 surfaced shortly after the basic TRC-77 was produced but before the requirement for additional radios (TRC-77A) that came several years later. To meet that requirement the Army contracted Arvin Industries to produce the TRC-77A with minimal, technically risky or costly improvements like SSB voice.

A Theory: In 1962 ARPA ordered ten TRC-77 sets modified for AM voice to be evaluated in the Thailand performance comparison tests. So it MAY be possible that ARPA also ordered only ten TRC-88 SSB sets to also be evaluated as part of the Project AGILE tests – for the same reasons they needed ten TRC-77 “AM” sets. We know of only two TRC-88 sets in existence, serial numbers 2 and 9 (could easily be more – but so far, no evidence). That possible small order of ten sets could explain a lot…

Further, the Project AGILE comparison tests were for voice communications, no mention made of CW work. The example I have, Serial # 2, shows CW as a selectable mode via the front panel Mode switch along with SSB, FSK and Tune functions. However the Mode switch itself is not built or wired to enable the transmitter to operate in the CW mode. Odd. But it is starting to add up.

CW was not required for the tests so it was not fully implemented, at least in this example. It’s beginning to look like there may have been only ten TRC-88’s produced and that they may have been built solely for evaluation in these ARPA voice tests.

Interestingly, this radio, Serial #2, also has a prominent #2 gummed label attached to the panel edge. Set 2 of 10? Maybe. Hmmm.

“CW is dead”: Below is the rear of the XMTR Mode wafer switch:

TRC-88 XMTR Mode Switch CW position

Above: The rear wafer of the XMTR Mode function switch. This switch, when rotated to the CW position, needs to ground the T/R relay return. This ground actuates the relay which applies operating voltage to the TX oscillator and CW generation circuits, turns on the high voltage power supply for the power amplifier tube, and also switches the antenna from receive to transmit when the T/R switch is moved to the XMIT position. The pencil points to the hole in the wafer where that contact pole should have been riveted.

There’s nothing there, there never was, no CW was ever possible. Why did they do this? Here’s the fix:

TRC-88 XMIT Mode Switch

“Necessity is the mother of intervention”

I installed a new switch contact in the CW position where it was missing. I found a donor switch in the junk box, drilled out the wafer rivet and installed the contact finger in the TRC-88 with a tiny “eyeglass repair” screw. Added a jumper over to the PTT line, CW is now operational. Phew! Tiny parts in a hard to access location. Note the amateurish soldering on that 10 ohm RF lamp shunt resistor. Someone has been in here, probably trying to figure out why it didn’t work on CW..

Back to the radio: See the comparison with the TRC-77A (on top) below.

TRC-77A / TRC-88 Comparison

These two sets are very similar externally but with significant internal differences to accommodate the single sideband voice capability found in the TRC-88. Note that the TRC-88 also includes an FSK (typically thought of as associated with radioteletype) capability. Odd for a manpack set. That may have accommodated some kind of AFSK burst CW or digital messaging device as ARPA was evaluating those as well. (Part of Project SEACORE). The TRC-77 was compatible with the AN/GRA-71 CW burst keyer.

For a detailed report on the TRC-77 set, take a look here: TRC-77 Radio Set

As these two sets are otherwise quite similar, I won’t go into that aspect. My TRC-77 research and actual field experiences with one apply as noted in the link above.

In the early 1960’s the Army continued moving towards easily operable, though short range FM voice sets which required little operator training. Airborne VHF FM voice relay was “The Heat”. High frequency CW and its inherent long range was still very useful, even required, for long range recon but its days on tactical circuits in the jungle were numbered. Hence the short lifetime of the TRC-77 and subsequently the TRC-88 despite its SSB capability.

Some TRC-88 specifics: Behind the front panel it is a significantly different radio than the TRC-77. Both sets use the 2E24 rapid-heating cathode power amplifier tube. However the TRC-88 uses a transistor PA driver stage instead of the 3B4 tube used in the TRC77 as its crystal oscillator / PA driver tube. The TRC-88 uses a transistor oscillator and buffer stage to drive the transistor PA driver. Power and space efficient. Both sets use a high voltage power oscillator to generate the PA tube plate and screen voltages.

Both the ’77 (non-A model) and ’88 use a similar tuning “link” arrangement in the receiver RF amp antenna and mixer (RF amp collector) circuits. These enable the wider band tuning range by adding small, fixed capacitors into the tuned circuits. The TRC-88 also uses moveable links to add/subtract inductance in the PA tuning inductor as needed to enable the wide tuning range.

TRC-88 packaging details

Above: The TRC-88 receiver RF/Mixer PCB on the right illustrating the tuning links to insert additional fixed capacitance into the circuits for lower frequencies. (That receiver RF/Mixer PC board is the same as the one in the original, TRC-77 “non-A” sets.) The TRC-88 IF/AF PC board is marked RT-665 ( ) TRC-88 and is therefore different. The RF/Mixer PC circuit includes a cylindrical IF “ladder filter”, probably a mechanical type. It is in the receive path on both CW and SSB operation. The two sets diverge markedly from there.

Below: The transmitter oscillator, driver, power amplifier and HV power supply are on the left; additional PA tuning links are under the hinged HVPS cover.

TRC-88 tuning links

Above: The adjustments in the upper left corner are for the TX crystal oscillator and buffer/driver stage tuning. The PA driver transistor has the round heat sink on it, the 2E24 PA tube is shielded with a black IERC-type heat sink. That tap on the PA tank coil is somewhat mysterious.

The TRC-88 uses TX and RX crystals in HC-6/U packages and dedicated ceramic sockets versus the larger FT-243 holder sockets and adapters seen in the ’77. Conserves space. Like the TRC-77, the ’88 will operate with FT-243 crystals in the receiver (455 kc IF, Low side injection) but it also will not operate with them in the transmitter oscillator. The plot thickens.

The left hand CR-18A/U crystal is stamped 4602.5 kc (suppressed carrier frequency) for a mid-channel freq of 4604 kc USB which came with this radio. It works. Note that the TRC-88’s SSB frequency generation/mixing scheme requires a TX crystal that operates at 9000 kc higher than the channel operating frequency. So that crystal actually oscillates in the High Frequency Oscillator (HFO) circuit at 13,602.5 kc despite its markings.

That HFO signal is then mixed with the 9000 kc output of the crystal filter to produce the desired TX USB output channel frequency to the antenna.
(The Balanced Modulator uses a 9000 kc crystal controlled oscillator to initially generate the sideband signals.)

TRC-88 transmitter crystal deck

As noted above, this radio could never have operated in the CW mode although SSB and Tune modes work as expected. So NO crystal would have worked in CW, even during this early FT-243 trial. (At the time I hadn’t realized that the TX crystal needed to be 9000 kc higher than the operating frequency. This is an SSB radio, not a MOPA TRC-77 that I had recently been conditioned with!)

That 4604 USB frequency is assigned for USAF/Civil Air Patrol use in the US Midwest and Rocky Mountains region. I hear frequent ALE soundings on that frequency at night, but so far no comms. It is very likely that this particular example did see some service with the CAP.

If I can find a schematic and if I can then gain access to the cramped circuitry, and if I can’t find a source of custom-made CR-18A/U crystals, I would need a dual frequency synthesizer for the TX and RX. Note that the crystal compartment is significantly smaller than that in the TRC-77(*). Limits your options for a future frequency synthesizer. Many if’s… Or maybe try those little custom made programmable oscillator circuit modules.

So, what kind of drive level is necessary to operate the receiver crystal oscillator?

As an experiment, I tried injecting the output of my ancient Heathkit VFO and then an LM-14 Frequency Meter into the receiver crystal socket. Adjust it to 455 kc above or below the desired receive frequency, adjust the Mixer/RF amplifier circuits accordingly.
Boom! Tunable receiver.

TRC-88 Tunable Receiver Hack

Or, with the LM-14 Freq Meter with 600 mV p-p output at the crystal socket.

TRC-88 with LM-14 RX VFO

Above: The receiver works fine with about 6 V / 600 mV p-p into the crystal socket, I’m “tuning around”. I’m thinking some of those Epson programmable oscillators should work fine. At $4.00 each that might be a good solution. This should also work just fine in the TRC-77 as well, same receiver circuits. Many thanks to Dave and Bob from ARMYRADIOS for the alert on these little devices.

I got an evaluation board from W9RAN last year, so I ordered some oscillators and try it out. (Stock item is Epson part number SGR-8002DC-PTB-ND)
Bob wrote a good article on these parts and ordering information in the January 2018 Electric Radio magazine. Thanks Bob!
Here is my initial “brassboard” lashup:

Epson programmable oscillator tests in TRC-88

I ordered 4 oscillator chips from Digikey and began evaluating them in the W9RAN prototyping board driving the TRC-88. The board includes a CMOS hex inverter to buffer the selection inputs and outputs of the oscillator IC’s and it also includes a 5 volt regulator to power them from 12 volts. Initial tests driving both the transmitter and receiver crystal sockets produce good results. The output at the receiver crystal sockets is basically a ringing square wave with this layout but the tuned circuits in the radio clean them up. The higher frequency HFO TX crystals deliver a nice clean sinewave.

The next step is to build 2 six channel circuit boards, one for the TX the other for the RX, fit them onto the crystal compartments and rewire the channel selector switches to operate the circuits. But first, more investigation into the TRC-88 power amplifier issues.

Some preliminary performance tests: (stock, with crystals) Like the TRC-77 the TRC-88 receiver has exceptionally good sensitivity. This example produces a very usable CW SNR at below 0.1 microvolts. It can hear that 100 nanovolt leakage from my URM-25 with no cables connected. The receiver draws about 20 ma, slightly more than the TRC-77 so with the stock NiCad or my gel-cell pair, the receiver will operate for over a month, 24/7. Amazing.

This particular transmitter currently only puts out about 5 watts PEP into a 50 ohm resistive load on voice (why?) so I have some more work to do. The 2E24 power amplifier tube and 650 volt plate supply should be capable of 15-20 watts output PEP. The output (once I corrected the Mode switch to make it work) is also about 5 watts on CW, still too low. I have found no evidence of a grid bias adjustment or screen voltage differences for the PA tube. There is no indication the PA is biased into Class C for more efficient CW when switching from SSB linear operation to CW.

The transmitter just uses a 2.5 kc keyed audio tone injected into the SSB microphone circuits to generate the CW and the Tune signals. That 2.5 kc tone above the “window frequency” is what many military and Collins FRC-93 USB SSB radios do for example. Too high for my liking, 800 cps or so is better and more conventional. Is that 2.5 kc currently inside the crystal filter passband where it belongs? Something out of adjustment causing low power output? Putting this transmitter CW carrier or SSB suppressed carrier “on channel” will simply involve picking the appropriate HFO oscillator frequency offset to make this set compatible with conventional radio nets.

Both the TX and RX sound quite good on a monitor or in either headphones or an LS-454 speaker which it drives well. This set fits the same DIY battery box that I built for my TRC-77, two 8 Amp-Hour SLA batteries in parallel for 12 volts. I like it!

This set came to a friend via an EBay sale, provenance unknown. It shows signs of probable service wear and there have been some circuit repairs done. These include a replaced power transformer apparently done by qualified personnel with access to unique parts like that. There are no obvious signs of “Ham-mering” by a Ham although the set was badly misaligned to the 4604 kc crystal frequency pair installed. There is also some questionable, non-factory solder connections here and there. Tuning links were incorrectly set, a broken piston cap etc. Otherwise in good condition for its age, everything is there.

I’m willing to bet this set spent some time in the jungle in Thailand.

The form, fit and finish of this radio is excellent, clearly not a prototype or pre-production sample (but maybe it became one). The designers did a good job fitting a lot of capability into this small package using the technology available in the early 1960’s. Fiberglass, silk-screened PCB’s, clear epoxy MFP (a real pain when soldering), chromate passivated aluminum parts, excellent environmental packaging. The module shield covers even include a nice silk screened photo of the PC boards with component symbols (but unfortunately no test point or function labels).

Getting this field set on the air will be tougher after the demise of International Crystal Manufacturing as a source for custom made TX and RX crystals. However those Epson oscillators hold good promise for these radios. Maybe Bob can build a 6 or 12 channel PCB that will fit over the crystal sockets in this or other similar radios!

Does anyone out there have a schematic or other documentation for the TRC-88? Are there any former Sylvania EDL or Stanford Research Institute employees out there who can shed light on any of this? It would be much appreciated!

I have made a request into the US Army Signal Corps Museum at Ft. Gordon, let’s see what they may have. So far, “Radio Silence” from them.

An inquiry made to the U.S. Army Communications Electronics Command indicates that they have no information in their historical files on the AN/TRC-88 save for one “portrait photo” of a TRC-88 set. Nothing on the AN/TRC-77 either. The mystery deepens.

Perhaps someone associated with Project AGILE or ARPA in general has some information on this radio in their archives. That may be all that’s left.

All for now, stay tuned.
AR