TRC-88 HF CW/SSB Transceiver

UNDER CONSTRUCTION 9/29/17

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..

RT-665( )/TRC-88 Radio

This TRC-88’s power output is about 10 watts; 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 (USB) voice capability in addition to CW 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 receive AM voice comms.

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 somewhat rare.

Operational use? The only reference I can find of any type regarding the TRC-88 is in Reference (65). That report indicates that a TRC-88 was evaluated alongside a TRC-77 (modified for AM), a Hughes HC-162, TRP-4 and GRC-9 in jungle propagation tests performed in Thailand in 1963. 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.

As with the TRC-77, I can find no evidence that the TRC-88 was used operationally by the US in Vietnam. A TRC-77 did undergo a brief 3 month comparison test in 1963 by Army Special Forces in Vietnam against the PRC-64. However after extensive research I’ve found no evidence that the TRC-77 was ever deployed there. That may indicate the future fate of the TRC-88 as well.

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 EDL/Sylvania quickly produced the TRC-77A with minimal, technically risky and costly improvements like SSB voice.

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 (radioteletype) capability. Odd for a manpack set. That may have accommodated some kind of AFSK burst CW device. 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 in CW capabilities 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 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 and mixer circuits to enable the wide band tuning range. They also use links to add additional variable capacitors in the PA stage tuned circuits to operate at the low end of the tuning range.

TRC-88 packaging details

Above: The 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 looks to be the same as the one in the original, TRC-77 “non-A” sets. The two sets diverge markedly from there.)
The transmitter oscillator, driver, power amplifier and HV power supply are on the left, additional 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.

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) but it also will not operate with them in the transmitter oscillator. Rats!

TRC-88 transmitter crystal deck

Above: A trial of an FT-243 crystal in the transmitter – Improvise, Adapt, Overcome. The circuit won’t oscillate – nor will it with others that I have tried. However the other HC-6/U crystals do work. The right one is a TV color burst crystal on 3579.545 kc. The left hand CR-18A/U crystal is on 4602.5 kc for a channel freq of 4604 kc USB which came with this radio. It works.

Note that the crystal compartment is significantly smaller than that in the TRC-77(*). Limits your options.

That 4604 USB frequency is assigned for 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.

As with the TRC-77, type CR-18A/U crystals in HC-6/U packages do work in both sets, TX and RX.

If I can find a schematic and if I can then gain access to the cramped circuitry, and absent a source of custom-made CR-18A/U crystals, I would redesign the transmitter oscillator to work with the FT-243 crystals that I have. Two big if’s…

Some preliminary performance tests: Like the TRC-77 the TRC-88 has exceptionally good sensitivity. This example produces a very usable CW SNR at below 0.1 microvolts. It can hear 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 gel-cell pair, the receiver will operate for over a month, 24/7. Amazing.

The transmitter puts out over 10 watts on 80 meters CW and similar PEP on voice. 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. Tuning links incorrectly set, broken piston cap etc. Otherwise in good condition for its age, everything is there.

The form, fit and finish of this radio is excellent, clearly not a prototype or pre-production sample. 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, chromate passivated aluminum parts, excellent environmental packaging. The module shield covers even include a silk screened photo of the components with circuit designations – I have not seen that in any other military radio. Great field tech aid!

Getting this field set on the air will be problemmatic after the demise of International Crystal Manufacturing as a source for custom made TX and RX crystals. It will be difficult to design a tiny frequency synthesizer or incorporate programmable oscillator circuits without even a schematic available. So now the search for suitable crystal pairs. It’s on 3579.5 kc T/R for now….

Does anyone out there have a schematic or other documentation for the TRC-88? It would be much appreciated!

All for now, stay tuned.
AR