AN/PRC-74B HF Manpack Transceiver

UPDATED 3/10/17, stay tuned.

A friend in Colorado recently gave me a veteran RT-794B/PRC-74 because he wanted it to go to a “good home”. I shall try… Many thanks to Steve! In return, and as promised, I will write up my observations, research and future field use as I go along..
This is the main Receiver-Transmitter unit, becoming the AN/PRC-74B when all parts are included.

AN/PRC-74 SSB/CW Manpack Transceiver

AN/PRC-74 SSB/CW Manpack Transceiver

Basic Specifications: (from FM24-24, December 1983)

Frequency Range . . . . . . . . . . . . . . . . 2.0 to 11.999 MHz (AN/PRC-74 and AN/PRC-74A);
2.0 to 17.999 MHz (AN/PRC-74B and AN/PRC-74C)
Planning Range . . . . . . . . . . . . . . . . . . . . 40 km (25 mi) ground wave; refer to sky wave
propagation chart to determine medium range
Number of Channels . . . . . . . . . . . . . . . . . . . 10,000 spaced every 1 kHz (AN/PRC-74 and
AN/PRC-74A); 16,000 spaced every 1 kHz (AN/PRC-74B and AN/PRC-74C)
Power Input . . . . . . . . . . . . . . . . . . . . . 10.5 to 17 V DC, 12 to 31 V DC or 110/220 V AC
Power Source . . . . . . . . . . . . . . . . . . . . . Battery (BA-30, 70 each or BB-418/U, 10 each);
vehicle power system (requires PP-4514/PRC-74); any appropriate AC power source (requires use of PP-4514/PRC-74)
Power Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . …. . . ..15 W PEP
Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AS-1887/PRC-74, slanted wire and dipole
Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . Detent
Type of service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -3 K00J3E, 100HA1A
Weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.8 kg (41.5 lb) with dry Battery BA-4386/U,
two each; 13.4 kg (29.5 lb) with wet Battery BB-418/U, 10 each; 22.7 kg (50 lb) with PP-4514/PRC-74

Operational history: Under development
I’ve done some preliminary research on its history and usage. It didn’t really replace any specific radio per-se; the new requirement was for a portable HF voice radio that could be used while being carried, something previous HF field radios could not do since the BC-611 Handy Talkie in WWII. A side mounted, tunable whip antenna made that possible for the PRC-74, it would also work with the usual wire antennas for fixed-portable operation.

PRC-74 Bush Antenna

PRC-74 Bush Antenna

The first fifteen PRC-74’s were issued to units of the Military Assistance Command Vietnam – Studies and Observations Group (MACV-SOG) in Vietnam in November and December 1966. They were then widely used by Special Forces there. This included SAAT teams whose unconventional warfare (U/W) mission was to infiltrate deep into enemy held territory to locate and rescue downed pilots or prisoners held by the North Vietnamese communists or their Viet Cong dupes.

Those missions required portable, long range HF radio but the assigned radio personnel were either unqualified or otherwise not proficient in CW. CW being the most efficient, mode of choice. So a special 11 week CW school was set up at the airborne training site at Camp Long Tranh to get the radio operators qualified on an essential skill for these missions (Reference 1 MACV-SOG Communications).

The AN/PRC-74 was nominally sourced as an interim stopgap for the AN/PRC-70 (which had not even appeared yet due to significant development delays). The AN/PRC-70, in turn, was intended to have replaced the AN/GRC-109 HF radio in some applications, although the GRC-109 could not be operated while being carried and was CW-transmit only. The CW and SSB voice PRC-74 provided a new capability in select man pack applications while the PRC-70 was still experiencing design problems. (Reference 1). The PRC-70 would provide HF SSB voice and CW but also VHF FM voice in one (heavy) package. As a consequence, note the out-of-sequence Model numbering; the 74 preceding the 70 in deployment.

The PRC-74 portable transceiver was ostensibly intended to provide beyond line-of-sight communications for U.S. Army long range reconn, air assault, Special Forces and other “mudborne” US Army units (FM24-24) . There was the basic unlettered, initial issue AN/PRC-74 that covered 2 – 12 Mc and then a similar “A” model; the B version that covered 2 – 17.999 Mc and also was interoperable with the GRA-71 code burst morse keyer.  The C model came still later with minor improvements to the B version.  It was adapted from a “commercial” transceiver (Model HC-162) made by Hughes Corporation to fulfill the Special Forces requirement and it became the AN/PRC-74, the first frequency synthesized HF manpack SSB/CW radio set fielded by the US.

It was eventually superseded “on paper” by the PRC-70 that had wider HF frequency coverage and also included VHF FM voice. Good idea, badly executed and some of the SF guys that I have read preferred the PRC-74 which they thought was quite good – although a battery hog and sometimes had problems with the heat and humidity. The manpack HF function of both the PRC-74 and PRC-70 was later replaced by the HF-only AN/PRC-104, still in use today. There doesn’t seem to be many PRC-70’s around as far as I can tell (I know of one being used by a Ham). Virtually nothing about the PRC-70 in the historical record.

This particular example:

What an adventure this one has been!   Seems this radio may have been that unfortunate one that the depot maintenance guys never got working – so they pulled any good modules from it and replaced them with ones that were too difficult to troubleshoot.  Enter FrankenRadio….It happens, get the radios back out to the troops – work on the “Too Hard” ones later on… eventually “beyond economic repair” and scrapped.

None of the module serial numbers match each other or the chassis, so this one has been “through the mill”.  It is a “B” model, 2 – 17.999 MC, wired for use with the GRA-71 code burst keyer. The particular example in this post was built in 1968. The PA module was overhauled by the Sacramento Army Depot in 1982.

 In addition to the PS module marked “Bad”, The PA stage and power control board  were similarly marked underneath.  (turns out the PS was OK, I was really worried about finding replacement 2N174 germanium switching transistors; there was just an intermittent contact in one of the fuse holders…phew!)

The real problem was the RF module, which is common to both receive and transmit modes.  It has a 6-wafer band switch with 12 rotary positions (4 are used, the other 8 are mechanically blocked by the gear train – figuring THAT out was an adventure in itself!) ).  The “D” wafer selects one of the 4 RF transformer secondaries in the RF amplifier stage.  The rotary switch D ring was installed in the switch 180 degrees out – it never could select anything!  It had to be assembled at the factory with this latent defect, the switch and wiring were all in otherwise pristine, “factory condition”, unmolested.  This module would have never worked and it took awhile to figure that out.  That’s one for the books.  It could have never passed Hughes QA or QC testing.  Very weird.

AN/PRC-74B chassis wiring, typical

AN/PRC-74B chassis wiring, typical

Above: Showing typical wiring on the underside of the chassis. Note the use of terminal strips and spade lugs to connect to the main wiring harness versus multi pin connectors. (At least these give you access to some power supply and control voltages.) That’s the RF module in the center, the transmitter RF power amplifier on the right.

I could not remove the RF module bandswitch, switch wafer or the drive shaft to reposition that rotary contactor – so I gently pried off the actual metallic rotating ring, rotated it 180 degrees and reattached it to the plastic drive ring.  All in a completely inaccessible space using dental tools. – bending and re-bending (by feel) 6 tiny metal tabs used to hold it all together.  Wow!  no wonder no one ever got this to work.  This was even more insidious than the infamous backwards-installed band switch knob on some GRC-9’s…..The module now works properly.

If that RF module was originally in this chassis, the radio would have never worked – that may explain why it is in such good cosmetic condition externally.  The internals have taken a beating…

I had to replace 4 transistors and 2 diodes on the gain control board.  There was evidence of some Blowtorch Soldering…..The RF stages and  synthesizer were badly misaligned,   So now the receiver works OK, better than 1 microvolt sensitivity.  But the transmitter power amplifier is not working and the ALC feedback loop that it drives is also whacky (and somewhat complex, the manual is not particularly helpful) .  Much more investigating to be done there…

PRC-74B Power Amp Testing

PRC-74B Power Amp Testing

Above: Typical bench project; locating where the transmit signal stops going. Looks like it might be bad PA transistors. I hate it when that happens.

Another problem was the mechanism that drives the 4 position band switch from the front panel.  The manual advises that “the band switch mechanism is properly set if the band switch changes from Band 1 to Band 2 when the MC selector knob is moved from position 2 to 3″. Thanks, that was helpful…. The MC knob has an extended lever to permit the operator to exert some additional torque while rotating these cams, pawls, gears and switches. The parts were sufficiently worn that it usually jammed during rotation.  There is just enough gear backlash coupled with worn cams and pawls that it doesn’t work reliably so I just mechanically disabled it by removing the drive gear.  I can change between the four major bands with a screwdriver for now…

Another bizarre problem: The wire that connects the front panel antenna Ground post to the chassis (actually to the PA module chassis) was broken – but the teflon insulation was intact OVER the break. Whiskey Tango Foxtrot? That almost sounds like a fault injected into a students radio to see if they could find the problem while in training to maintain this set. Hmmmmm.

The Manual: TM11-5820-590-35-1. Not particularly good, it jumps around a lot, the sub system schematics are not near the associated text, the schematics are multi section – with the “down range” circuitry in the manual ahead of the their associated input stages. Grrr. Very poorly organized.

A more immediate concern is that the radio cannot be aligned as a operational radio. Each module must be removed, powered up by up to 3 independent power supplies, some external adjustment pots, inputs provided as specified, outputs loaded properly and measurements made while adjusting. Makes sense for a module assembly line but tough to get a complete radio figured out and tuned up at Depot level shops, not to mention field repair sites. Very few signal test points available while the modules are installed in the chassis.

I’ve spent about 60 hours on it so far but it’s a challenge – I am learning a lot along the way and that’s the “fun” part… HaHa !  Fortunately no unobtanium IC’s, microprocessors, LCD displays etc.  All straightforward mid-1960’s analog transistor design.  Even lots of NPN silicon transistors among the germanium PNP ones.  most are easy to replace as necessary…Except those stud-mount PA transistors that sport a Hughes in-house part number. Probably a stock JAN type of the era, but graded out by Hughes for similar performance in the push/pull output stage. Someone suggested 2N2876’s. Hmmm.

Maybe the thing is to Hammer this set and design a new PA stage using modern 12 volt silicon RF transistors. Fit it inside the existing module, disable the 40 VDC power supply oscillator and feed it directly from 12 V. Some impedance matching changes would be needed but should be simple enough.

Functionally, the radio’s kind of like a mini PRC-47 with extended frequency coverage but without the 100 watt power level function. One significant issue for use on the ham bands is the way it generates a CW signal. Like many SSB radios, this is accomplished by injecting a keyed audio tone into the mic circuit – which causes that tone to appear as a sideband (above) the suppressed carrier frequency, the indicated “Window” frequency.

Standard stuff, but the PRC-74 uses a 2 kc +/- audio tone to generate the CW signal. Will be very confusing to the receiving station as well as trying to figure out which frequency your signal actually is on. The Collins KWM-2 (FRC-93) does essentially the same thing with a 1.7 kc (appx) signal. Had those sets used an 800 cps (+/-) tone instead, they would now be “forward compatible” with modern ham gear. HiHi.

Some of these sets seem to be around but I think they are not particularly attractive to most hams as they are only USB. Amateur radio convention has chosen LSB operation on the 75 and 40 meter phone bands. Of course USB operation on those bands is perfectly legal although many “appliance operators” these days seem to think otherwise.

I understand that “converting” one of these sets to LSB is quite invasive. Probably not widely done. Of course the basic model “plain” and “A” versions would not operate above 12 mc so they would be even less interesting to many Hams.

On Primary power: In addition to the stock wet-cell battery there was an alternate manpack battery box which included holders for 70  D Cell batteries. Wow…A pragmatic logistics-supply consideration… The receiver only draws 95 mA.

UPDATE: I just received this NIB Battery Box and whip antenna clamp from Mark K1HF as a very kind donation to “The Cause”. This CY-6121/PRC-74 was as “NIB” (New In the Box) as you can get – it was still in the original vapor proof packaging from July 1971. Many Thanks Mark!!  It’s gone to a Good Home.

NIB CY-6121/PRC-74 in original packaging

NIB CY-6121/PRC-74 in original packaging

By the way, Mark is looking for a PRC-74 RT as he gathers all the accessories. Please help him out if you can, or if you have a lead!

CY-6121/PRC-74 Battery Box

A rather complex “box”, it contains the inner clamp assembly for ten BB-418/U wet cell NiCad batteries. (The “/U” denoting Universal application) These batteries do not fill the entire volume of the box as you can see. This probably indicates the requirement to use this existing battery type (also used in the AN/TRC-77) even though the footprint of the RT could have accommodated somewhat larger batteries of a different geometry.

There is another battery box for this set, the CY-6314/PRC-74. It was designed for use with two BA-386/4386 dry cells, a battery also used with the AN/PRC-25. This CY-6121/PRC-74 sports a 1968 Contract date from Hughes.

Clipped to the RT, it makes for a rather large package, but like the earlier AN/PRC-10, it is proportionally quite thin, keeping the weight closer to the soldiers back. A “more comfortable” carrying arrangement when lashed to a standard packboard. (There is no issued web gear carrying harness for this set.) Also note the swing-out feet on the bottom of the battery box to facilitate standing the radio face-up. Also as seen on the AN/PRC-10. Good idea.

AN/PRC-74B Ready to Go

AN/PRC-74B Ready to Go

Above: A complete field package, ready to go with dipole/wire antennas. In my use, the battery box will accommodate two 12 volt rechargeable SLA gel cell batteries wired in parallel for operation. (That internal clamp assembly for the wet cells is removable). Note the different paint colors used.

PRC-74B with Whip Antenna Bracket

PRC-74B with Whip Antenna Bracket


Above: The PRC-74B with the MT-3613/PRC-74 whip antenna bracket and an AT-271 three meter whip antenna installed to test the receiver. I need to find or fabricate an electro/mechanical equivalent for the AS-1887A loaded whip antenna. That M-80/U mic was standard issue for these sets although an H-189/U Handset is also appropriate.

I am using a 12 volt, 7 Amp-Hour SLA battery in the battery box; it works well. It will eventually have two batteries in parallel once I get the transmitter PA fixed.

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Further work on the PA power output problem. Pull the module, power it externally, drive and load it properly, look around. Also look for any collateral damage caused by this – or something else that caused this short. That wattmeter still hasn’t moved off zero….

PRC-74B PA Transistors

PRC-74B PA Transistors

As I feared, one of the PA transistors was shorted. Now to find suitable replacements or rough equivalents. That’s the dead one, laying on the pad.

The TM11-5820-590-35-1 maintenance manual dated July 1968 shows the 6 digit Hughes part number for these transistors as 703075. The one that failed in my set is stamped with TRW as the manufacturer and it has 7230128038C printed on the side .

However my PA module was overhauled in 1982 so that may be a newer number for a replacement, possibly better transistor. It is likely that those PA transistors were replaced during overhaul as they are apparently prone to failure. The tables also show that Depot level maintenance activity spares allowance is 20 transistors for each 100 radios. That suggests a high failure rate compared with other transistors in the radio. Those last 4 digits printed on the failed transistor may be a date code: 8038, the 38th week of 1980.

My PA module was overhauled by the Sacramento Army Depot so it is possible that a different (better, hopefully more robust) transistor was available (and installed) at that late date. Hard to say. The Federal Stock Number for the PA transistors is listed as 5961-999-7341. Manufacturer code 01281 which is TRW. Manual page B-40

Not knowing what performance parameters Hughes required for their “house numbered” parts, this will take some investigating via reverse engineering. Then finding available parts that would meet the as-built circuit requirements as well as the mechanical, stud-mount package. It’s tight in there. One key parameter would be a pair with similar Beta’s or hfe performance, all other specs also being suitable, especially a high GBW product capability. Dissimilar Beta’s/hfe would cause bad waveform distortion in this push-pull output circuit which already has minimal harmonic filtering.

More to follow….