I was a Marine Ground Radio Repairman 2841. If the OP brought me these radios I'd swap the mic components in the handsets first. I wish I had a buck for every spit slinging LT. or radio operator who complained about no transmit on his set. The mics have a cellophane membrane covering the carbon mic to help control moisture; however, humidity, rain, spit and long trips in the back of any vehicle can pack the carbon in those mics to where they barely work or fail altogether. Field Radio Operators 2531's for the Corps are taught to speak away from the mic for two reasons: to avoid over driving the pre-amp audio circuits (analog wave clipping) and the practical reason of not loading the mic up with spit. Unfortunately, there are plenty of folks who haven't received this basic instruction who misuse or abuse the handsets.
That said- here's what I remember as the most common failure points of PRC-25's and 77's.
For TRC, VRC and PRC radios-
#1. Mics For the named various reasons above (and then some), they were fragile.
#2. Handset cables dry rotting. The insulation can tear, crack and expose the soft wires inside the cable. To prevent dry-rot and hardening of the rubber around the cables, I used a little brake fluid on a rag, and rubbed it on the cable. It softened the rubber and made the handset cable more pliable; thus, extending it's life. In desert conditions, this is a must!
#3. Handset audio connection- the brass would corrode or tarnish. Using a pencil eraser will get you by, but it really needs to have the set turned off, then steel wool rubbed on the contacts. Shine them up, then blow away any loose wool 'hairs' laying around the connection. A shot of WD40 on a rag and wipe the connection to keep moisture off the contacts. DO NOT USE SPIT ON THE RUBBER O RING ON THE AUDIO CONNECTION!
#4. For PRC radios- Bad batteries. Over heated batteries (desert environment), batteries that are too cold- Arctic conditions, (I've seen both) Will rob the radio of needed voltages to operate properly, We used to store them in the refer, but let them come up to room temp before use.
#5. RT524, RT246's. Cooling fan for the transmitter magnetron going bad due to MUX transmit demands. Contrary to what the TM's say, (TM's never lie, do they?) these radios have a transmit duty cycle that should be adhered to, but unfortunately, multiplex operations require a constant transmit carrier function to maintain data links and an audio link. Used for Hawk Missile digital data links, multi-voice channels, Teletype. Usually field op's requiring uplink services to a mainframe in data collect mode. Fan replacement in the VRC12 family is a PAIN IN THE RUMP! While you're in there, you might as well swap out the magnetron too. Dispose of the old magnetron properly (think Tritium weapon sights disposal).
6. Power supplies/amplifiers/radio mounts in vehicles. Mostly failed due to vibration, poor connections, components often rattled loose inside of mounts on a hard field exercise. Bent pins from forced insertions, bad power cables at the battery due to neglect.
7. Low transmit power. Bad magnetrons (RT246, 524) bad transmit tube PRC-25. Never had to replace the PRC-77 transmitter transistor, not once! However, low power can come from VCO voltage controlled oscillators- causing drifting carrier frequencies and it will affect both transmit and receive sections. Varistors that control the synthetic frequency (VCO) are temperature sensitive by design. They can and do go bad from extreme heat.
8. Poor receive sensitivity- commonly the IF modules in the receiver (PRC 25 and 77). The RT's and R442 were always great for reception. Very little maintenance on them for that problem.
9. Antenna connections bent, broken, chaffed, or antenna cable connections frayed and broken.
Know this: I never once used all the portable radio diagnostic tools mentioned in previous posts. We always used a bench mounted HP Freq counter, HP Sig Gen, a Watt Meter, and a Tektronics O'Scope to perform our troubleshooting. There's nothing wrong with the portable tools, but like the radios, they have been bouncing around mount out boxes for 30+ or more years. Most likely they've rarely been calibrated, if at all. We had a regular calibration requirement annually for all bench test gear, including the portable stuff. Calibration services are available on the open market, but dig deep, it's not cheap.
Therefore, use with caution.
Semper Fi,
Dave
