PRC-77 help...

major519

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Nipissing, Ontario, CA
I have 2 PRC-77 radios. They were bought "as-is" but apparently work, or are supposed to...
I have fresh batteries for them, NOS antennas and bases and a used handset for each.
Both radios will fire up, volume controls work, lights light up etc.
I have tried to transmit with both, using the squelch and on settings. When I depress the talk on one set I do get an interruption in the rushing noise on the second. When I depress the talk on the other set I get the same result.
I do not get any voice communication when I try it.
Do I have bad handsets? Bad connections or other gremlins?
Looking for ideas to try to fix this.
 

tennmogger

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You may have a low battery voltage situation even if the batteries have never been used. Are your 'fresh' batteries the magnesium primary (non-rechargable) batteries?

Here's why I suggest that: If each receiver hears the other transmitters signal enough to break squelch, then both transmitters and both receivers are working, to some extent. If battery voltages are low, the frequencies may not be locked on correctly but are hearing each other "in the vicinity" of the right frequency. These receivers are broad band. The transmitters may not have enough power to modulate enough, too.

These primary batteries are unusual in that after using them for a while the voltage actually increases. The solution to your problem might be as simple as using the transmitters more (transmit loads the battery more...a good thing in this case). You might find that both will recover enough voltage and current capability to be able to talk to each other fine after you use them a while.
 

maddawg308

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Get the aftermarket battery adapters that use 10 D cell batteries, they will give you lots of power. The "freshest" original batteries are 15 years old, so they are tired even if they are the newest ones out there and are new in the wrap.
 

ke6rwj

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i have several of these, dont even touch a tool until you put HOT batteries in them... they will drift all over the place unless the power is correct..

then put a scanner on the other side of the room tuned to the same freq, if you dont hear yourself, verify the mic. you should be able to hear them fine with the scanner.

from there check frequency with a counter.. a handheld job is fine (got mine at radioshack)...

it gets ugly from there...
 

papakb

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One of the problems that these radios and the 25s have is that when they sit for long periods unused the contacts in the modules oxidize and this can cause off freq problems. If they've been sitting for more than a few weeks run the tuning knobs from limit to limit several times to wipe the contacts clean so they make good connections. The A40 modules are particularly susceptable to this. The same thing happens in the 442, 246, and 524 radios.

You also want to cycle the band change switch several times too.

Kurt
 
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renodogs

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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
 
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renodogs

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A few other items of interest on the FM radios PRC-25, 77, RT 246, RT524, R442.

Receive sensitivity was always set for a sig gen to inject a 3 micro volt signal between 30-75.995 mhz via a coax cable to the coax antenna connection on the radio. All units were required to have this minimal sensitivity. if it didn't pass this minimum standard, I tore into it to find out why. Alignment was usually 'on' in sets; it was usually an IF module that went South. And yeah, I went to the component level if I was at garrison, but module swapped while in the field. Our issued Pace kit was nice at garrison, but not part of a mount out T/O when in field op's.

A rule of thumb for transmit power and actual transmit distance for this band in FM- 1 watt= 1 mile.
PRC 25 transmit power was a nominal 1-1.5 watts.
PRC 77 transmit power was a nominal 2-3 watts. Much better than the 25!
RT246 & RT524 low power 3 to 5 watts hi-power 28 - 35 watts (there is an 'in theater' trick to tuning the RF output wattage (antenna load matching coil) section to bring it up to max output for each frequency; but other freqs will suffer. Think inductive reactance for you radio geeks.) You'll need a watt meter, freq counter, and an insulated screwdriver. NOT RECOMMENDED FOR THE UNTRAINED). Put your hand in the wrong place around that area and you'll see smoke coming out your ears. A little caution for those who want dabble in the RF AMP section- the magnetron (the metal cased power tube) is RADIOACTIVE. DO NOT take it out and play with it, put it in your pocket, etc. I'm not kidding, it really is radioactive. Glowing in the dark should be left up to the nuclear forces.

Transmit frequency tolerance is a big one. If your unit is drifting, you have a problem with the oscillator circuits and you need immediate attention or you will fry a lot of other stuff inside. That's just the nature of the beast! If you want your radios to last, an accurate frequency counter and watt meter is mandatory equipment to maintaining these radios.

Antenna coax cables were a huge source of grief due to shielding breaking, corrosion on connections, and RF insulation leaks causing RF amps to fail. Get a cable checked out by using what we called a 'Megger'. A hand operated meter that generates enough power to check marginal insulation properties of cables. You hook up your cable and you literally had to hand crank the device much like the ol Ma Bell hand crank on an early Bell telephone. Effective and accurate! I found more garbage cables and intermittent transmit problems due to cables and mics than anything else.

FYI- RT246- uses a 'Wheatstone Bridge' matrix to select frequencies (button panel). These radios are primarily used in tracked vehicles wherein a selector knob like the RT524 is impossible to operate and you can't see the freq dial readily due to cramped spaces inside. Those of you in Amtracs, Tanks and APC's know what I mean. The Wheatstone Bridge idea worked well, but it too can develop its own unique maintenance problems that a RT524 won't ever suffer from,

All parts from an RT 246 and an RT 524 will interchange except the push button panel (Wheatstone Bridge).

I'll post more as I think of it. This is really cleaning out the attic cobwebs for me. It's only been 35+ years since i worked on this stuff. :drool:
 
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