testing the PRC-25 and PRC-77 radios

[maddawg308]

Well, got the test bench, well ok it's the dinner table, set up the other day to test a few PRC-25 and PRC-77 radios that customers wanted me to realign and test out, make sure they were good functioning radios. I have all the tools needed to test these out, but it is a complicated process that requires special tools.

The first pic below shows the test setup. In the middle, is AMGeneral's PRC-25 radio. The battery pack on top of the radio is made by Brooke Clarke, from PRC68.com, it uses 10 "D" cell batteries instead of the more common lithium or magnesium batteries that the 25/77 uses, as most of them were made long ago and are now dead as a doorknob. Simply snap in 10 batteries, insert as normal, and you're set to go.

On the left is the AN/GRM-55A module test set. These are test sets that use a probe and a set of easy-to-follow, but numerous instructions to test all the modules of the radio set and thereby narrowing down what is wrong with the radio's receiver, transmitter, or synthesizer decks. These test sets are uncommon, but could be found on eBay once in a while for $50-75 each. The instructions on how to use it are actually part of the unit, screened on aluminum plates bolted to the test set.

On the right is the rare AN/PRM-34 test set. This is not often found on eBay, or anywhere else. They are rare as hen's teeth, and I don't know anyone else who has one, much less for sale. The only way I got one was lucking out and finding one in a batch of surplus from GL about 6 years back. Still had a working battery in it too. Anyways, this test set is a combination wattmeter, frequency meter, and signal strength meter for any VRC-12 radio set, including the PRC-25/77, RT-524, PRC-68, etc. It is able to quickly diagnose what the transmit freq. is compared to what the radio dial says, test the forward and reverse transmit power in watts, and check the signal strength of the radio with the antenna in place. Very useful, although other test gear could be used for this purpose as well in place of the PRM-34.

Front and center, are the manuals for everything I had to use. If you do ANY radio testing, having manuals, in printed form (not .pdf) are irreplaceable for proper operation and maintenance of the gear.

Second pic shows same setup with a commercial frequency counter to more accurately check out the broadcasting freq. versus what the radio dial says. In other words, if the radio dial says 50.00 MHz, and the freq. counter says 49.99916 MHz, then the radio is only 840 Hz off frequency. This may seem like a lot, but even if the radio is 1 or 2 kHz off center, this is within spec, since the freq. bandwidth of the 25/77 is 9 kHz wide, 1 or 2 kHz off would still put the majority of the signal around your desired center freq.

Third pics shows the -25 taken apart and the GRM-55A test set plugged into a module. This is radio surgery, and this is what it looks like. Avert your eyes if it make you queasy.

Oh, and AMG's radio? Passed the transmitting test, but failed numerous receive and synthesizer tests, which would explain why he can't hear anything and noone can hear his voice on transmit. Out of about 30 tests the GRM-55 covers, his radio failed 14 of them. This doesn't mean that 14 modules are bad, it could only be one, which affects a bunch of other tests. We'll get it all sorted out and fixed for him soon.

 

[panzerwillie]

Great work thas why i send you my radios

 

[BillF]

Do you work on the Prc 10 radios as well?

 

[Wile E. Coyote]

FSS problems are the worst with those things, and the GRM-55 will only allow indirect tests of certain modules. Fair Radio Sales had a bunch of the GRM-55s and still might - and Mike Murphy has (or had) the PRM-34s.

Whenever I've seen those things go south, they usually don't track the correct frequency in one band or the other. Typically you'll get good comms for a couple of exchanges, then the radio will go off-frequency - usually on receive - so you won't catch any replies. Sometimes you can go off frequency a few Mhz and then go back to your assigned and do another exchange or two, but ultimately you have a problem. I've seen many of them do this.

When the radio oscillator goes off frequency, the FSS will generate a 'hunt' voltage which is supposed to correct the oscialltor drift provided the oscillator hasn't drifted too far out of FSS capture range. Many people notice the problem and try to realign A9 (A39 in the Prick 77, if memory serves) but that generally won't work unless A9 is *just* outside the FSS capture range - neither will exchanging it with another one out of another radio for the most part unless you go through the whole realignment routine too (which you don't want to do because that module has to go *back* in the other radio.)

There are several modules in the FSS chain, some of which you can only diagnose indirectly with the GRM-55 (actually I believe *most* of them fall into that category.) You can swap some of those modules out with 'known good' ones out of a working radio (make sure you label which is which), but anything adjustable will be tuned for the radio it came out of for the most part and probably won't be too helpful swapped out as a troubleshooting aid.

I've spent hours on those way back when but now when I come across one exhibiting those symptoms I just don't bother. Outright failures are very easy to diagnose on those sets, but intermittents like that off-freq problem are a complete b*tch. I did all the voltage measurements, swap-outs, signal tests -- you name it -- but no dice. If I'd had a second signal generator at the time I probably could've worked it out, but all I had was my CSM and a few spare evenings at work.

If you do find a fix for this, please post it, as I have...lessee...probably three or four sets around doing it right now.

 

[SJBDOE]

I have a PRC-77 that transmits fine, at least a half mile but will only receive a short distance ten feet. Beyond the ten feet, the audio begins to distort and I have to turn off the squelch to make anything out. My other radio is a PRC-25 and it is working 100%. Any help would be greatly appreciated.

 

[Wile E. Coyote]

My first question is "How do you *know* your PRC-25 is working 100%?" If you're just using those two radios alone to do your testing, I'd be more inclined to believe there's a low transmit or off-freq output condition on the PRC-25 than a receiver problem on the PRC-77.

If you know someone else locally with a known-working military set (packset or jeep radio) you could test both your radios against *that* set and subtract an awful lot of variables right out of the gate. But if you're absolutely dead-certain your PRC-25 is 100%...try receive on the PRC-77 just outside of your test frequency; for example:

PRC-25 set to transmit on 51.00 -- try set the PRC-77 to receive on 51.05 and 50.95. If the received signal is stronger/ less garbled on either of the 'wrong' frequencies it *probably* means the PRC-77 needs at least an alignment.

One word of warning though, and that's to *never* change frequencies on a radio while you're holding the mic or handset transmit button down. Bad things happen

This is where I learned the hard way that one or two pieces of test equipment really help out, as I spent more than a few pointless weeks trying to troubleshoot one bad radio using another bad radio I thought was okay.

 

[SCSG-G4]

AN/GRM-55 Test Set (available from Fair Radio, e-bay and others) will test all functions of a PRC-25 for correct operation. Does not work as well on a PRC-77, but can test some functions. Let us know what it says is not working properly.

 

[SJBDOE]

I should have mentioned that I did test the PRC-25 with a third radio a 50 meter amature radio and got about 3/4 mile out of it before the signal got scratchy. I also have a AN/GRM-55 and tested both radios of which both passed the tests.

 

[maddawg308]

Half a mile transmit is not good for the PRC-25 or -77. There's something else in there, could be any of 3 or 4 different problems.

First, make sure you have a NEW CURRENT made battery pack, I like the PRC68.com made unit, uses D cell off the shelf batteries, the old magnesium batteries that were military contract are all over 15 years old and even the best ones are probably crap. Second, get the GRM-55 test set, it's worth it for about $60 on eBay. The PRM-34 isn't necessary to diagnose which module is bad, only power output and signal strength, so the GRM-55 will tell you most of what you need to know.

The hard part of repairing these isn't the tools or the know-how. It's finding fairly priced modules. Some of the modules are rare as hen's teeth and the only place you'll find a replacement is another complete radio. These radios haven't been made in 20 years for the -77. The -25 hasn't in 40 years.

If you are a tech-level radio user, best bet is to do what I recommended above, and swap out the bad modules with good ones if you can find them. If you can't find a good module, bad news, might have to REALLY get down into the module and replace individual components. That's a PITA, but it might be your only option short of sending it out to a specialist for repair and alignment.

 

[papakb]

PRM-34s

Mike,

About 20 years ago Cincinatti Electronics shut down the production line they had building PRM-34s in Mexico. I bought 8 palletts of PRM-34 stuff from them including about 50 finished units in their field cases. I still have my original 34 and 2 others in their field cases ready to go. PM if interested. There's still boxes of cases and boards out there in the garage but I'd be hard pressed to find it to build up any more of them.


The biggest problem with them is finding a decent 5v battery to run them. I have a few originals and a lithium or 2 but they're probably dead by now.


I also have all the ICs and schematics for the 34s.

The radio techs used to call the 34s the "God box" because it could do power, freq, field strength and was a signall generator all in one box. It was the sole piece of test equipment that could help you etermine if you really had a good radio in the field. The GRM-55 was only a simple go-no go tester for the 25s but will only check a few modules on the 77s.


Kurt

KG6KMJ

 

[maddawg308]

For a battery for the PRM-34, here's a pic of one done up, I copied this gent's idea. All it needs are the connectors and battery holder, which Radio Shack carries, and 4 AA batteries to provide the juice.

 

[SJBDOE]

Thanks for the replies, I do have a 55 and have run the tests on both of the units and they both pass the test sequence. Unfortunately I don't know of anyone with a working PRC-77 that I could just swap out parts however,I have found and ordered some modules that I suspect might be the problem, found them cheap so no big lose if they don't work. I was also thinking of playing around with the A21 mod to see if i could tune it in to receive better any advice there would be helpful, I know the adjustments are fragle.

Kurt, I might be interested in a PRM-34 if you are looking to sell. I definitely enjoy diving into these older radios.

 

[papakb]

module swapping

Going back over your post you may have an issue in the antenna match unit located just inside the front panel. This is the aqua blue thingy behind where the antenna mounts. Look for a thin coax cable and see if it hasn't broken off the terminal it attaches to. It might be coupling just enough power to get your half mile but wouldn't do anything for receive. These radios usually put out about 2 watts when they're tuned correctly and even with the short whip should get you a mile or more. With the long antenna I've had reliable comms at 10+ miles. Check this before doing any module swapping.

Keep in mind that there are only a few modules that can be easily swapped without some retuning. The A39 VCO almost always needs to be setup for the radio it gets put into and it's the heart of the FSS and it's a major pain to do. Most of the transmitter tank modules will interchange but need to be tweaked afterwards for best power out. Almost all of the modules on the receiver tray can be swapped without must work.

Batteries are another source of headaches with these radios. The magnesium batteries are all getting old now and the first thing that usually dies is the 3v section. This is used in the 25s as the filament power supply and no filament = no output power. In a 77 it isn't used. If the 13 volt section of the battery is still good it will power the receiver in either a 25 or 77. The magnesium batteries need to be kept cool when stored and if they have any swelling at all they're history. They definitely don't like the heat.

The lithium batteries that are out there are good for use in a 77 but discouraged in the 25s. Again, the 3v section is the culprit. A lithium battery can supply enough power fast enough to burn out the filament in the 25s output tube. Most of the battery adapters out there that use regular "D" batteries should work fine in either radio.

Kurt

KG6KMJ

 

[SJBDOE]

Thanks Kurt,

Unfortunately all the wires were intact so no easy fix, I do have about six modules coming all in the receiver chain A32 through A35 along with A22 and A55. For now, I am not messing with the A39. One other thing to mention about the 77 is that the unsquelched audio is very weak even at full volume it is about a quarter of the volume strength of the 25 I have. I am hoping it is just a bad module. One last thing about my range test with the PRC-25 (the working radio). I said the range was only about a half mile but the signal strength was reduced because I had the radio sitting on the front seat of my car with the short antenna and the windows up and amoung buildings. That being said, I would suspect I would have much greater range in the open.

Thanks again,

Steve

 

[F18hornetM]

Ahhh PRC-77. Well I dont know much about fixing them, but my back knows how much one weighs. That was my job in the Marines, PRC-77 humper! [30 yrs ago].

Thanks for pics.

 

[papakb]

Well I tell you F18hornet, they aren't getting any lighter! I have a couple Sincgars sets and a PRC-117 and they weight in about the same. Lucky there's a seemingly unending supply of Grunts willing to hump these things around! Now they real new stuff like the PRC-148 MBITR and the PRC-152s are the babies. They'll talk to just about anything out there and other than being built like bricks are small handhelds.


http://i1204.photobucket.com/albums/bb402/papakb/PRC149pic.jpg

Kurt
KG6KMJ

 

[KaiserM109]

Give that PRC-25 a kiss for me, just for old times sake!!

Formerly Striker 35

 

[JoeyAirborne]

Austria kicked them out of service so many prc25 and prc77 parts are aviable right now.Just check out german or austrian ebay for modules.
I got a cheap battery solution made three of em by now only thing i need is a good supply of old magnesium batteries for the connector heads to built more.Filament save through resistor and converter so you dont have to worry.
http://www.airsoftteam-weekendwarriors.de/include.php?path=forumsthread&threadid=165

 

[HOWEY]

Low volume

One other thing to mention about the 77 is that the unsquelched audio is very weak even at full volume it is about a quarter of the volume strength of the 25 I have. I am hoping it is just a bad module.

Steve

I have this problem on one of mine thought it might be the volume control potentiometer? , any signal technicians care to give their

 

[papakb]

The pots do fail occasionally and aren't too hard to replace. Symptoms include low audio, wobbley shafts, and endless turning.

The PRC-25/77 radios have never been known for great audio and unfortunately their A25 and A55 modules don't interchange. This is the audio amp module located on the right side of the receiver tray. Luckily, it's an easy swap and doesn't require any retuning.

They never did have enough power to drive an LS-454 speaker well even though lots of guys mounted one on th top of their packs to keep from having to hold the handset to their ears all the time.

I built a batch of amp'd speakers that plugged into the 13 pin connector on the front panel and were powered from the radio but that connector is a bear to find.

Kurt

KG6KMJ