testing the PRC-25 and PRC-77 radios

[scottgs]

Typically, in an FM radio, if the unsquelched static volume is low, it's often an IF section problem. It could be detector (discriminator) or audio section too.

If you have test equipment, the way to tell is to connect to a signal generator and generate a very strong on frequency signal, and if the audio level is as loud as normal, but the static is weak when there's no signal, then it's the IF section. If it's still weak either way, then it's probably a detector/audio problem. Of course if this is an IF problem, the receiver's sensitivity (to weak signals) will be very poor.

You can substitute for the generator by just taking another working radio and transmitting close by, say a few yards away. Talk with a normal voice as you transmit, and if it comes out the bad radio weak, it's an audio problem. If it comes out the bad radio good and loud, it's probably an IF problem.

I'm not a PRC 25/77 expert, although I have repaired them. However, I have repaired far more than my fair share of FM transceivers in general.

Garret

 

[JoeyAirborne]

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

Thought the problem was, that most of the LS 454 are used with other pin configurations for vehicular intercom or mounted use.I have to change two LS454 229/U connectors also by swapping hot pin E and solder it into pin B on both and now they are working with the PRC25 or 77.

 

[JoeyAirborne]

Got a PRC77 on the desk with dead battery control bulb.The one that lightens up the dial section.Any idea where to get those mini bulbs?

westinghouse #328 6V .20A
written along the side wall of it.

Best regards

Joey

 

[papakb]

Mouser, Digikey, Allied, Newark, Jameco, 1000bulbs, almost every surplus electronic store in the country................

 

[packinheat2]

Hey everyone new here! Just had a question. I just recently bought a prc-77 and want to make sure everything checks out, is it worth buying a grm-55 if it only tests some of the modules? Thanks for any input.

 

[Wile E. Coyote]

Very generally...no. They're pretty hardy sets, and usually if they work on air - they work. Just make sure you have a good battery. I would say every BA-386/ BA-4386 out there is now worthless even if the E-place auction says "Brand New!" or whatever, so you'll need one of Brooke's battery adapters (www.prc68.com) or put it in an AM-2060 amp tray and power it/ check it that way. I've "fixed" many for people who wouldn't believe me when I told them their "brand new" BA-4386 batteries were no good. That's all that was wrong with their sets.

If you have some gearhead amateur radio friends you can probably get them to put it on at least a wattmeter for you (again, batteries, batteries, batteries) and maybe do a basic receiver test for you if they have the equipment.

One "never-never-NEVER" do with these sets is change the frequency with the transmit button held down.

 

[packinheat2]

Thanks for the input. Yes I already bought a battery adapter I wouldn't trust battery that's probably 10 years old. Also in the mean time I bought a frequency counter to hook up to it. I tested it on 50mhz along with a few others. With the radio on 50 the counter said 49.99971 and the others were all very close as well so I would say it is tuned good.

 

[packinheat2]

Thanks for the input. Yes I already bought a battery adapter I wouldn't trust battery that's probably 10 years old. Also in the mean time I bought a frequency counter to hook up to it. I tested it on 50mhz along with a few others. With the radio on 50 the counter said 49.99971 and the others were all very close as well so I would say it is tuned good.

 

[darukight]

hello and thank you for reading this lowly noobs post ^_^, so i was lucky to find a prc-25 (name plate was missing but it has a bulb vacuum tube) in a garage sale's left over pile and i managed to fix it and restore it, i think ^_^. it was mainly in parts and the case was bent and well it looked like junk so they gave it to me for free, so i reheated and beat the case back into shape and put back all the parts that had to go in it by looking at pictures online, the only things that i am missing are: a battery, long antenna and base plus the flex tape antenna base, i ordered a tape antenna from Israel on ebay. my question is: how and where do i get battery's? are they at all available since an adapter is really out of range money wise or is there a work around and do you know where i might find cheep antennas since the tape one cost me 50 dollars... ouch... im kinda not to wealthy that's kinda why i was digging in junk pile and hate to let nice things die...sorry i know i sound stupid but old things are amassing to me. ^_^

 

[SCSG-G4]

hello and thank you for reading this lowly noobs post ^_^, so i was lucky to find a prc-25 (name plate was missing but it has a bulb vacuum tube) in a garage sale's left over pile and i managed to fix it and restore it, i think ^_^. it was mainly in parts and the case was bent and well it looked like junk so they gave it to me for free, so i reheated and beat the case back into shape and put back all the parts that had to go in it by looking at pictures online, the only things that i am missing are: a battery, long antenna and base plus the flex tape antenna base, i ordered a tape antenna from Israel on ebay. my question is: how and where do i get battery's? are they at all available since an adapter is really out of range money wise or is there a work around and do you know where i might find cheep antennas since the tape one cost me 50 dollars... ouch... im kinda not to wealthy that's kinda why i was digging in junk pile and hate to let nice things die...sorry i know i sound stupid but old things are amassing to me. ^_^

Dead batteries are a waste of time and money, and all the old (or new old stock) batteries are dead, or will be within five seconds after turning the power on to the radio. Purchase the battery adapter from Brooke (see link above) and use "D" cells. Faster, easier, and "D" cells are a lot cheaper. You only need one adapter per radio.

 

[tennmogger]

Hi,

You would be surprised at how many of us got started in military 'stuff' collecting by dumpster diving and going through junk piles. Some of us still admit to it! You will never find a bigger bunch of tightwads, either, so don't be concerned about trying to keep cost down.

Did you get the battery box for the bottom of your '25? If so, that makes Mike's good advice even better.

But there are ways to save a buck. Your PRC-25 with the tube requires 3 volts filament voltage, too, and that makes operating off a simple 12v external gell cell not quite so easy. You could use a 12 v gell cell and a couple of D cells in a pack you make yourself, to operate the radio. Look on the web for the battery terminal pinout because a word description might lead to problems.

You don't say what area of the country you are in. Know of any collectors near you? You might be able to find a dead original battery from which to 'save' the battery connector. You might even be lucky and find an original battery that has life in it.

The original magnesium batteries (BA-4386) have a curious trait. Take a new one out of it's wrapper and measure the voltage and it'll show very low voltage. Put the battery on a load of a few hundred milliamps and let it work for a few minutes and the voltage actually comes up. A little 12v bulb makes a good test load. Note 'little' bulb. You can't use a car tail light bulb because even those can draw an amp or more.

As an example of how these old batteries behave, I just took the battery out of my PRC-77 (where it has been for a couple of years) and it shows 6 volts (expiration date 6 of 2008. I put it on a test load and applied a tiny amount of load. Voltage came up. After watching it for the time it took to type this, voltage is now showing 14 v no load and about 12v at an amp of current load. It will get better and is ready to operate the radio.

If you can get an original battery, receive mode on the radio can be your 'rejuvenation load'. Obviously this method does not work with worn out batteries, just unused, or lightly used ones.

I'm going against good advise and saying you could be ok to buy one of the NOS batteries that are abundantly available. I have never used anything but new-old-stock BA=4386 batteries on my PRC-77 because I got them cheap.

These original batteries are not rechargable so really are money down the drain if you use your radio a lot. But, if it sits most of the time like mine does, the BA-4386 works.

Bob
WB4ETT

 

[darukight]

Thank you all for the good advice and I will try to save up for the adapter and I will also buy a battery just to try it out if it does not work I can at the least salvage the terminal box, now as to the battery cover I found one with it but the vent was missing the mesh grill so I made one from an old screen and as for my location I'm in California in Santa Rosa, do any of you know of any one close by that might want to sell or trade some antennas or parts? eBay Is super crazy expensive.

 

[maddawg308]

Thank you all for the good advice and I will try to save up for the adapter and I will also buy a battery just to try it out if it does not work I can at the least salvage the terminal box, now as to the battery cover I found one with it but the vent was missing the mesh grill so I made one from an old screen and as for my location I'm in California in Santa Rosa, do any of you know of any one close by that might want to sell or trade some antennas or parts? eBay Is super crazy expensive.

I recommend Murphy's in El Cajon, CA. It's a ways south of you, near San Diego, but they specialize in military radios and equipment. Might be able to help you out with any parts you need.

http://www.murphyjunk.net/

 

[SCSG-G4]

darukight, you need to change your location to reflect the town and state you are in. It's part of the rules you agreed to when you joined.

 

[darukight]

Sorry i thought it was automatic but i fixed it ^_^

 

[darukight]

Here are some pictures of what I made it look like, ^_^ I painted the letters and most of all cleaned it we'll plus I painted the case after un-crunching it and unbending it.

 

[T. Highway]

Looks like it cleaned up nicely.

Bert

 

[renodogs]

Aha, a 2531! I was a 2841 (Radio Repairman and i too got to hump 77's, 25's, and those nasty little KY38's. No fun, no fun at all. But- they worked!

SF

Dave

 

[Armando]

Fss problem prc77

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

Well, first of all, greetings to everyone!, sorry becouse my english is not perfect but, ill try my best.
I want to talk a little bit about PRC77 FSS problems.
I think that those false lock or unlock failures youre talking about, have to do with two parts only.. A39 FSS VCO with its associated tandem capacitor. But at the end i think its a mechanical failure!. The frequency selector adds an error to the tanden capacitor couse it doesnt block the tandem always the same position when changing to another channel, so... it creates an aditional capacitance drift and frequency error that, at the end, may cause the phase comparator and discriminator to false lock or unlock.
Service manual says you have to adjust a39 to 40.5 and 64.45MHz, but it wont help!, try to make this and then move around up and down, and see how long fss last without unlocking hehe.
And, then, how to fix it?... thinking in long term solution, change the frequency selector !!!, or try to adjust a39 away from the above frequencies to add an error to the actual selector error xddd, for example 64.300 40.200... its not an easy task and you may have to repeat this procedure each month....
Just lock at the control voltaje at a39 j4, should be 4... 4,2... 3.9... not under 3.9..
Let me see what you have!

 

[JoeyAirborne]

Well Armando just search for the pdf "PRC-25 & PRC-77 Problem Log by N3OC" on the internet.It helps a lot.Tuning the A9 (prc25)or A39 (prc77) will help you not out!
Best regards
Joey