testing the PRC-25 and PRC-77 radios

[Armando]

I have seen it, but it dont help for this anyway, what they say is that if you use diferent brand of tandems with an a39 they could send you 1mhz low or high becouse of the diferences between manufacturers. I thing thats not that matter. Alignment is very clear as i have said at the manual, but it didnt work for me.. Sugestions?
Best regards.

 

[Armando]

Not an easy task guys, that was my theory but if you have yours, i will be greateful of reading them. Fss unlocks, even if you cal it like the manual says, i close the radio, the, days pass... then radio unlocks one day and when i open the radio i find out that fss is now locked haha... sick!

 

[Wile E. Coyote]

I had a 77 like the one you describe that I just couldn't fix. Changed out everything that could be changed with a known, good radio and aligned as per book...still unlocked. Got frustrated with it and gave up - as evidently had the radtechs at the previous holding unit because it was green-tagged as 'UNS - CAN'T CORRECT FREQ DRIFT' or similar. I hate admitting defeat, but it just wasn't worth my time to pursue it further when I could just grab another operational one for $150 or whatever at the time and move on. I still have fantasies of a 12 ga. in my hand and that radio flying through the air upon the command "PULL!" Some sets would benefit more from...exorcism...than oscilloscopes.

 

[papakb]

You know after 40 years or so cold solder joints and bad contacts and connections can play havoc with these radios. Half the time just running the tuning knobs around wipes the contacts clean and they start working again. If you can find a can of "tuner lube" or "contact cleaner" you might want to give the modules a shot through the triangle area where the band change switch mechanism lives. Radio Shack used to sell it but most of them are gone now too. Shoot the A40 module too but you should pull the cover off first to really get down onto the rotary switch.

http://comingsoon.radioshack.com/ra...eaner-and-lubricant/6400148.html#.VbbzFrNVhBc

Kurt
KG6KMJ

 

[Armando]

Yep, if they are not damaged enough....
I only have found partial solution, if you calibrate 39 the book way... it wont help, couse probably your problem is on the frequency tuner gears that dont fix the capacitor the same point all the time, at least it seems to create too much error for the fss... Cleaning or reemplacing the contacts... it could be a long term fix... but its so hard for me to go on this crousade hahaha.
Sometimes as is said at the prc77 problem log page, the tandem capacitor is not aligned or is not compatible with the a39 and or a38 fitted in, becouse it dont match the correct vco capacitance range...
Anyway, i think there are two possible ways to go:
Adjust first a39 so it is not at the actual frecs the book says but for example a litttle bit lower at 40.150 and a little bit higher at 64.450 to compensate the panel gear error.. then, look at the control voltaje, make it almost equal at both ends, i sugest not to go under 4v becouse the fss is too relaxed here to compensate future errors, go for 4.3-4.5v.. go up and down frequency several times and measure again and again at a39 j2, voltaje should not go under 4v.
Second way is to replace frequency selector meachanism or just cleaning it up, then, adjust like the book for 40.150 and 64.450 and, go to 52.95, force a little bit the mhz knob, but it should not afect sooo much to the voltage.

 

[Armando]

Did you change the tuning capacitors?, the frec control mechanism? assure the correct capacitance ratio? Im not sure capacitro and vco of different brands are interchangeable from one radio to another, if you have a RCA ill put an rca a39 a38.... Capacitance ratio could also afect loop stability of the pll, there is c10, another cap that is not tweaked in the book, but it has effect on the varicap diode and on the vco gain...... this cap should be taken in to acount.

 

[turnkey]

We just with our R-2368 radios....and the AN/URT-23Js....HF gear .....Got lucky and have a few spares...use to work on the PRC-25 FM radios in the 80s for uncle sam.......great stuff to work on....lots of bad solder joints and loose wires....Still its a bunch ofwork onthem....

 

[Armando]

Hey guys, ive tried my best and the prc77, looks fine now, but i dont trust her so much... fss problem may apear again another day.
Ive got a PRC77A (25khz version), is someone here familiar with it?, couse despite i know the basic changes and new fss system with only a xtal reference to tweak (this is good), i dont have found any manual of it.
Kind regards.

 

[Armando]

Beat heard at some frequs

I can hear a whistle in some frequs in one band for example 53 and the same noise at 30mhz.. ¿is something beating with the synth?.. ¿maybe a small audible delta between the tx side step osc and the reference fss that modulates the tx vco?.

 

[Armando]

Oh.. sorry, this is only a tx problem

 

[Armando]

It only happens during tx at some frequs... still dont know...

 

[ThirdGear]

I have a couple GRM-55 test sets that don't pass self test / calibration.I can reproduce the same symptoms on two different GRM-55's using multiple different PRC-77's. Both GRM-55's are behaving similarly -- can't get a solid green on self test / cal mode. It's strange that both boxes fail nearly identically on multiple PRC-77's -- with and without either of the two probes connected to the cal test point, the red light comes on, will flicker to green but won't stay green as you vary the calibration knob from full CCW to CW.
Here's a link to a youtube video I posted showing my test procedure: https://www.youtube.com/watch?v=nu5SDGJd4sY
If any of you in the higher echelon out there have seen this before, please let me know if I've followed the self test procedure correctly or if you know any commonly seen causes (operator error?).

Thank you!

 

[papakb]

Since the GRM-55 wasn't designed to be used with the 77 that may be your problem. There are enough internal differences between the radios that a test that works on a 25 may not work on a 77.

 

[papakb]

Just looking at the info for the change 2 manual for the GRM-55 and they are now saying to alternately push and release the test button as you adjust the cal knob rather than holding the button down. So possibly holding the test button down steady is the problem. Give it a try and let us know what you find.

Kurt
KG6KMJ

 

[papakb]

Any updates?

 

[ThirdGear]

Update: Out of three GRM-55 sets I have, only one of the MX-4622 Test Prod/Probes works between them. I am now following the pulse method to push and release the test button between cal knob adjustments as pointed out. Even so, after trying all combinations of probes and test sets, only one probe will successfully calibrate on all of the GRM-55 test sets. The other two probes won't pass cal on any test set. I have tried adjusting the only potentiometer in the non-cal probes, and can't find a passing window of operation. If anyone comes across a 35 level depot maintenance TM for the GRM-55 test set, please send a link.

 

[ThirdGear]

For the PRC-77, you just can't beat the PRM-34 "God Box" for getting a quick characterization of receive sensitivity, squelch, transmit frequency, and transmit power. Here's a video demonstration of using the PRM-34 test set on a PRC-77. The RF output power measured way too high in this video. I recalibrated the test set afterwards to match a NIST traceable power meter.

https://www.youtube.com/watch?v=bUwwMtGscNo