PRC320 power supply repair

My military HF manpack radio PRC320, 3 months after I’ve bought it, started to make a whistle and was not able to tune on any frequency. After few minutes it went KO. No audio at all. Damn! How to fix it? Let’s take a look inside!

Opening the radio is an easy task:

  • Disconect battery, antena connector and headset cables
  • Unscrew all the allen screws
  • Take apart gently, front and back pannels. They slide out the aluminium enclosure.

This is what I seen inside it

Inside PRC320

The Unit n° 5 is the first suspect. This module is the power supply. As a general rule in electronics servicing, check the power supply first! Let’s open it to verify if it’s doing it’s job.

Now, with extreme care I’ve reconnected front and back pannel and powered the radio via its external power supply cable. After measuring the voltages on 6 and 112V points my suspects where confirmed. No way to have some power supply, there’s no output voltage. I’ve removed the module from the radio to troubleshoot it.

Repair that module is not an easy task because it’s semiconductors are all obsolete and hard to find parts. Still at the moment I don’t have a suitable replacement for TR1: the main 24V to 12V stabilizer, that fried. I’ve found also the BC107 on the step up section KO. Giving a chance to a BD139 in place of TR1 resulted in a semi funcional radio: I could listen to radio stations but i couldn’t transmit! Ok, let’s search for a complete new module! This is not a trivial task: this modules are very hard to find, I’ve found one new, serviced from a guy that does this job, radio servicing, during his military career, on ebay for 80€! It’s not cheap.

Now with this new module in place the radio comes to life again in RX and TX! Nice… but I have still to deal with the old broken module. I’ll update this post when I’ll fix it. At the moment this file could help you, Module 5 schematics