This (soldering) guns for hire, even if were just faultfinding in the dark...

I have a certain amount of sympathy with the lyrics of the song from which I have paraphrased this post title - as a night shift worker the first few lines are very familiar! So apologies to Mr Springsteen.

Earlier today, I popped along for a nosy at Johnsons of Leeds, and came away with a Clansman PRC-344 UHF manpack and antenna. I always knew I was taking a big gamble on an untested set, so was only minimally disappointed to find it doesnt work. Physically it is complete, and no damaged parts other than the lamp/call switch, which does still work but the toggle is loose.

So, with nothing more than a basic block diagram and 25years of experience, I set about trying to find the problem...

Firstly, the fact that it is totally dead (not even the dial light works) is actually a good thing! A part working set is always more of a worry, but a totally dead unit immediately points to a power supply problem.

Opening it up, I discovered a few of the plug in modules to be loose, but seating them back in correctly didnt cure anything. It may be that someone had previously started fault finding? Who knows!

What I can say, is that the build quality of these is amazing!About half of the electronics are in the form of plug in modules, but the interconnecting wiring harness looks a nightmare!

The first job was to ensure that the battery supply was actually reaching the electronics. So with the set off, a check at the incoming battery terminal showed 19v. Hmm, ok so the power is there, but thats a recently charged 24V battery! So, battery off and trace the supply wire, expecting to find a fuse blown.

What I found, in a fuse holder, was not a fuse. I think its some form of Varistor, but either way, there was no voltage after it. Having removed it from the circuit, I tried metering out the supply wiring to other parts of the radio, and made an interesting discovery - there was a dead short!

Now, in a radio as complex as this, a dead short could be a real basket to locate! But, here we have the advantage of plug-in modules! So, after making a sketch of the module positions and numbers, I pulled all the modules I could. Checking again showed no continuity to ground, so it was reasonable to assume that the short was in one of the modules.

Putting them back in one by one, and checking the continuity after each, the short returned when I replaced module 6.

Module 6 is the 6V stabilised power supply. I suspect that much of the radios systems run on 6V, and so this being faulty would almost certainly account for a dead radio!