PRC-344 Module 6 repair

With module 6 removed from its shield, the next task was to track down the failed component(s). The large TO-5 can is the regulator IC, an LM100H. Very much obsolete and available only at extortionate prices!

Armed with the datasheet for the LM100H, so I knew which pins connected where, I first traced and drew out a rough circuit diagram. There are four connections to this module. Three make immediate sense - raw 24v, regulated 6v, and Ground. But the fourth im not sure of! But it does seem to rule out the plainly obvious repair of simply replacing the thing with a modern three-terminal device!

The first suspect, and the easiest to isolate and test, was the 22uF capacitor across the raw 24v input line. I lifted the +ve leg of this and metered it...

Bingo! Bloody thing was dead short! I lifted the other leg to take it totally out of circuit and confirmed this was the case.

Now, im not at all sure what type of capacitor this is! But its value, voltage rating, polarity, size and the fact it was across a supply rail leads me to suspect a tantalum. My next job was to physically remove it. Although these modules look normal, they are in fact washed over with a tropicalizing varnish, and the capacitor was stuck down solid. It required a careful application of a wood chisel to prize it off!

A modern standard electrolytic capacitor fits neatly in its place. I would have liked to have fitted an axial device, but the only 22uF 50v devices I have are radials, so I had to bend the legs around a little!

At this point, I havent yet put the module back in the radio and tested it. First, ive to check this beastie over -

This is mounted in fuse holder contacts in the supply line. My suspicion was that it was a varistor, but a bit of research online suggests it might actually be a Schottky diode!