I'd like to thank Dr.Frank for sending me the solid state comparator board, to replace my mechanical chopper based one that died in my 332A.
My unit had been working prior to the chopper failure, but it did have some other issues.
One was that when I'd bought it (ebay, 'unknown condition') it turned out to be missing the inner chassis top and bottom covers.
The 332A case is constructed with three independent layers of electrical shielding:
1. Outer metal case and front panel, at mains ground.
2. A 'guard' shield layer, brought out to a front panel banana post. This shield is constructed by lining the outer case with PCB fiberglass, with an inner copper layer that is made electrically contiguous via wires soldered between each plane.
3. The main internal chassis, which for some reason I don't understand, is connected to the '+' output rather than the minus.
The inner chassis is supposed to have top and bottom metal covers, electrostatically shielding the circuit boards from the Guard layer. But some bozo in the instrument's past before it came to me, had lost them.
Replacing them had the slight complication that they needed insulating spacers to keep the inner chassis separated from the Guard layer. Also the sheets needed were bigger than any of the scrap aluminium I had lying around.
I'd never got around to making replacements.
Now I had a new comparator board I decided I should fix up all the other little problems too.
Starting with making those shields. That's done now. A bit rough, but should work. For spacers I used some things intended for supporting glass table tops. The side spacers which are present are teflon I think, but I'm not sure if that's really necessary. The inner chassis can be up to 1100V above ground (scary machine to work on!) so maybe those spacers should be ultra low leakage? It remains to be seen if the clear PVC spacers I used are adequate. If not, they'll be easy to replace.
While visually checking out the other boards before trying to power it up, I noticed a very singed area on the 'current limiter' PCB, with the component that had overheated now replaced with a wire link. What?!
Took me ages to find that in the schematics, since the relationship between schematic sheets and boards is a bit random. Turns out the front panel used to have filament bulbs for the decimal points, but at some stage a mod changed them to LEDs. This burned resistor was the current source for all the bulbs. Now the individual LED current limit resistors are in the wiring loom, hence the wire link on the board.
That's where I was up to when distracted by other chores. Still haven't powered it up with the new board.
Planning to make an extender board with the spare pins and socket Dr.Frank sent me, before doing more on the unit.
Edit to add: Incidentally the messy felt pen writing on the inner chassis is not mine. Perhaps by the guy that lost the covers. It's on the list to clean off and redo neatly.