Unusual HP-21 ?

[jebem]

(11-26-2017 08:12 PM)jch Wrote:  Vss and Vdisp are filtered by small tantalum capacitors, would they classify as electrolytic capacitors which may dry?

Tantalum caps usually fails catastrophically by going into a short circuit, which is not the case here. However, I have seen these caps presenting leakage, in other words, they act more like resistors than filtering components.

When i see ripple above the acceptable value and there are just a few low cost caps to look for, and i can't find any other obvious causes, i don't think twice, and to save time, i just replace them.
Obvious causes would be:
- Any component in parallel with the output that is draining current above the power supply specification (a defective cap is the usual suspect when the power supply was disconnected from its target load, as you did);
- Any defective power supply component. I would start looking to the rectifier diodes, they can present reverse current leakage above the acceptable. Again, when in doubt, i just replace the diodes and move on.

Edit:
I see a lot of pcb traces corroded by battery electrolytes.
Did you try to remove the capacitors?
I would not be surprised if one or more solder joints in the power supply were affected too.
That would isolate one or more components.
It is easy to spot such solder joints: It is very hard to unsolder them.