It is rather difficult to guess what cap belongs there and what else is missing. Form the position close to the power transistors, this could be a afterthought (and thus on the back) cap to reduce oscillations. These caps can be tricky to guess, even if you have a circuit diagram. The capacitance can matter and could be anywhere from 10 pF to about 10 nF, depending on the circuit.
Without the schematics its really hard to tell what is missing, and even than one might need to do a simulation or/and measurements to see, what is missing / wrong. If you are lucky the cap is just extra buffering and less critical, at least that could be seen from the schematics.
The resistor (100 Ohms ?) that is still there at the under side looks damaged too. At least this one is easy.
Thank you for sharing your knowledge! I suppose I should invest in a scope which isn't 80 years old and do some testing. It is very relieving to know these aren't completely critical, as the person before me used this continuously, and I had assumed that they really might have damaged it.
But thank you for letting me know; this will be a good way to start reviving and continuing my passion.. Thank you again for the input.
Good day for now
Why would they put large components on the back side of the board like that?
If you have any trimmer capacitors you could experiment with the value untill the circuit functions properly.
One reason to put the cap on the back side is the need to adjust it, with no easy access to the other side. Variable caps in the nF range are really rare and large, so not that easy.
If the capacitor is for a stability adjustment, it is well possible the supply works with well behaved loads even without the capacitor. But without the right size cap it might show more ringing or could oscillate with a more critical load. So it may kind of work without, but could fail unexpected with different loading.
The first thing would be to get a schematics, at least for the part just around the cap. Today one can simulate the circuit and this way get a clue for the right size of the cap, maybe even better than it was possible by measurement.
For those who'd like to see the pen scope, I believe it's the same model that Dave showed in Mailbag #860
Sorry to bump the thread again but I replaced the red capacitor and everything is happy [not hot, etc] but I replaced the 100
+\- 10% with an equivalent one, only with +/- 5% tolerance, and the voltage gauge slowly rises even when its knob is at 0% when powered on, so I think I may have quite a bit of simulation / trials to do