Quote Originally Posted by daver2 View Post
You say above that you replaced C51 with an electrolytic. The parts list calls for a low leakage electrolytic - or a tantalum.

If the capacitor is too leaky - it may not charge up enough to operate the 555.

Either that, or a faulty PCB...

When I had to change C51 (because the tantalum failed short-circuit), I always opted for a 1uF ceramic as replacement. I think a regular aluminum electrolytic should work there unless it's really of very low quality. But yes, that would be my last choice.
Also, if the original tantalum wasn't short circuit, there was no need to replace it in the first place.

Frank (who currently has a PET universal board on the repair bench too).