Strange Electronic Properties

[garagebandking41]

My 5-pin 1978 vibrato has intermittent problems with the preamp. The basic effect is that the vibrato indicator stays on when in off, and cuts out when the speed is too slow.

I don't touch the preamp for say 20 mins, and this problem will show up/go away depending, sometimes fooling with the switch and speed settings will cause it to work again and i will hear a small pop.

The service section says "the most common preamp failure (even then it was rare) was in the 4558 IC's."My simple question is, does this sound like a IC problem or something else?

I've ran through the preamp with a multimeter trying to find and cold joints or anything weird, but nothing really.

[Electroharmony]

When the vibrato indicator is on , in the off position, can you still hear the vibrato?

[garagebandking41]

No, the light just stays on and the rhodes sounds normal. Adjusting knobs does nothing to the sound until you turn the vibrato switch to the on position. If I test capacitors (C13 for example ) for continuity while the preamp is on, this sometimes brings the preamp back to it's normal state, not sure if that's safe.