My band has an ancient Sunn PA head that has been very reliable. Last week, one of the clubs asked if they could play music on an Ipad through it during our breaks. We said "fine," and used the "tape in" RCA inputs. But then trouble started. We couldn't get sound from the PA when we resumed our set. We fiddled with the main volume and the thing began to work again. Then, last night, we turned it on and nothing happened. We thumped it and twisted knobs, and suddenly it began to work again. Then toward the end of the first set, it quit. We discovered that by turning it off and then back on, we could revive it. We had to do that again later in the night. This seems very mysterious. It doesn't seem to be a potentiometer problem. I sprayed contact cleaner into the main volume knob and it had no effect. Do you have any idea at all what might be causing this?