Hi all, I have an interesting question that someone on here may be able to answer - I have a Fender Deluxe 112 Plus combo on the bench which is causing a few sleepless nights, as it has a fault of the type I have not ever come across before...

Randomly it will suddenly make a loud, short buzzing sound, no longer than a second in duration, which resembles the sound of a bee in a bean can. It usually occurs about a minute or so after switch on, and can happen at any time after that. All earthing checks out OK.

The fault is there whether the volume control is up or down, suggesting a power amp problem, but having monitored dc conditions in that area, nothing changes when the annoying sound occurs.

This is the baffling part - most of the electroytics have been replaced, including the two large smoothing caps, as have all of the FETs, and U1 (an MC1436 opamp).

The output transistors have not been replaced, as they appear to be in good order. Intermittent breakdown ocurring in one of those maybe?

As you can appreciate, a fault of this type is very difficult to trace - would anybody here have any experience of this fault, or some suggestions as to what to check?