problem with dunlop hendrix wah

[acorkos]

this dunlop hendrix wah came to me for repair....bad switch and 'weird noise,' according to the owner. i replaced the switch, and discovered the weird noise to be a loud upper mid-high frequency howl when the wah is engaged. when the pot is rotated, the sound goes from nothing to the high pitched howl. no guitar signal when engaged.

i swapped in a new pot....no change. swapped in a different board from an old dunlop wah....no howl. so something on the board is toast. i swapped the inductor...no change. not sure where to look next. does anyone have any experience with a unit like this? what component(s) is(are) likely to be the problem? i don't want to start swapping all of them (not yet) without a little understanding as to what might be the problem.

thanks for your help.

http://www.freeinfosociety.com/elect...iew.php?id=352

[52 Bill]

All dunlop, vox, foxx, etc. are basically the same circuit. They are a two stage amp with tone filters and a lot of negative feedback. When the pedal toe is all the way down the second stage in completely out of the circuit (input shunted to ground). Does it pass signal at all this way? If not check the first transisitor and all of its related components. Check for bad grounds and solder joints and traces around this stage. If it has the pin connector strip, check it for bad solder joints, etc.

Because it howls when you sweep the pedal, it seems that the second stage is working. Does it look like someone has modified or "fixed" anything?

Hope this helps.