Fender Hot Rod DeVille 212 screeching problem.

[Bigred]

I have a Fender Hot Rod DeVille 212 that works fine on the clean channel, but when I hit the drive/more drive buttons either on the switch or the amp itself, it lets out a nasty screeching sound. I have checked the boards and wires for any obvious signs of a malfunction and have come up with nothing. Any help would be greatly appreciated. Thanks.

[bluesfreak]

Ever checked the +/- 15Vdc supplying the OpAmps and Switching circuitry?
Especially the 330Ohm resistors R78/R79 and the Zeners CR11-14 surrounding it are worth a look. Using the search function will give you a lot of hits on this topic...