Shredmaster oscillation

[slideman82]

It happens when contour control is set at min (full mids, no scoop) and gain at max. I used the schematic posted here http://www.harpamps.com/schematics/shrdmstr.pdf and the one in GM Arts, that I believe it has errors. When I use the Gm configuration, a 220n cap and a 47k resistor in series come to the - input (+ input goes to 4.5V), while in the one in Harpamps lug 2 of treble pot goes directly to + input, and the 47k resistor plus 220n cap in series goes to 4.5V from the - input of the 3rd op amp. In the first case, oscillation it's softer, also is distortion, and I think it doesn't sound like the original. I had to rise 8.2k resistor to 100k before the 2nd op amp and oscillation disapears, but I have modded with tonestack bypass, so oscillation rises again! I'm working with Haramps' configuration, but was different with GM's, because oscillation went off with bypassed tonestack... this mod sounds quite well.
I'm using my own layout, I hope this won't be the cause of this self oscillation that's driving me nuts!

[John Lyons]

I'd say that it's a layout issue.
Can you post a pic of the inside of your box?

Short wires and a linear layout as well as keeping the input and output wires well apart will help.
Shielded wiring may help as well with longer wiring runs.

john

[slideman82]

Thanks for those ideas, John! I'll try the shielded cable connections but I'll try some ideas before, like lowering that 680k resistor.