Boss SD1 Repair

[Paladin2019]

Hi everyone, been reading this board and finding it an invaluable DIY resource for a long time but this is my first post.

I've picked up a broken SD1 for cheap, hoping to get it working and use it as a base for some modding experiments but the repair is proving more trouble than I expected.

The problem is that the tone and volume controls work fine, and up to about halfway the drive control is fine too. But after halfway the signal cuts out and a deep, rumbling oscillation takes over which changes in pitch and frequency as you roll the drive knob up and down. Strangely the problem persists even when the pedal is in the off position.

Some mods had been attempted at one point by a clueless amateur. The IC was replaced along with the clipping diodes, but I've restored these to near-enough original spec and cleaned up the soldering. I also replaced all the transistors and FETs, but to no avail. I've measured the drive pot, it's working as it should. I can't find any solder bridges, broken traces, dry joints or loose wires. The previous owner claims to have no clue about the nature of the mods and couldn't (wouldn't?) tell me about when or how the problem started.

Can anyone tell me the next place I should be looking?

Thanks for any help you can offer!

[PBE6]

I know you said you've already checked for dry joints and loose wires, but I suggest re-flowing the solder joints where the wires connect to the PCB. All of them. I had a similar issue on one of my first SD-1 mods, and it drove me absolutely nuts looking for the problem. Turns out one (or more) joints broke invisibly just from gentle handling of the board. Re-flowing the joint fixed it right up!