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Yeah it's primarily for reducing noise, it's also a good place to star ground if you don't want to use a stereo jack as EYG describes

It's got a stereo jack and it's (mostly) star grounded at the chassis screw point. Continuity tests well. Next time the iron's hot, I'll redo some ground connections and if that fails I'll run two separate batteries in there to see if that makes a difference. If that doesn't work, I might just forget it and get used to having an always-on SHO in front of my fuzz (doesn't sound *bad*, but it's different).

I would try to make sure it's ALL star ground. Any ground loops could be your problem

The wah board goes to the chassis ground at two points (the blue and red wires on the far end of the board in the pic), does that look like a potential loop? It wasn't particularly noisy until I added the booster circuit.