#1
So this is been doing my head in. A while back I decided my SGV800 needed a phase switch put in, because the standard dual pickup sound was too muddy with gain. I copied out the diagrams from online and carved a hole in the scratchplate for it, in the space just under the toggle switch that clearly was just asking for it: drilled the screw holes and soldered it all up.

Before I put on the rest of the scratchplate and stuff I tap the pup poles with something metal to make sure they work in the right switch positions.

Now here's the trouble. When the switch is hanging loose from the scratchplate, everything works fine in all settings. As soon as I screw the phase switch in postion, at least one half of the range stops working! And even the half that works is intermittent.

Now, I realise that my soldering is piss poor. It could well be that. But I've tried fixing all the contacts several times and every time it fails on installation. As soon as I unmount it to troubleshoot everything is instantly working fine again.

The switch is very close to the large pickup toggle switch, and in fact the topmost screw bracket of my phase switch has to sort of nestle into a gap between the pickup switch housing and the scratchplate, a gap formed by the pickup switch collar, to fit where I've put the mounting holes. So, the switches may be touching there. I've tried covering that area of the pickup switch housing with insulation tape but it doesn't seem to make any difference.

The only other thing I can think of is that the switch could be shorting on the scratchplate foil. Is that possible? It's the only thing that'd kind of make sense, because the screws I've fixed it in with aren't quite the right size and the switch sometimes slips down half a thread either side - that'd account for half of it working sometimes on installation - the side that slipped down wouldn't be touching the foil. But if that were a common problem the switches would be plastic-capped or something, surely?

Also, yeah. If anyone knows the right sized screws for this kind of cheap DPDT switch that'd be really helpful too. The best screws I've currently found for it I've salvaged from a toy voice-changer thing.

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#2
Can't tell a lot from your pictures, but the description sounds like it's shorting out on the foil backing or maybe touching the other switch. My first idea was tape between the switches, you already tried that. Try cutting away the foil with an exacto knife, just the small area where the new switch is. Depending on how it's glued in place, once you cut out the area to remove it might peel off fairly easy. The only reason I can think of is the switch could have a metal plate inside that's allowing the screw threads to ground against the foil. I've never seen a switch do that, but from your description that's the only thing I can think of. If it works before being screwed into place, something is changing when you install it. Either the switches are touching, which it sounds like has been ruled out, or the foil is causing a direct short. It might also be causing a short between the two switches - same solution, eliminate the foil under the new switch. It shouldn't affect the shielding of the foil enough to worry about.

And if you're not good at soldering, find someone who is or learn how. Bad solder joints will fail at the worst possible time...like onstage in the middle of a great song...been there done that...learned how to solder...
Hmmm...I wonder what this button does...