Weird ground problem with on guoitar mounting

[edo1123]

I know I may be a pain with all those problems related to wrong off board wiring. This time I have some real trouble. I've built the great destroyer and I truned it on.. It worked as it should.. Then I decided to put it on my guitar (really small board/ big Muse fan) replacing the tone and the drive pot with two separate jumpers. I've already mounted a green giant booster/distortion on this guitar therefore I've used the same connection.. When I've finished mounting the board I've turned it on.. No sound.. I took my multimeter and I've started taking the voltages. Here's the big problem: how is it possible when I put the battery in all  grounds get 7 volt? I mean when I measure a ground (sleeve or back of pot) multimeter shows 7 instead of -0.. It will be surely a off board mistake, but I don't know how is it possible to have positive ground on a negative ground circuit.

[induction]

Post photos and we'll see what we can spot.

[edo1123]

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Messy and low definition. Sorry for that. The gray wire from the input is connected to the battery ground..

[edo1123]

I've tried to put back the green giant and I have the same problem now (the ground value rises up to 7 volt).. How is it possible? The off board wiring is the same as the time I took it off..

[induction]

Are you measuring these voltages with the guitar plugged into the amp? The guitar itself has no ground connection, it relies on the amp to see ground.

Where are you connecting the black probe when doing these measurements?

[edo1123]

Unplugged.. The green giant works now.. The great destroyer is in stand by. I've made such a newbie mistake. I was measuring the values with the jack unplugged so I had the "positive" ground, am I wrong?

[induction]

That's probably correct. When the guitar isn't plugged in, it's ground wire isn't attached to a reference, so it floats. What voltage you read will depend on what you're comparing it to (ie. what was the black lead from the DMM touching when you took the measurement).