Fender Starcaster Leaking Pick-Up Signal

Ché

Fender Starcaster, 2 vol, 2 tone, earthed at switch with vintage braided cable and CTS pots.
Just got it back from my tech and had a phone call with him, we're both flumoxed. I'm taking it back tomorrow but wondered if anyone had any ideas as to what could have gone wrong.

The Problem
Bridge pickup selected, bridge volume off, neck volume on - faint signal from guitar that goes away when neck volume is rolled down
Bridge pickup selected, bridge volume on, neck volume off - sound, ground hum/inteference
Neck pickup selected, bridge volume on, neck volume off - faint signal from guitar that goes away when bridge volume is rolled down
Neck pickup selected, bridge volume off, neck volume on - sound, ground hum/interence

The ground hum fades a bit when I place my hand on the guitars strings and switches but still reacts to interference. Walking towards my printer with my hand on the guitar still makes a horribly loud hum/buzz the closer I get to it.

It's wired like a simple Les Paul, both pickups go: pickup to vol knob with capacitor to tone, vol knob to pick up selector, pick up selector to output jack.

Any ideas?

ICBM

It sounds like either a ground loop (most likely since it seems common to both pickups), or a poor ground.

The most likely culprit for the loop is the cable from the jack to the switch making contact with any of the other wires, or the two pickup wires touching each other, or the four controls having a common ground. Ideal, you want full 'branch' grounding - similar to 'star' grounding except without a central point - each pickup ground to go *only* to its own volume and tone controls, then the volume controls to the switch, then from the switch to the jack, with none of those grounds being in contact with each other. You will have to sleeve the braided cable with heatshrink tubing to achieve this. The string ground needs to be ideally connected directly to the jack, but in practice any nearby ground will do since it's not part of the signal path.

The most likely cause for the poor ground is either of the volume control ground terminals having separated slightly from the track as they are bent back to solder them to the casings, which strains the riveting. Ideally you want to only fold over the top part of the terminal at a right angle with a pair of pliers, not push the whole thing. You can usually fix this by squeezing the terminal hard onto the track board with a pair of fine-nose pliers.

The interference from the printer is nothing to do with the grounding and is simple electromagnetic noise from the switch-mode power supply in it.

Ché

Thanks @ICBM. I'll re-read that in the morning as I'm a bit too tired for it to process. Really appreciate your assistance in input though

The point about the printer was even with both volume knobs turned down it was till causing interference. Not a ground issue but still not something that should happen.

MattFGBI

Yeah, something is leaking! I think it's going to be one of the pots but if you return it to the dealer they'll send it here and I'll find out. 

Ché

Thanks, it's a second hand guitar though and it's come back from my tech like this. I'm taking it back tonight.