Hi, I'm new :P I've done all my own guitar work for years and have never had a problem wiring and shielding a guitar so that it's as quiet as possible... Until I converted one of my guitars from active to passive pickups. The guitar came with EMG humbuckers which I removed and replaced them with Dimarzios, CTS pots, orange drop cap, etc as I've done for my other guitars. The cavities were pre-painted with conductive paint (why I don't know considering it came loaded with EMGs--the PU cavities weren't connected to the control cavity's shielding). I direct-mouted my Dimarzios, and while the guitar has never been as quiet as my others, it was relatively quiet after that. I suspected a poor bridge ground connection to be the reason it wasn't as quiet as my other guitars with similar configurations. It has a Floyd in it, and since it came from the factory with actives, there was no connection from tremolo claw to control cavity shielding/vol pot. I added that, but doubted the quality of my connection with a 25-watt iron on the stainless steel claw. I resoldered it repeatedly once I got a 45w iron and noticed no improvement, so I went looking for other issues.
I noticed that the pickup mounting screws (drilled directly into the pickup cavities) were not grounded. Yesterday, I removed the pickups and put copper foil tape over the four mounting holes to ensure connection between screw and the surrounding conductive paint (connected to the control cavity shielding). I then remounted the pickups and confirmed that the screws were now grounded. The screws went from registering nothing on my multimeter to registering something--I thought any number registered on the meter meant the screws were now grounded... Maybe not.
I fired up my amp and noticed the noise was the same as always when my hands aren't touching any grounded parts of the guitar. A slight buzz that goes away when I put my hand on the strings or bridge. However, now there's a new, much louder noise. Whenever my hands near the pickups, or touch the plastic faces (the term escapes me), there's an enormous amount of noise now. This goes away when I lay my hand on the bridge... I read here that checking two grounded points and registering 3-4 ohm or less means those points are properly connected, but registering a higher resistance means there is a problem. I put one probe of my multimeter on one of the mounting screws of one of the pickups (activated via toggle) and the other to the sleeve of my cable and found a resistance of 640 ohms. Same when I touched one probe to the conductive paint of the pickup cavity. Compare to less than one ohm when touching one probe to the strings, bridge, any other grounded part of the guitar and the other to the sleeve.
What's the problem here? Are my cavities + mounting screws acting as 'floating grounds'? I seem to have two problems here: the new cavity issue I apparently created with the copper foil between the screws and the conductive paint and the new connection between the control cavity and PU cavities; and the older problem of the guitar being loud when nothing grounded is touched. Regarding the older problem, is there anything I can do with my multimeter to verify the quality of the tremolo claw solder point? Touching one probe to the strings and one probe to the sleeve of the cable and finding it registered under 1 ohm tells me this connection is good, right? Regarding the new problem, should I dismount the pickups again, tape over the factory conductive paint with electrical tape, and then remount the pickups so the mounting screws aren't grounded (these are 7-string pickups and Dimarzio's 7-stringers have fiberglass back plates rather than brass, so there's no electrical connection between mounting screws and back plate)? Is there an easier fix?
If any of this is unclear, I'll be around to elaborate. I'm pretty desperate and am hoping someone can help me diagnose this fully with my multimeter. I know the control cavity wiring is solid because it's the same setup I've used in other H-H guitars of mine that are whisper-quiet (some with higher output pickups), and I've actually resoldered everything inside five times in an desperate attempt to mimic that quietness :(
No takers as of now and I'm not a trained expert, self-taught and not completely available to do research at the moment.
And although I taught my son, I can't go running to him with every question I would like to answer, since he surpassed me with 2 years of electronics so far in college, and he was building synthesizers before that.
Anyhow, what did you do with the jack? You no longer need a stereo jack which the active pickups used, with an additional positive lead to switch the power from the 9V battery to the pickups. A stereo jack would still be okay, as long as that positive connection is removed and is defunct. Find that wire, trace it and remove or kill it. :shock:
Also, I believe that is carbon paint or shielding paint in the cavity, and not a conductive substance or part of any circuit. It is to hush the thing, and quiet the hum. Adding the copper foil probably compromised that and brought your pickup screws into play in an undesirable manner, and into a possible ground loop situation. At any rate, your bobbins or poles got noisy. IMO, you should re-configure the jack to the bare essentials, positive and negative, or replace it if you want.
Remove the shielding, check to see there is no solder out of place at the toggle or blade switch, and double-check DiMarzio's instructions for start and finish at the coils, connecting only the correctly colored wires, and taping off or heat shrink closing the two which are unused, but are twisted or soldered together conducting current. If you have erred on just one pickup, it can be out of phase with the other, and produce unwanted noise.
I don't think you have an issue grounding the trem claw either, and like you said, redoing it didn't make much difference.
Like a bird on the wire,
like a drunk in a midnight choir
I have tried in my way to be free.
