Mystery hum.

[griff]

A customer brought a Harmony Rocket in for a number of issues, one of which is a persistent hum when plugged in, which he reduced when he grounded the jack nut to the Bigsby mounting screw using an old guitar string. I unfortunately didn't bother to check the hum without the make-shift ground wire attached. I assumed it was a broken ground, and pulled out the harness. But everything looks fine inside. Solder connections are strong. I cleaned the pots with De-Oxit and am getting good readings from the multimeter.

So before I put it all back together and hope that something fixed itself, is there anything I can do with the harness out to look for clues? This harness has the metal spiral wire sleeves that join every pot and then ground to the jack line at the three-way switch, exactly as pictured here:

http://harmony.demont.net/documents/schematics/standard_2pickups.php

[Russell Vance]

check the string grounding wire from under the bridge to the system earth (back of pot, earth lug on output jack or similar). The strings should be earthed to the system through the bridge. R.

[Tom Krebs]

Some of those never had a string ground. Should run from under the tailpiece mount.

[FRANK]

are those two single coils ?then of coarse you should hear a bit of hum i wounder if a pick up was turned woops?dose it clean up when playing with the tone pot ?because i wounder if you hear system noise I did like an answer that I just recently came across it was to test every part and find the bad part .beautiful guitar

[Len Biglin]

This setup will hum , 1 . They are single coil pickups . 2 . the pickups are wired to the wiper of the pot , so when you turn down the volume , the system is still at a hi impedance and susceptible to noises .
The cure may be to wire the pickups to the hot side of the pots and take the output from the wiper (centre) , then if you turn down the noise drops too.But in mid switch pos'n volume effects both pickups ( as per gibson ). Also try reversing one pickup (if you are able and capable, it may not be easy)as they may be wound as to not hum cancel in mid pos'n . If this works you will then have to flip the magnet(s) in one p/up to avoid sounding out-of-phase .And as Tom says you need the strings grounded.If this is unclear to you check gibson L.P. wiring diag, Len