MIMF

Hey folks. I've got a Gibson custom SG with an aftermarket Bigsby that's having a 60-cycle hum which gets worse depending on where you're pointing the guitar. The problem is reproducible on multiple amps in different physical locations, so I don't think it's a power supply problem. The guitar is NOT receiving radio transmissions. The ground to the bridge is solid. I'm considering shielding the cavity but would like some input as to whether this is likely to fix the problem. Thanks in advance.

The problem is in the wiring and pickups and not the bigsby or any of the hardware. What kind of pickups are on this one?

'57 Classic humbuckers.

So if you have humbuckers in there and you're still getting hum like that, something is wrong with the wiring. That should be a pretty quiet guitar. Plug any other humbucking guitar with no issues into your rig, and see how quiet it is- your SG should be just as quiet.

Is everything in phase? (Both coils on both pickups.) Does the noise drop when you touch the strings?
A quick way to check if shielding will help is to wrap the guitar in aluminum foil and ground that to the jack. If it quiets down then internal shielding will help.

Paul,

I usually start to trouble shoot these at the output jack and regret it. Disconnect the pick-ups at the volume pots and get a resistance reading. If you're not getting about 8Kohm, something is wrong in the pick-up. Make sure the cover and frame are tied to the shield. Tie the individual pick up directly to the amp. If you've got hum, you've got an issue with the pick-up(s). If the pick-ups read correctly and are quiet, then you've got a wiring issue (pots not grounded or the like). You see where I'm going.

Is the output jack wired correctly? In my day, SGs had plastic jacks so it wasn't obvious which was the hot or ground lug. Also, you say the bridge ground is good. These guitars were usually grounded at the tailpiece, so there might be an issue there .

Sorry for the delayed reply - got tied up in other projects. David - phase checks out. No change in noise when strings are touched. I'll try the foil trick.
Joshua - I'll try your troubleshooting method after that. I get ground continuity from tailpiece to bridge to tuners, so that seems to be in working order.
I have noticed that the hum is significantly worse when in single-pickup mode, and almost absent in dual-pickup mode. Thoughts?

http://www.guitarelectronics.com/produc ... Tones.html

This is the wiring diagram that most closely corresponds to this guitar's. The only exception is that the lugs on the tone pots are reversed, i.e. the cap is wired to the outer pot lug and the center goes to ground.

I've checked the pickup ratings - 7.8K on both. The aluminum foil trick did not work. I've got the electronics pulled out so I'll shield the cavity anyway.

Paul Burnside wrote:Sorry for the delayed reply - got tied up in other projects. David - phase checks out. No change in noise when strings are touched. I'll try the foil trick.
Joshua - I'll try your troubleshooting method after that. I get ground continuity from tailpiece to bridge to tuners, so that seems to be in working order.
I have noticed that the hum is significantly worse when in single-pickup mode, and almost absent in dual-pickup mode. Thoughts?

Unless these pickups are "offset wound" I see no reason for any of this behavior. Who made them?

Now before we go any further. Are the said bridge, tuners and tailpiece connected to the jack ground?
Touching the strings should make an audible difference (making the guitar quieter when touched).

David King wrote:

Paul Burnside wrote:Sorry for the delayed reply - got tied up in other projects. David - phase checks out. No change in noise when strings are touched. I'll try the foil trick.
Joshua - I'll try your troubleshooting method after that. I get ground continuity from tailpiece to bridge to tuners, so that seems to be in working order.
I have noticed that the hum is significantly worse when in single-pickup mode, and almost absent in dual-pickup mode. Thoughts?

Unless these pickups are "offset wound" I see no reason for any of this behavior. Who made them?

Now before we go any further. Are the said bridge, tuners and tailpiece connected to the jack ground?
Touching the strings should make an audible difference (making the guitar quieter when touched).

As far as I can tell, they are stock (baseplate logos and all - unless someone got crazy and rewound them). I get continuity of ground from all of the above to the jack ground. Touching the strings makes no difference at all. I shielded the control cavity - no difference.

I don't believe that you have told us the resistance of those pickups yet. What do they read? Oh wait- I see it now, 7.8K...
You need to connect them directly to an amp and see what happens. Is the wiring all shielded wire? Can you send a photo of the wiring, and has it been messed with before?

It's odd that you are getting some hum cancellation with both pickups on.
According to Gibson's 2010 catalogue the following specs should be present:

Gibson 57 Classic - Alnico II
6.5 Output 8.15Kohm 4.5khz
Gibson 57 Classic Plus - Alnico II
7.5 Output 8.52Kohm 5.12khz

You may have some shorted windings in one of the coils in each pickup giving you the lower than expected DCR and the noise but I wouldn't expect such a small offset to make that much noise. Note that it's impossible to get an accurate DCR reading with the pickups in the circuit unless you are measuring the PU that's off at the selector switch.

For sure disconnect the pickups and hotwire them to the jack one at a time. Also listen to the harness with the pickups out of the circuit.
The directionality of the noise might indicate that the coils are involved here but I don't have a lot of experience with guitars. A simple ground<-> hot reversal somewhere in the circuit could account for all the problems.