Gibson Chet Atkins Tennessean pickup and wiring saga

[dehughes]

I know...this is the AMP garage...but you guys are the most knowledgeable group I've encountered on the web....

I replaced the pots (one MV, one master tone, and two individual pickup volume, Gretsch-ish setup) on this guitar with 500k all round...and it was quiet beforehand, but now it's hummy. Hummy like "my instrument cable is plugged into the amp but not the guitar" hummy. It quiets down when I touch the metal parts on the guitar (or the chord...), but not totally quiet. Turning down the volume on the guitar doesn't really help either....still sounds like the cable is plugged into the amp but not the guitar. The guitar sounds good, so I know the signal is getting through the pickups to the amp, but something is up.

Ground loop? I wired up the pots EXACTLY as the old ones were (including solder points on the pots, wires touching other wires, etc..). I tested out the pots before installing them to make sure they were the proper value. I even went back and re-flowed the solder points to make sure they were solid. The pots are CTS spit shaft 500k with solder-able backs, so they're good quality.

Any ideas? I just dropped a Lollar low-wind Imperial in the neck and have a TV Jones Classic coming for the bridge, so you can imagine I'm dying to get this thing working. Funny, as I've built amps, but am totally stumped by a simple guitar wiring harness...

[selloutrr]

Recheck the grounding, cold solder?
make sure the bridge wire is soldered to the ground.

It sounds like right now you are the ground.

[dehughes]

Recheck the grounding, cold solder?
make sure the bridge wire is soldered to the ground.

It sounds like right now you are the ground.

Thanks for the reply.

Bridge ground tests good. Solder joints look good.

Might an over-heated pot cause such an issue?


david

[Structo]

You didn't happen to get the wires reversed on the output jack did you?

Although if that happens, about all you get is BUUuuuzzzzzzz.

Sorry don't know what else to suggest without the guitar in hand.

[dehughes]

Thanks, Tom. No, the wire is the old cloth-covered, braided shield type, so there is only one conductor and then a braided ground. Hard to reverse those.

Hmm....I'd rather not take this into the 12th Fret and pay the money to fix what it most likely a simple issue....kinda frustrated that I can't figure this one out.

Is it possible that the cloth insulator could get saturated with flux and cause a hum issue? Or perhaps some flux seeped into one of the pots? I'm trying to consider the oblique, as the obvious doesn't seem to be the problem....

[selloutrr]

if you have some caig fader lube you could spray the pots but I suspect they are fine. If it was pot related it would be a scratch or a noise directly related to using the pot. unless you have a pot that does nothing when you turn it.

even though the grounds look good. reflow the solder and see if it helps. try holding down the wire with a screw driver while you reflow the soder.

check continuity through the guitar. plug an instrument cable in to the guitar clip a test lead to the ground/ shield and meter your way through the instrument, once it's confirmed good. go back and do the same thing but testing for the ohm load and see if anything meters above average if so reflow the solder and try to bring it down. Give the metal plenty of time to cool down before you test or resolder.

[Structo]

That is what I would suggest as well but it will be hard to do as it is a hollow body.
When you re-wired it, did you have the pots and caps on a jig so that you could solder it up, then transfer it to the guitar?

Double check your grounds with a ohmmeter looking for ground loops.

I believe the ground is working as it should. When not touching the strings, there will be a bit more hum but it should quiet down when touched.

Does this guitar use the pot ground where you bend a lug back and solder it to the case?

Do you have a ground buss wire connecting all pot backs together?

Do the tone control operate as they should?

And lastly, this is why I hate working on hollow body's.
You just can't see what's going on. I suppose you can use a dental mirror but it isn't like a solid body with a control cavity.

[dehughes]

Good points, selloutrr. I'll go do just that.

Funny how the "simple" things can be the hardest!

[dehughes]

That is what I would suggest as well but it will be hard to do as it is a hollow body.
When you re-wired it, did you have the pots and caps on a jig so that you could solder it up, then transfer it to the guitar?

Double check your grounds with a ohmmeter looking for ground loops.

I believe the ground is working as it should. When not touching the strings, there will be a bit more hum but it should quiet down when touched.

Does this guitar use the pot ground where you bend a lug back and solder it to the case?

Do you have a ground buss wire connecting all pot backs together?

Do the tone control operate as they should?

And lastly, this is why I hate working on hollow body's.
You just can't see what's going on. I suppose you can use a dental mirror but it isn't like a solid body with a control cavity.

Thanks Tom.

1) The pots and jack are soldered up on a "jig" of sorts...that is they're all in their "proper positions" when soldered.

2) What would a ground loop "look" like on a meter? I know what they sound like, but not what they'd look like...

3) Pot grounds are indeed the lug bent around and soldered to the pot case.

4) The shields of the wires are the "ground buss", and all pots are connected to each other and to ground in this manner.

5) There is only one tone control (master tone), and it works as it should (quite well, with a Sozo .022uf cap!).

The pots and the jack are all out of the guitar, so I have free access to all of them at this point. I want to make sure they're working properly before I put them back in (again).

Thanks guys!

[Structo]

Well a ground loop is going to be a ground point that has a higher resistance to the main ground. That is why people use star grounds in guitars, which may be something you want to try, as described on GuitarNuts.com

http://www.guitarnuts.com/technical/noisebucket.php

[dehughes]

So far:

1) Tested all the ground points, and all measured the same (about 1.3 ohms) when the jack ground point was used as the reference.

2) Re-flowed all the solder joints throughout (backs of pots, tabs, etc...).

3) Re-wired the grounds so that it now is a star-ground scheme, with all points converging on the tone pot back (where the ground to the jack lands as well).

Can't think of what else to check...pots all work as they should, pickups sound good (buried in the hum), and the bridge ground is solid.

One thing that's odd is that when I turn the volumes down the hum in a way gets louder....it's less "buzz-saw-y" and more deep hum. Don't know what that means....hmm...

The schematic is identical to the one in the attached PDF.

[Structo]

Man, I'll bet that is frustrating.

Sounds like you hit all the points that would cause it.

I suppose it could be a fault pot, cap or switch.

If you can, wire each pickup, one at a time straight to the jack to see if it is the same or different.
Be sure to maintain the same grounding so that is included in the test.

Is the wire to any from the switch shielded and properly grounded at the pots?

[dehughes]

Man, I'll bet that is frustrating.

Sounds like you hit all the points that would cause it.

I suppose it could be a fault pot, cap or switch.

If you can, wire each pickup, one at a time straight to the jack to see if it is the same or different.
Be sure to maintain the same grounding so that is included in the test.

Is the wire to any from the switch shielded and properly grounded at the pots?

Yup and yup. The switch is grounded to the tone pot.

I disconnected the cap and there was no change.

I think I'll just take this in to my local shop. I just wanted to install some new pots and throw in some new pickups, but this is beyond what I wanted to get into. I didn't want another "project" so much as I wanted a simple, noticeable improvement. Funny how that happens, huh?

We'll see...maybe if I get a second wind I'll try more troubleshooting...but for now I'm going to give it a rest and get back to it.

Thanks for the help! Much appreciated...sorry I didn't fix it. I'll post back if I do, or if I end up having a local shop fix it, so we can all learn from this experience.

[selloutrr]

have you tried it with a different instrument cable?

is the toggle switch wired properly? is it grounded?

[dehughes]

have you tried it with a different instrument cable?

is the toggle switch wired properly? is it grounded?

Yup and yup.

Here's the chronology:

1) Get the guitar...love it, but realize it needs the stock pickups replaced and the stock pots swapped with 500k.

2) Replace all the pots with split shaft CTS 500k pots. Hum is presents itself.

3) Try basic trouble-shooting...to no avail. Still humming...

4) Remove pickups (was going to sell them anyway). With pickups removed, guitar still hums.

This is where this thread comes in. So, I KNOW it's not the pickups, and I didn't touch the switch, so the culprit is definitely the pots. Dang. I wonder if I have a bad pot (or pots)?