Weird volume pot behaviour on Epiphone Les Paul

[Ice-Tea]

Hello all,

Long time amateur guitartis and long time not so amateur electronics engineer here but to be honnest I'm a bit lost with a Les Paul a friend gave to me to check out. It does some weird things with the volume pots. When measuring resistance between the signal from the pickup and the connection to the switch on the pots, it would go from zero to, say, 150k and then hover there or even go down again (rather than to make it to 500k as I was expecting.

I first thought it would simply be a bad pot but it does so on both. I've learned to be suspicious of such things. If one thing seemingly broke then that's a good explanation. If two things seemingly brake you're probably at fault.

So, did I miss something obvious?

[Alex M.]

The pickups are soldered to the volume potentiometers. By turning the potentiometers to maximum, the resistance is shunted by the pickup coil. A multimeter may read 4-20k ohms depending on the pickup. At 50% volume — DMM shows 2 halves of the pot folded in parallel, with a small addition of resistance from the pickup.

Check at the wiring diagrams for the Les Paul.

I have big questions about the level of your engineering education.    

Edited November 5, 2023 by Alex M.

[curtisa]

15 hours ago, Ice-Tea said:

So, did I miss something obvious?

Other than the resistance looking back into the output jack not being the value of the pot (which isn't, strictly speaking, an issue in itself) is there something else that's faulty on the guitar?  

[Prostheta]

5 hours ago, Alex M. said:

I have big questions about the level of your engineering education.    

This adds nothing to the discussion. 

[Ice-Tea]

Thanks for the (mostly) kind pointers

Yeah, I was missing the pickup from my mental image. In addition, I must have had the selector switch in the mid position for some of these measurements. When disconnecting the switch from the rest I get readings between zero (makes sense) to 140k or so (makes sense) to 13.8k (ie the pickup dc resistance. So far so good.

The reason I'm poking around in the first place is because the selector switch doesn't function the way I thought or think it should. Ie: when the selector switch is on rythem the neck volume control knob controls the volume (bridge does nothing). When the selector switch is on treble the bridge volume control know works (neck does nothing).

But in the middle position I was expecting both to work but only the bridge pickup and its volume control work. So I obviously figured bad switch and replaced it but it's still the same.

[Prostheta]

Post photos of the cavity and the switch wiring. It may be obvious from a visual inspection.

[henrim]

To me it sounds like a classic Les Paul wiring. As it has always been. It is a bit strange in the middle position. You either get used to it as is, or rewire to a configuration that makes more sense.

The original wiring is not all bad. But you have to play around a bit with to find the tone you like in the middle position.

[Prostheta]

This is true. My own wiring is always as weird as it needs to be. I only ever wire my tone pot to my neck pickup for example. That and me using active circuits wherever possible tend to exclude most of the weird interactions that are judged as being "the character of..." or a plain old weirdness.

Factor into this the "vintage" and the "modern" variants in how the tone pots are brought into the circuit.

[henrim]

Oh, if the neck pickup is not working at all in the middle position then there is something wrong obviously. 

[curtisa]

If you raise both volumes to 10 and select the middle position do you get output from both pickups? If you touch something metallic, like a screwdriver, to the pickup pole pieces you should hear a pop from each pickup.

[Ice-Tea]

Short youtube vid capturing behaviour

Note1: no, I haven't checked if the switch is upside down, so that's a 50% chance for you

Note2: yes, my friend is a leftie. No, I'm not the friend in question.

[curtisa]

That inline connector in the control cavity - is that standard in an Epi Les Paul?

Hard to tell in the photos, but I'd be checking that the wires on both sides of the connector go to where they're meant to be going as they pass from one side to the other:

• Red wire on switch -> bridge volume pot middle lug
• Green wire on switch -> neck volume pot middle lug
• White wire on switch -> output jack tip lug
• Braid/shield on switch -> ground

[Ice-Tea]

All as you describe. Also:

Measured the (new) switch. Center lug is connected to both outer lugs in the center position. So: switch is good.

Volume pots work respectively on rythem and treble settings of the switch.

Both pickups work on their respective switch positions.

Doesn't leave a lot of unknowns, does it? I'll take her to the lab tomorrow and hook her up to an oscilloscope and see what happens.

[curtisa]

52 minutes ago, Ice-Tea said:

Doesn't leave a lot of unknowns, does it?

Very odd. Do you get output in the middle position from both pickups when the switch is removed from the cavity? Is there something fouling the switch mechanism when it's stuffed back into the guitar perhaps?

You're right - there's not an awful lot that can go wrong with those kinds of switches, but their open-frame construction does make them a little vulnerable to damage. I've personally not had much luck with the import-style three way toggle as the contact metals appear to be of lower quality compared to the Switchcraft equivalent, and go intermittent over time. Usually if I found one that was cutting out I could temporarily get it going again by quickly flicking it back and forth a few times. But it's weird that you've had two switches fail the same way, one new from the packet, yet it test correctly.

 

52 minutes ago, Ice-Tea said:

I'll take her to the lab tomorrow and hook her up to an oscilloscope and see what happens

Will an o'scope tell you anything useful that plugging in to an amp doesn't? The neck pickup is either going short-circuit to ground or open-circuit in the middle position, leaving you with no output from the neck pickup. All that will do is visually show you what your ears already know.