Help!!! Epi LP Studio Wiring Problem?!

[osdave]

Hi -

I just bought a new Epi LP Studio. I put on new strings and set it up to my liking. Then I plugged it in and the only position that works is the bridge position. I know I could just return it, but I really like this particular guitar and I'm pretty good with electronics, so I'm hoping I can fix it myself. I really hope someone here can help me? Here is what it does, and what I have done:

I checked the wiring/operation of the switch, and everything looks fine. I also checked the wiring of all the controls and everything looks okay there. I used an ohm meter on the outer lugs of each of the volume pots to measure the pickups (it appears this would be the correct way to do this, as they are the - and + of the pickup wiring). The bridge reads 13.08k. But the neck read .10k. However, when I removed the + lead of the pickup from the pot and measured it, it read about 8.3k. When I hooked it back up, it again read .10k.

Here is the interesting part. I was messing with the switch and controls while playing, and I found that: with the switch in the middle position and both volumes all the way up, I got no sound, but when I rotated the neck volume between 1 and 10, I was getting sound between about 2 through 9 - but only from the bridge pickup. When the neck volume was all the way up, or all the way down, I got no sound. And, if I turned the bridge volume all the way down, I got no sound at all.

Could it be the neck volume pot? Or the pickup?

Any info/help/ideas are GREATLY appreciated, as I hate to send this guitar back...

Thanks in advance!

[hammeroff]

Howdy man! Welcome to the greatest site ever!

I guess my main suspect would be the pot that the + off your neck pup goes. It may be a dodgy pot, or it could just be a solder joint that is shorting to the ground somehow.

I highly doubt there's anything wrong with your neck pup.

I'd inspect the neck volume pot more, and then if it looks good on the outside, go buy a new one.

[sumgai]

osdave,

Hi, and to the forums!

hammeroff is correct, but he's also too conservative - your pot isn't just dodgy, it's downright out to lunch.

Unless you have the time, or the lack of budget, don't bother tearing into it, just go get another one. Be sure it's the same value as the current unit, or you may get an equally big surprise when you're done!

The clue is, your pickup measures the expected resistance when the pot is not connected. Following up, the pot changes unexpectedly when you rotate it - up, down, it jumps around. Not what the engineer designed it to do.

Some OJT with the soldering pencil is in order, after a trip to your local electro-outlet (or the web). ;D

HTH



sumgai

[osdave]

Thanks for the info (and the welcome)! I appreciate it! And I hope to spend more time here in the future!

Thanks again!

[UnklMickey]

hammeroff said:

...it could just be a solder joint that is shorting to the ground somehow....

just what i was thinkin'!

+1 for you.

[sumgai]

unk,

I'm curious as to how a solder short to ground can kill the signal at minimum or maximum pot rotation, yet when between 2 and 9, osdave still gets sound?


sumgai

[UnklMickey]

well, i prolly oughta let Hammeroff explain. he seems to be honing his skills, and explanations are a skill of their own.

but it's gettin' close to the weekend, and i don't know if he unplugs all weekend, like i do, so rather than wait, here goes:


this is one of those 2 volume forward wiring schemes.

since the wipers of the controls both go to the output either volume being set at zero will shunt the signal from the other to ground.

you already knew that, but i thought for anyone else who's following this, i'd explain.


so, if the top end of the neck volume is also shorted to ground, when both pickups are selected, the signal from the bridge gets shunted to ground, when the neck volume control is a minimum or maximum.

you probably figured that out too.

but, you need to look closely at what happens between those 2 extremes.

let's just take the best-case-scenario first.

when the neck volume is at the middle of it's resistance value, we have 2 paths for the signal from the bridge to get shunted through the neck pot. one is though half the pot's resistance and to the low lug. the other is through half the pot's resistance to the high lug (the one with the solder blob)

so for a 500k pot, we have 2 - 250k resistors in parallel, = 125k.

125k will dull the tone, but it won't kill the sound.

as we move away from the equal resistance position, the total resistance decreases, until near either end it approaches zero.

whew, i'm outta breath.


L8r,

unk

[ChrisK]

Well, if..

I just bought a new Epi LP Studio

...and it didn't work, I might just return it and get a good one (warranty is)?

[osdave]

Thanks again! When I went to my local music store and explained the problem, he informed me that he is an Authorized Epiphone Repair Shop and would fix it for me free. So, as much as I like doing things myself, you can't beat that!