Epiphone LP bridge pick up problem

[David Shore]

I have a Epiphone LP that is not getting any sound from the bridge pick up. Here is a list of things I checked/done:

- Check continuity in the circuit. Everything checked out.

- Tested the resistance of the pick up. Fine.

- Replaced the 3-way switch.

- Tested the pot. It's working fine.

 

Though I'm not getting sound from the pick up, I do hear a slight "hiss" when the volume pot is turned between 3 and 8. No sound at all when on 0 and 10 which would seem like there's a grounding issue but I just can't seemed to locate it. HELP!! 

 

Thanks for your help in advance everyone! First time poster..be easy on me! haha

[Rob Mercure]

David,

 

Everything has to eventually return to ground so hook one lead of your ohmmeter at the ground of the output jack and work your way either from the pick up to back to the jack or from the jack to the pick up.  For example, with the volume turned up all the way you should be able to hook the other ohmmeter lead to the "hot" lead of the pickup and get a reading of slightly less than the pickup's resistance (the volume control in parallel will reduce it a tad).  Then if you don't get a reading test the grounds all the way up to the P/U ground.  If that's OK then hook the lead up to the "hot" side of the output jack and start at the pickup again - this time you should have very little resistance if things are working, but if not then move the prove to the point where the hot lead is soldered, if it reads it's the wire between the pickup and volume or tone or whatever comes next.  Then just continue until you get some continuity and you'll find where the circuit is interrupted. 

Or, like I said, you can start at the jack and trace the continuity until it breaks - either way.   But one possible culprit right off is a shorted tone control capacitor - unsolder one end and hook the ohmmeter across it - if should read open circuit but if not then it's bad (and there's no "magic" to the striped caps now referred to as "bumble-bees" - they're just plain old foil and paper caps and often leaky on older equipment).

Rob

[David Shore]

Thanks for all the info and great ideas. I'll be attacking this thing again later this afternoon and will let you know the outcome. Cheers.

[Paul Verticchio]

David: best of luck with this. Rob gave you superior advice & instructions.

Rob: Thank you for bringing-up the capacitor "quality" issue (e.g.: bumble bee's, vitamin whatever, high end magic dust impregnated..etc).  It's the biggest scam being run in the industry right now.  Tone caps ARE NOT signal caps. Any ceramic disc or poly cap will work great.

 

[Paul Verticchio]

"Tone caps ARE NOT signal caps." add: in this type of application. 

[Pierre-Antoine Roiron]

About caps : agreed!

[Joshua Levin-Epstein]

Have you connected the pick up directly to the amplifier yet?

 

Joshua

[David Shore]

I did indeed. Works just fine.

 

[Joshua Levin-Epstein]

Tested the pot. It's working fine.

 

Though I'm not getting sound from the pick up, I do hear a slight "hiss" when the volume pot is turned between 3 and 8. No sound at all when on 0 and 10 which would seem like there's a grounding issue but I just can't seemed to locate it. HELP!! 

 

Though you say the pot is OK, your follow up remark says it isn't. I'd replace the volume pot (leaving the tone control out for now). Assuming that works, I'd replace everything else (tone pot and jack) with US components, because it's the right thing to do.

 

Joshua

[David Shore]

I tested the resistance of the pot and got a reading of 4.46. It being a 500k pot this reading seemed ok. I did change the pots out for the better CTS pots. Same result.

[Alexander López]

I'd suggest to do the entire circuit OUTBOARDS the guitar. Use new potentiometers if you have them (I always keep a handful around for these scenarios) and run a wire from the pickups to the new circuit. So, if all components are new, and the circuit is right, then it's the pickup.

[Rob Mercure]

If the potentiometer isn't grounded all it will do is add some resistance to the signal line - you will still get signal to the amp.  While I suppose that it's possible that a pot could internally short between ends to the resistive strip internally in over 40 years of electronics work I've never seen it and I've very rarely experienced the strip opening up - usually when a pot goes "bad" the electrical contacts just become too noisy to use.  Is the "hiss" happening as you turn the pot or are you saying you get the hiss between "3 & 8" with pot just setting there  (hint: percentage of rotation is a more helpful description)? If you're getting the "hiss" as you turn the pot that's just internal noise and tells you that the pot's probably functioning (albiet it could use some contact cleaner). 

I pulled up the Gibson Les Paul wiring diagram - I assume your's is wired the same, and the volume pot is wired directly across the pickup so turn up the volume control all the with the output jack plugged into an amp and touch the point where that pickup and the pot are wired together - you should get a hum.  Then touch the wiper (middle terminal) of the pot - again you should get a hum.  If you don't get a hum at either place the let's reverse the proceedure and with the switch in the bridge P/U position you should get a hum at either side of the contacts - the contact not connected to the output jack connects directly to the pickup. At some point the hum will stop and you'll know where the problem is.

But this is a very, very simple circuit.  You mentioned you'd tested the pickup - how did you do that?  Then what did you do next?  The very worst manner of repair is to just change parts until something works - you never really know what was the matter, you won't be able to fix a similar problem and you waste your money on parts (and if you make money on repairs like I did for many years you go broke).

Ah well, one last alternative:  Solder a wire from the pickup "hot" lead to the switch and unsolder the volume control where the P/U attaches and also where the wiper attaches - this takes the volume control totally out of circuit. Then unsolder the end of the cap not connected to the tone control (probably soldered to the "top" of the volume pot). Now that the wire from the P/U and solder it to the switch - if no signal then the problem is in the switch.  If you do get a signal then hook your ohmmeter across the outside terminals of the volume pot and it should read 500 kilo ohms - then move one of the leads to the wiper contact and turn the pot control to see if there are and "dead" spots.  Do the same for the tone control as long as you've got it disconnected.  And then hook your meter across the cap, again just testing while you're in there, and there should be no reading on the ohmmeter - while this circuit may generate as much as 0.5 VAC peak any cap leakage is only sure to get worse so replace it if it shows any DC current flow (if your ohmmeter is a DMM it puts less than 0.5 VDC across the leads - an older VTVM used 1.5V).  (Note: this could also be done by ungrounding the vol and tone if that's easier - I've just not been into a LP for over 10 years as amps and acoustic guitars are my primary interest).

Lastly, do you have a schematic diagram of the circuit and can you read it?  There are many available, just "google" it, if you're not familiar with the symbols (and none of this post is intended to be imply anything about your abilities - I just "don't know what you know") here's a quick, specific, primer:  The pickup is represented by a series of coils of wire- the solid lines drawn besides denote an iron core; resistors are drawn as a ^^^^^^^ sort of symbol and if there's a <- it's a variable resistor which with both sides connected forms a potentiometer; the capacitor is represented by || or |( mostly representing the scribe's preference; Gibson draws their switches more like a picture of a switch and doesn't use standard electronics symbols; and ground is represented by a three parallel horizontal lines with the upper most longer, the next shorter, and the last shortest.  Again, if you're already ahead of this good maybe someone else can use the description. 

Note: These symbols were standardized during WWII and remained fairly constant until recently when computer aided circuit design became common - now resistors are shown as a simple rectangle with leads coming out a dumb idea which gives absolutely no idea what the component does - my 2 cents.

If this still doesn't get you fixed up e-mail me.

Rob

[David Shore]

Rob,

 

Unfortunately, I was unable to get into the shop today to try any of the advice. However, I can at least answer some of  your questions. Again, thanks for taking the time to offer your expertise on this subject. Troubleshooting guitar circuits have not been my strong suit.

 

You asked how I tested the P/U?  I set my meter to the 20 ohms setting and touched the leads to the hot and ground wire of the p/u getting a reading of 14.38 ohms. After that I hooked the p/u directly to the jack and it worked fine. I couldn't agree with you more re: changing out parts until it works. I am guilty of this method too often! 

 

I'll get back to you as soon as I've tried the new methods you've added. Thanks Rob.

 

David