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| Diagnosing a faulty humbucker circuit http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10123&t=50710 |
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| Author: | Tim Mullin [ Mon Jun 04, 2018 6:55 pm ] |
| Post subject: | Diagnosing a faulty humbucker circuit |
I knew there was a reason I build acoustics rather than electrics, but I’m reasonably good with wiring and understand the basics of electric guitar circuits. So why is this humbucker circuit giving me grief? (Repairs seem to be lucrative in general, but not this guitar!) Seymour Duncan neck humbucker cuts out periodically. When it does work the tone control doesn’t — bleeds treble completely regardless of position. The original wiring was done by a student and was pretty bad, but I’ve rewired and now have confidence in the quality of the joints. How to diagnose? I guess the easiest is to stuff another pot in there. Even if it’s not the same spec, if it does anything at all we can conclude that the original pot is faulty? Of course, even if that is the case, the PUP might also be wonky. There was evidence of cable pinching against the edge of the cavity. While the outer insulation is damaged, the innards show no obvious damage, but these are tiny wires. Any way to test the pup itself. Probably not relevant, but the guitar also has push-pull switches on the volume pots for coil splitting. Any advice on trouble shooting this appreciated! Sent from my iPad using Tapatalk |
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| Author: | Conor_Searl [ Mon Jun 04, 2018 7:07 pm ] |
| Post subject: | Re: Diagnosing a faulty humbucker circuit |
Sometimes a wire inside the control cavity can intermittently come up against another part which will short the whole thing out. I'll often leave the control plate on and wiggle each wire, including the caps to see if that causes the short. Otherwise you can likely just remove the tone pot from the circuit, typical les paul wiring has one tone pot attached to each volume pot, removing it will simply take away your tone control, but you should still have signal and so you can easily see if that's your problem. I find if I end up super frustrated and confused the best thing is to take the whole mess apart and start from scratch. It's never as complicated as it seems in those moments, and sometimes that's the best way to get some perspective back. |
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| Author: | Tim Mullin [ Mon Jun 04, 2018 7:18 pm ] |
| Post subject: | Re: Diagnosing a faulty humbucker circuit |
Conor_Searl wrote: Sometimes a wire inside the control cavity can intermittently come up against another part which will short the whole thing out. I'll often leave the control plate on and wiggle each wire, including the caps to see if that causes the short. Otherwise you can likely just remove the tone pot from the circuit, typical les paul wiring has one tone pot attached to each volume pot, removing it will simply take away your tone control, but you should still have signal and so you can easily see if that's your problem. I find if I end up super frustrated and confused the best thing is to take the whole mess apart and start from scratch. It's never as complicated as it seems in those moments, and sometimes that's the best way to get some perspective back. All helpful, Conor, thanks. Indeed, taking the mess apart was my first approach. I’m pretty sure the wiring is ok and there are no shorts when the wires go back into the cavity. Sounds like taking the tone control out of circuit is a first step and see if the neck pickup cut in and out. I’m highly suspicious at this point of the PUP and the damage to its cable. Of course, the damage is right close to PUP housing, so a difficult splice. Sent from my iPad using Tapatalk |
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| Author: | Freeman [ Mon Jun 04, 2018 9:04 pm ] |
| Post subject: | Re: Diagnosing a faulty humbucker circuit |
My background is in electrical and electronic design and I've spent a lot of time trouble shooting things. One of the easiest ways to do this is break the problem in half and see which part seems to be acting up, then divide that until you've finally diagnosed it. You didn't describe the wiring so I'll make a couple of assumptions You say "neck" pickup, I'll assume there is also a bridge. I'm also going to guess that there is a switch between them and in fact, lets assume that each has its own volume and tone controls (Gibson style wiring). First wiggle, pull, push and play with all the wires looking for a loose connection or cold solder joint. Next switch the two pickups - probably where they connect to the volume pots but might be at the switch. Unsolder the hot wire from each pup and switch them (a couple of short wires with alligator clips makes it easy). Does the problem follow the pickup or does it stay with the controls? If it follows the pup, bingo! If it stays with the controls then you've narrowed it down to two pots, the cap and the switch (neck, if it stayed with the neck position it can't be the bridge controls). Disconnect the tone pot and cap - the volume control should still work. Does it? If it does it must be OK. Capacitors (almost) never fail, consider replacing the tone pot. Three way switches in LP style circuits can be problematic. Normally they are wired after the volume and tone shaping stuff - the signal comes from each pickup control circuit to the switch, then to the jack. Try unwiring each signal at the switch and hooking it directly to the jack with your alligator jumper - does it work? Consider replacing the switch. If you really suspect the pickup and know approximately how it was made you can disconnect it and measure the resistance with a good volt-ohm-meter. Typical values might be from 7 to 9000 ohms (7-9 Kohms). If you try to measure it while its connected you can get bogus readings from the other components. While you have your ohm meter hooked up to the pickup wiggle the wires - the solder joint between the fine coil wire and leads is a potential problem. Be systematic, rule out whatever you can and keep dividing the problem. edit to add, changing the value of a potentiometer will still work but it will change where on the dial it starts attenuating the treble and how much it cuts it. another edit, the coil splitting could also be the problem - normally that works by grounding one of the two coils at the mid connection. Leave it in the series position while you do the other tests, then once you've ruled out everything else investigate it. |
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| Author: | B. Howard [ Tue Jun 05, 2018 5:58 am ] |
| Post subject: | Re: Diagnosing a faulty humbucker circuit |
Something else I didn't see mentioned, overheating while re-working wiring kills components, especially cheap ones. If it is a cheap enclosed switch and not a quality open type just replace it now. Same for those tiny metric Alpha pots. |
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| Author: | dzsmith [ Tue Jun 05, 2018 8:42 am ] |
| Post subject: | Re: Diagnosing a faulty humbucker circuit |
Does the cavity have copper shielding? If so, pull the pots away from it. I’ve seen intermittent shorts fron the lugs on the underside of the pots to shielding. Another thing to try is to pull the pots outside of the cavity for close inspection. Best of luck. |
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| Author: | Tim Mullin [ Tue Jun 05, 2018 7:54 pm ] |
| Post subject: | Re: Diagnosing a faulty humbucker circuit |
Thanks all for your input here. I’ve hit my wall for today, what with the “other” job (half-time) and a Takamine classical brought in for a panic repair to its preamp connections, I spent little time today with the troublesome electric. One thing that irks me is how all the bits are mounted on the pick guard - a very awkward thing to get off and on with the strings in the way avoiding pinching wires as everything is tucked into the cavity. I hate this guitar. Sent from my iPad using Tapatalk |
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| Author: | Tim Mullin [ Wed Jun 06, 2018 3:55 pm ] |
| Post subject: | Re: Diagnosing a faulty humbucker circuit |
Progress today. Switching the PUPs demonstrated that they’re OK, but the neck volume pot is not. As someone suggested, overheating Alpha pots could be a problem — give the man a prize! Should have switched the PUPs earlier, but went down the wrong path — as you do late in the day! Have new parts ordered. Would have been simpler to just rip out the original pots and wire and redo with new. I guess we’ll get there in the end. The client will complain he needs to borrow a guitar for this weekend, but I warned him that would be the case if a part proved faulty and new ones required. I’m gonna go now and carve some top braces for a couple of 00s to get my sanity back. Sent from my iPad using Tapatalk |
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