NUCCIO Posted July 18, 2015 Share Posted July 18, 2015 My '94 LP Custom has developed a loud hum especially on the bridge p/up and when playing on a high gain amp. The noise does not go away when I touch the strings. I tried w/different amps and cables and the noise is still there. I tested continuity with a multimeter (1 probe on the strings and the other at the end of an inserted jack) and it was ok. Should I test continuity between the wires in the control cavity? What is the correct procedure for that (I don't want to short anything)? Thanks in advance for your time. Link to comment Share on other sites More sharing options...
JpGeo Posted July 18, 2015 Share Posted July 18, 2015 My '94 LP Custom has developed a loud hum especially on the bridge p/up and when playing on a high gain amp. The noise does not go away when I touch the strings. I tried w/different amps and cables and the noise is still there. I tested continuity with a multimeter (1 probe on the strings and the other at the end of an inserted jack) and it was ok. Should I test continuity between the wires in the control cavity? What is the correct procedure for that (I don't want to short anything)? Thanks in advance for your time. The test you did with the multimeter confirmed that the bridge is grounded, but it doesn't mean everything else in the cavity still has continuity to ground. You can't damage anything in the guitar measuring continuity with a multimeter. It's possible that here is a broken or shaky ground connection in the control cavity. Your bridge pickup may have shorted windings, that is, your humbucker is no longer bucking hum. You can measure the Kohms of the bridge pickup and compare the reading against what a good one should be. If the reading is much lower, then the pickup is at fault. Jp Link to comment Share on other sites More sharing options...
NUCCIO Posted July 19, 2015 Author Share Posted July 19, 2015 The test you did with the multimeter confirmed that the bridge is grounded, but it doesn't mean everything else in the cavity still has continuity to ground. You can't damage anything in the guitar measuring continuity with a multimeter. It's possible that here is a broken or shaky ground connection in the control cavity. Your bridge pickup may have shorted windings, that is, your humbucker is no longer bucking hum. You can measure the Kohms of the bridge pickup and compare the reading against what a good one should be. If the reading is much lower, then the pickup is at fault. Jp Thank you JpGeo. Would I need to remove the p/up from the guitar to test it or can I leave it in place? Link to comment Share on other sites More sharing options...
JpGeo Posted July 19, 2015 Share Posted July 19, 2015 Thank you JpGeo. Would I need to remove the p/up from the guitar to test it or can I leave it in place? I'm not a Les Paul expert, but from the schematics available from a google search, the bridge p/u has a volume pot (500K probably) in parallel with it, so that will effect the reading slightly (about -1% assuming a 8Kohm p/u and 500K pot). You will be looking for a much bigger error than that, so no need to disconnect or remove the p/u. Make sure the volume pot is at it's maximum value. and the p/u selector switch is set for the Neck pickup, to isolate the bridge p/u. Connect one side of the multimeter to ground and the other side to the bridge pickup non-ground wire. I found a list of various LP pickup resistances on this forum, reprinted below. Good luck. Jp Posted 25 July 2010 - 06:11 PM Only Creating this thread so I can place it as a link in the Epi Lounge "Do-it-Yourself" Sticky, under the "Guitar Pickup Info" sub-section. The Poster I "borrowed" it from indicates location as Stuttgart, Germany... EDIT: The data contained herein was ORIGINALLY compiled by one of our OWN members - Ricochet! Gibson/Epiphone Pickups DC Resistance Values Gibson Burstbucker Pro (sim: BB1,Alnico5, potted) 7.4 KO Gibson Burstbucker Pro (sim: BB2, Alnico5, potted) 8.0 KO Gibson Burstbucker 1 (Alnico2) 7.5 KO Gibson Burstbucker 2 (Alnico2) 8.0 - 8.4 KO Gibson Burstbucker 3 (Alnico2) 8.2 - 8.8 KO Gibson '57 Classic (Alnico5) 7.44 - 8.5 KO Gibson '57 ClassicPlus (Alnico5) 9.0 - 13.8KO(the 9.0 appears to be a screw-up or faulty PU) Gibson Dirty Fingers (2xCeramic) 16.0 -16.6 KO Gibson Mini-humbucker (Alnico2) 6.0 - 6.4 KO Gibson 490R (Alnico2) 7.2 - 7.83 KO Gibson 490T (Alnico2) 7.9 - 8.53 KO Gibson 498T (Alnico5) 12.32-13.46 KO Gibson 496R (Ceramic) 8.3 - 8.63 KO Gibson 500T (Ceramic) 14 – 15 KO Gibson Angus Young (Alnico5) 13.5 KO Gibson Tommy Iommi (Alnico2+Ceramic) 16.5 KO Gibson Joe Perry(BB2&3?) (Alnico5) 8.4(neck)-8.8 KO(bridge) Gibson Jimmy Page (Alnico5) 7-8 KO(not found yet, but I reckon within these values) Gibson P-90R (Alnico5) 7.7 - 8.5 KO Gibson P-90T (Alnico5) 7.8 - 9.35 KO Gibson P-100R (Alnico5) 6.1 KO Gibson P-100T (Alnico5) 9.4 - 16.0 KO Gibson P-94R (Alnico5) 8.1 KO Gibson P-94T (Alnico5) 9.1 KO Gibson HB-R (Alnico5) 7.06 - 9.13 KO Gibson HB-L (Alnico5) 13.39 - 15.3 KO Link to comment Share on other sites More sharing options...
NUCCIO Posted July 19, 2015 Author Share Posted July 19, 2015 Ok, I tested the bridge and neck p/up and they read ok at about 8.5 however the middle p/up is reading at about 4.5. Is that the way it's supposed to be? Link to comment Share on other sites More sharing options...
JpGeo Posted July 20, 2015 Share Posted July 20, 2015 That sounds about right. The middle pickup is wired in parallel with the neck p/u (I think) when the selector switch is in the middle position. So 8.5kohms in parallel with 8.5Kohms is ~ 4K. Is the bridge p/u output level ok, just with hum, or is there little output and big hum? If you are comfortable disconnecting a couple of wires, you could swap the bridge and neck p/u wires and see if the hum is still occurring in the bridge position. If the problem moves to the neck position, then the bridge p/u is probably bad. If the problem stays on the bridge side, then there is either a wiring problem in the bridge p/u circuit, maybe, but probably not a bad pot. If you are going to go to the trouble of desoldering you could also swap in a different (known good) bridge p/u. Sorry can't be of more help here. Hard to troubleshoot without actually having the guitar in front of me. Jp Link to comment Share on other sites More sharing options...
NUCCIO Posted July 20, 2015 Author Share Posted July 20, 2015 That sounds about right. The middle pickup is wired in parallel with the neck p/u (I think) when the selector switch is in the middle position. So 8.5kohms in parallel with 8.5Kohms is ~ 4K. Is the bridge p/u output level ok, just with hum, or is there little output and big hum? If you are comfortable disconnecting a couple of wires, you could swap the bridge and neck p/u wires and see if the hum is still occurring in the bridge position. If the problem moves to the neck position, then the bridge p/u is probably bad. If the problem stays on the bridge side, then there is either a wiring problem in the bridge p/u circuit, maybe, but probably not a bad pot. If you are going to go to the trouble of desoldering you could also swap in a different (known good) bridge p/u. Sorry can't be of more help here. Hard to troubleshoot without actually having the guitar in front of me. Jp The output level is ok on all 3 p/ups. I'm definitely not comfortable disconnecting wires or soldering so I guess I'll have to take it to a shop. You have been very helpful and I really appreciate your time Jp. Thanks again. Link to comment Share on other sites More sharing options...
JpGeo Posted July 22, 2015 Share Posted July 22, 2015 No problem. Wish I could've been more help. There's a fair amount of references to the hum your searing if you do a google search. Most seem to be related to a poor or open ground somewhere in the control cavity. Good luck. Jp Link to comment Share on other sites More sharing options...
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