2 Vol / 1 Tone; Tone acting as volume control?

[Lester]

Sanity check? Additional electronic knowledge?

TL;DR: Below is my wiring diagram. Tone control is acting like a volume control but only on the bridge pickup.

The guitar is an Epi SL with two single coils. The pickups are not strong, resistance measures 5.6K and 5.5K Neck and Bridge. (I'd guess they have half the output of my mini-humbuckers when played through an amp). The bridge is weaker than the neck despite it being set very close to the strings and the neck being almost flush with the pickguard. I'd guess the bridge is about 1/2 to 2/3 the volume of the neck.

The tone control is cutting volume significantly on the bridge pickup. With the neck pickup, it only seems to change tone as expected. One the bridge, 10 gives me bright, but 0 gives me almost no volume.

It's not the cap (tested by replacement, and working fine on the neck pickup). It's not the basic wiring... the wiring was actually much more interesting with phase and serial/parallel switches. I rewired ground-up to this simpler layout to test and it didn't improve things at all.

Is there some interrelation between the weak pickups and 250K pots that might be happening? Those interactions are well beyond my electronic knowledge. Or maybe the bridge pickup has issues?

 

[CB91710]

I don't see any issues with the wiring, and the fact that it's not causing a problem on the neck pickup points to it not being a problem in the tone circuit itself.

Try to isolate the problem by first reversing the connections between the pickups and pots (temporarily wire the neck to the bridge pot), and if the problem remains, then reverse the connection from the pots to the switch.
Verify the value of the pots while the pickups are disconnected.
Verify the operation of the switch with your ohmmeter.... should be 5.6/5.5 on the bridge/neck positions, and around 2.75-3k in the middle.

 

[Lester]

Thanks, will give those a shot. The last one I did already and all seemed normal.

 

[freefrog]

A tone control acting like a volume with one single pickup normally presupposes some series capacitance before the tone pot (between pickup and tone pot). This series capacitance generally comes from a break in the pickup coil(s)... the wire is cut but stays aligned and behaves like a series capacitor.

Such pickups are normally easy to spot: they have a tightened bass range and don't allow a normal DCR reading but exhibit a capacitance @ low frequency (around 4nF, usually).

So, I'd start by checking if the DCR of your bridge pickup is still readable...

NOTES

-I've repaired this problem several times (and twice for a same player in two different guitars) so it's not that rare;
-the guitar stays playable as long as the tone control is not used. Roy Buchanan had a capacitive bridge pickup in his Tele nicknamed "Nancy" and it contributed to his unique tone. Some say that the first Frankenstrat used by EVH had also a broken / capacitive coil (but as he used no tone control, it wasn't a problem);
-I know that I've already explained this in a PM to you. Sorry to ramble: I do that just to share what I consider as useful knowledge... I hope you'll find a solution, anyway.

 

[Lester]

freefrog: Yeah, this is a new issue vs. the ice-pick issue that we discussed at length. Perhaps being in there so many times working other issues resulted in a coil problem. However, it does still have dc resistance that appears the same as before. It's just operating oddly now. Or maybe it was before and I just didn't notice with the overall ice-pick issue hitting me in the face.

 

[freefrog]

freefrog: Yeah, this is a new issue vs. the ice-pick issue that we discussed at length. Perhaps being in there so many times working other issues resulted in a coil problem. However, it does still have dc resistance that appears the same as before. It's just operating oddly now. Or maybe it was before and I just didn't notice with the overall ice-pick issue hitting me in the face.

I don't know if there's a relationship between your ice-pick problem and your tone pot behaving like a volume control but... knowing that it affects only the bridge PU and having seen how it's wired, I don't see how to explain the problem without supposing the presence of some series capacitance. Maybe there's a faulty/partial contact behaving like a series capacitor somewhere in the signal path between pickup and tone pot... maybe it's in the selector... If I were you, I think that I'd put some cable with alligator clips between the bridge PU and the output of the 3 positions switch, just to check if the issue remains when the signal path is physically duplicated (switch on bridge position + parallel cable from the selected PU to the output of the switch).

Anyway, here is some food for thought about the problem as I understand it: https://www.mylespaul.com/threads/on-“anti-mud”-caps-in-series-with-pickups-a-side-note.446786/#post-9713427

 

[dpgumby]

Looking at the circuit, I think it could be a bad ground connection on the bridge volume pot, leaving the bottom floating and so it's only acting as a series resistance. Tone control could act like a sort of volume control in this circumstance. If the wiring looks ok, try waggling the grounded lug on the pot, as I've had instances where the lug connection to the trace becomes intermittent (probably due to overheating when soldering).

(Edit: the above may depend how you have actually wired the grounds).

Just my thoughts, worth checking I think.

 

[diavolo]

Hello,
it seems like youre bleeding all your signal to ground.
Is the tone cap shorting out? Perhaps the leg on the pot is touching something allowing it to ground.
I dont think its a problem with a component or schematic since you checked all that.
Likely just a misc wire or solder touching where it shouldnt. A picture would help.

Check all your grounds. One of them is not making a good connection.

 

[Lester]

Def not the tone cap shorting. Also, works fine on the neck and if it was shorting it would be klling the neck pickup too. But, I'll recheck all the grounds. They were redone on the pots when I redid the wiring, but they do run in a star... maybe the hub for the star has issues.

I'll do some resoldering tomorrow when the sun rises. I need daylight to work where I am.

 

[freefrog]

Def not the tone cap shorting. Also, works fine on the neck and if it was shorting it would be klling the neck pickup too. But, I'll recheck all the grounds. They were redone on the pots when I redid the wiring, but they do run in a star... maybe the hub for the star has issues.

I'll do some resoldering tomorrow when the sun rises. I need daylight to work where I am.

Let us know what it gives...

BTW, is the volume pot of your bridge PU working normally? I was presupposing it to be the case, hence my previous answers.

EDIT:
-A fully functional volume control would put its resistance to ground in parallel with the pickup. It might hide the problem of a broken capacitive coil by allowing your DMM to read a resistance IF you measure DCR with the pickup mounted in the axe and paired to the harness...
-No abusively expansive pride or faith in my previous posts. I was just trying to share past experience and remain open to other explanations. Reason why I'll read your testimonial with interest.


Anyway and as I've forgotten to react to post 2 : + 1 about what CB91710 recommended. It would be a good way to know if the problem is due to the bridge pickup or not.

Good luck again.

 

[diavolo]

i reread everything again. im fairly confident its a ground.
if youre having problems with the bridge pickup only, and it sounds weak when the selector is on bridge pickup position (meaning its not a phase problem in the middle) then check grounding for the bridge pickup.

or is there a way you couldve accidentally demagnetized the pickup?

 

[judson]

this might be a long shot but had a similar issue and i swear i had everything correct, new pots and wires but i grabbed a used capacitor in my parts bin...

did a google search and read somewhere it could be a bad cap....on the tone pot

so i had nothin to lose, swapped out a new cap and boom....perfect...trashed the old cap so i never do that again....

its takes you 1 min to find out..

good luck !

EDIT: JUST READ ABOVE YOU TRIED CHANGING CAP.........oh well

 

[Lester]

Welp, I fixed it.

I tried every test, jumpered this to that, tested resistance across wires and connections, etc. (A bit of a challenge when there can be multiple installed current paths). I couldn't find anything specific. I reflowed most connections but still found no joy.

So, I started clipping and rebuilding. Redid all of the wiring. Added back in my phase switch and series-parallel switches that had been removed earlier when this problem first appeared. It's working.

I think the issue was a poor connection somewhere limiting overall output. The bridge pickup, which isn't that strong anyway, is very treble oriented. Tuning down the tone and cutting the treble from that pickup, with the lower output, was effectively shutting it off.


Thanks for the help.

 

[diavolo]

ironically I had the same thing happen today on a guitar I was modifying.
the leg from the cap was going through the lug on the pot and touching the side of the housing. i saw it right away and fixed it.

Is your bridge pickup still treble-y? is the output weak?

 

[freefrog]

Welp, I fixed it.

I tried every test, jumpered this to that, tested resistance across wires and connections, etc. (A bit of a challenge when there can be multiple installed current paths). I couldn't find anything specific. I reflowed most connections but still found no joy.

So, I started clipping and rebuilding. Redid all of the wiring. Added back in my phase switch and series-parallel switches that had been removed earlier when this problem first appeared. It's working.

I think the issue was a poor connection somewhere limiting overall output. The bridge pickup, which isn't that strong anyway, is very treble oriented. Tuning down the tone and cutting the treble from that pickup, with the lower output, was effectively shutting it off.


Thanks for the help.

Glad it worked!

Hope I didn't / don't appear to insisit too much because my goal was to help and not to argue.

Now, I'm still on the idea of a faulty contact behaving like a capacitor rather than like a limiting resistor. Electronically, the symptoms described couldn't be attributed to a purely resistive (short) cut (or lack of).

Maybe I'll share some 5spice sims to illustrate my thoughts, if time permits.

 

[Lester]

ironically I had the same thing happen today on a guitar I was modifying.
the leg from the cap was going through the lug on the pot and touching the side of the housing. i saw it right away and fixed it.

Is your bridge pickup still treble-y? is the output weak?

I wish I could have found something that. I even used a probe to run between contacts and look for minute solder threads and similar.

The Bridge is better now but has been weak and trebley I think by design or lack, I believe. But, I think it's now doing all it can do. It's giving me the same output as when I wired it direct to the jack, best I can tell.

Glad it worked!

Hope I didn't / don't appear to insisit too much because my goal was to help and not to argue.

Now, I'm still on the idea of a faulty contact behaving like a capacitor rather than like a limiting resistor. Electronically, the symptoms described couldn't be attributed to a purely resistive (short) cut (or lack of).

Maybe I'll share some 5spice sims to illustrate my thoughts, if time permits.

Yeah, I think you might be right. Most of the components and paths I was trying to measure were in circuit so I couldn't be sure that a test was necessarily valid. But I think high cap in some connection would be the answer why I couldn't find it with simple DVM measurement. Maybe someone more talented could.