Odd interaction between volume pots - advice neded

[slidincharlie]

I have just replaced all the pots and the neck pickup in my '66 Starfire V.
Everything works fine, but I note an odd thing:
when both pickups are engaged, rolling off one volume pot a little bit excludes almost completely that pickup. In other words when the neck pickup is on 10 and the bridge pickup is on 9, I hear almost exclusively the neck pickup (and viceversa). If I tap on the 'weak' pickup poles I can still hear it, but its contribution to the blend is almost null.
Thus I can't blend the two pickup appropriately.
Have you guys ever noticed this with your two-hb guitars?
I have an Epiphone 335-style guitar whose pickups I can blend freely.
My Starfire is wired as in the schematic below: the pickup hot lead goes to the volume's center lug, in order to have 'independet' volume controls. No treble-bleed cap for now.
--Carlo

 

[capnjuan]

Hi Charlie:

Does each p/u, by itself, put out a good signal? Is the volume from the neck p/u at 10 about the same as the volume from the bridge p/u at 10?

When rolling off one pot 'a little', does this phenomenon applies to both p/u's? That is, if neck = 10 and then roll bridge slightly, the effect is the same as if bridge = 10 and neck is rolled off?

cj

 

[matsickma]

Souns like the pickups are wired out of phase.

When both pickups are on and at "10" does the guitar sound get louder when one of the pickups is turned down to 9 or 8.5?

It would be best to do the following test. Have both pickups on and at 10. Turn the Bridge pickup down to 8.5 to 9. If when doing so the sound get louder and bassier then the PUPs are wired out of phase. You will need to swictch one around. In addition if you start to pick up hum or noise when your touch metal parts of the guitar then you also wired the "hot" leads to the "ground" location. You will then have to revere the wiring to make it less sensitive to proximity noise pickup.

M

 

[capnjuan]

...........when both pickups are engaged (Blend position), rolling off one volume pot a little bit excludes almost completely that pickup. In other words when the neck pickup is on 10 and the bridge pickup is on 9, I hear almost exclusively the neck pickup (and viceversa).

Hi M; sometimes talkin' about this stuff in this format can be awkward but, if I read Charlie's post correctly, with the selector switch at 'Blend' and p/us 'A' and 'B' at '10', nudging the volume down on p/u 'B' from '10' disproportionately dimishes 'B's output, not increases it. That is, assuming he's wired it like the drawing, they can't be out of phase because 'B' is losing volume as it's being turned down, not increasing.

The problem seems to be losing too much volume in proportion to the amount of shaft rotation. We beat up the subject of linear v. audio taper pots over in Dreamlander's speaker thread; any chance one (or more?) of the pots are the wrong taper? cj

 

[matsickma]

Hi capn,

Yes it is tricky discussing audio technical subjects with a key board.

My experiance when the PUPs are out of phase is that when both PUPs are of equal volume the output is weak. As soon as one of the PUPs are turned down a little bit (10 to 9 or 8.5) the other Pup changes tone, gets louder and dominates the sound.

If they are in phase and one is adjusted a little lower (10 to 9 or 8.5) the blended tone is only slightly altered.

I agree it could also be a bad Pot with a blind spot.

M

 

[capnjuan]

And, no offense to Charlie, but we're starting from the assumption that it's all wired the way his pic indicates. We should get on a plane and fly to Italy ... at his expense of course, and check into it ... only kidding ... mostly. No more ideas tonight, maybe something will occur to me tomorrow. cj

 

[slidincharlie]

Hi Charlie:
Does each p/u, by itself, put out a good signal? Is the volume from the neck p/u at 10 about the same as the volume from the bridge p/u at 10?
When rolling off one pot 'a little', does this phenomenon applies to both p/u's? That is, if neck = 10 and then roll bridge slightly, the effect is the same as if bridge = 10 and neck is rolled off?
cj

Thank you guys for advicing.
cj, yes to all your questions, you got it right.
Each pup and pot work perfectly well alone. When they are both engaged, as soon as I roll either volume pot its volume tends to disappear. This applies to both pup's. With one pot rolled off at 8-9 the relevant pup shuts almost totally off yet the overall output volume does not change. The unaffected pup does not increase, as suspected by matsickma. I don't know if my explanation is clear... :?

 

[capnjuan]

No Charlie; ya did good; will have to think about it some more but suspect either wiring boo-boo or maybe the wrong taper; linear v. audio taper pot installed so that small increments of shaft rotation produce disproportionate amounts of rolloff / reduction.

More when brain gets a little better engaged (less rolled off :wink: ) cj

 

[slidincharlie]

I'll double check the wiring again.
The pots are all audio taper (unless one is defective...). I'll test them, though.
Thanks,
Carlo

 

[capnjuan]

Hi Carlo; here's a general discussion of pot functions and other stuff: Information on Pots Assuming the drawing is right and your wiring is right, then it must be one of the p/us or pots.

This drawing is to define terms for location of resistance testing:

Pickups
First thing, I think you have to test the resistance of the pickups first; 'cold' - amp off - and then 'hot' - amp on. When testing with the amp on, turn the volume down to its lowest point. If you don't have a 'dummy load' to replace your speaker, touching the leads will cause some click and pop from your amp; try not to flinch when you hear it; no danger to any of your equipement. Test both pickups - hot and cold - and see if the resistances are within range of each other; you said one p/u was replaced; I guess this test applies more to the one you didn't replace.

Vol / Tone Pots
Don't need amp on but you need to lightly apply some masking tape near the vol and tone pots; because the pots only turn about 325 degs, you need to mark on the tape where full off, 50% on, and full on are to get some idea of how your pots behave. This chart represents, along the X axis the amount of shaft rotation from full volume/tone off to volume full on and, on the Y axis, the proportion of resistance of total resistance remaining between the wiper and ground.

Measuring Points assuming 500K volume and 250K tone controls; expected readings at full off/50%/full on

Full off...............50% rotation.............full on
Volume
NVR 500K ................450K....................50K
BVR 500K ................450K....................50K

Tone
NTR 250K ................225K....................25K
BTR 250K ................225K....................25K

If the complaint is that, when blend switch in middle, both pots when nudged from full on towards off, 'roll off' faster than expected, then one or the other or both pots, use volume as example, reaches a measured 450K or so at a point much sooner than 50% rotation. If the pots are all audio taper, and even if I have the 500K/250K values wrong, you can substitute in the table for what values you have and still continue. If the guitar were in front of me, this is how I'd go about testing the pots; there might be a better way - I just don't know what it might be. If you wish, I (and matsickma) can fly over from the States and help you ... at your expense of course.... :lol: good luck, I hope this helps! cj

 

[slidincharlie]

cj, this IS what I call a thorough explanation!
Thanks a lot for sharing your experience.
I'll post later as soon as I do the testing.
--Carlo

 

[capnjuan]

You're welcome Carlo; I hope it works! cj

 

[slidincharlie]

cj,
1) I had measured each pot before assembling the harness. Each measured as expected, and - keeping an eye to the approximate % rotation - they acted like log pots.
2) I have re-checked the wiring and it is exactly like the drawing above (looking at the pots from the back). There is continuity between all the metal parts of the circuit, including the tailpiece and jack ground.

[edited to correct for wrong ohm readings]
Now I have just followed your resistance reading instructions: I did all readings with the complete harness pulled out of the body.

- Pickups:
I took readings with the guitar plugged into an amp, reading directly from the lugs on the back of the pickup. With the amp on or off I had the same readings: 6.8k NPU, 6.3k BPU.

- Pots:
each tone pot reads about 300k on 10, about 30k on 5 and a few ohms on 1.
Each volume pot, disconnected from its pickup, reads about 500k on 10, about 50k on 5 and a few ohms on 1.

Everything seems ok, right? :|

I repeat that the guitar plays well, with a strong sound, each pup separately works perfectly and the pots work fine when only one pickup is engaged. The only problem is that, with the switch in the middle position, either pup disappears almost completely from the blend when I roll it off around 8-9.

--Carlo

 

[capnjuan]

cj, ... the approximate % rotation - they acted like log pots. Good.
2) I have re-checked the wiring and it is exactly like the drawing above (looking at the pots from the back). Good.
...there's something very strange going on here... I think so too.
Pickups: readings 6.8k NPU, 6.3k BPU. Good.
Pots: each tone pot reads about 300k on 10, about 30k on 5 and a few ohms on 1. Tone pot should read not more than 200K at full on per drawing.
Each volume pot reads about 6.8k on 10, stays high (about 5k) until rolled off to 3-4, then decreases to a few ohms. Not enough resistance; should read nearly 500K at full off --Carlo

Hi Carlo: the resistance on the p/us looks good. It's the pots. But first things first: I have messed up the chart above and reproduced it below but it isn't going to change anything:

Measuring Points for 500K volume and 200K tone controls; expected readings at full on/50%/full off

Full on ...............50% rotation................full off
Volume
NVR 500K ................ 50K ......................... 0
BVR 500K ................ 50K ......................... 0
Tone
NTR 200K ................ 20K ......................... 0
BTR 200K ................ 20K ......................... 0

The principle is that at off, there is nearly no resistance between the wiper (audio out) and ground. At full on, there is lots of resistance forcing the signal to the amp because the ground leg offers more resistance than the path to the amp. The pot doesn't really control volume, it controls how much signal is 'wasted' to ground.

According to the SF wiring diagram above and on the volume pots, you should have had nearly 500K of resistance in the on position; +/- 50K at the 50% on position....nearly zero K at full off. Same on the Tone pots; diagram calls for 200K which would be full on; 20K at 50% rotation, and nearly zero K resistance at off.

Agree, the pots behave like audio taper - not linear taper - pots. But if you were trying to match the SF diagram, the pots, though new, should have the resistances as indicated....????

Your Volume pots only have highest resistance rating of 6.8K, not approaching 500K. According to your readings, the highest resistance on the Tone pot of 300K exceeds the highest rating on the diagram for the Tone pot of 200K.

I suggest you switch the Volume for the Tone pots and see what happens. I also don't think you have enough resistance the Volume pots to have them work well as Tone pots. Drawing is looking for 200K at full on on the Tone pots; if you switch -as indicated - the Tone for the Volume pots, you will get closer to spec for the Volume but not have enough resistance for for the Tones.

I think you may have1) the Volume pot in the Tone position and (2) possibly the wrong pot for the guitar in the Tone position; insufficient resistance; drawing wants 200K at full off for Tone; you're not even close.

Suggest switching the Volumes for Tones; see what happens.

I apologize for the clumsy first pass; I too switch things back and forth but the Volume pots - reading either way - offer too little resistance to ground so the signal goes to ground. I don't think you have the right values of pots - hard to tell with me messing the chart up but it's your readings that don't match the SF diagram. cj

 

[slidincharlie]

cj,
you're spending so much time for my problem... please bear in mind that I won't get offneded if you stop replying! You sure have many other things to do in your spare time...

Have you read the edited version of my post? I have posted there the volume pots readings with the pup disconnected from the pot. The 6.8k value shown in the first version was taken with the pup connected to the pot (thus I was reading the pup resistance actually)...

I don't understand where is the problem with my pots:
I have max resistance to ground in the full on position (knob on 10) and a few ohms resistance to ground in the off position (knob on 1). Isn't this the expected behaviour of a volume pot? (BTW I have never met a pot whose resistance to ground was zero, and I've measured quite a few: they generally have 5-10 ohms to ground in the full off position).
Same for the tone pots (I have used 300k instead of less common 200k pots in this guitar).

Furthermore:
if there is a problem with the pots, how comes that they work fine when each pup is engaged alone? :?

Have a nice weekend!

 

[capnjuan]

Hi Carlo; it's been said here and on other forums that troubleshooting electronics over the web can be tough going. I respect the fact that you took it upon yourself to do the work and that counts for alot. It probably shows by now but I'm not doing so good with guitar pots. If someone really knowledgeable looked at it, they'd say 'oh, it's just this and that'; but it doesn't work that way for me.

Same thing occured to me; if the ground leg is connected elsewhere, we're just measuring all around the rigging at not just the pot. No; didn't mean exactly 0 ohms but lots closer than at 50% or full rotation.

The idea is that, at full off, not much resistance between the wiper/center lug and ground; all the signal is grounded. At 50% rotation, you should read only about 10% of the total rated resistance and at full on, you should read most of the rated resistance.

"I have max resistance to ground in the full on position (knob on 10) and a few ohms resistance to ground in the off position (knob on 1). Isn't this the expected behaviour of a volume pot? Yes; should behave this way - at least it's how I understand it. Yes, should have some resistance at full off.

"If there is a problem with the pots, how comes that they work fine when each pup is engaged alone? " I don't have an answer.

Instead of making things more confusing for you; you might try PMing DKL, matsickma, and maybe Walter Broes ...remember to include a link to this thread ... these guys really know something; like I said, one of them might look at it and say ' oh, just do this...'. I hate to 'punt' on you but if I can't help you, maybe it's time for the A team. I wish I could have been more helpful. cj

 

[dklsplace]

Ahhhh, yes. This same thing irritated & aggravated me for some time when I did my '64 restoration. If you recall Carlo, you also posted in that particular thread;
http://letstalkguild.com/phpBB2/viewtop ... =11&t=2016

Based on my experience & some off-forum conversation with others on this matter, I believe yours is behaving exactly as intended. Ultimately, I changed my harness & wired it like a standard X-150 or M-75 so all of my electrics behave consistently. I like it much better this way!

 

[slidincharlie]

cj,
you HAVE been helpful for sure, especially in the troubleshooting approach and with those details about pot functioning .
Now I have ended with a solution that works fine for me:
I have inverted the wires on the two non-grounded lugs of each volume pot. This makes the two pots 'non-independent', i.e. when I roll off totally either pot, the total output shuts off. This is what I did NOT want originally, BUT it has a very positive result: I can blend effectively the pots until one of them is on 10 and the other between 10 and 7 without any output loss. This is enough for me - I can blend the pups to some useful degree and adjust the output with the master volume. Now I'll have to get accusotmed to this setup...!
Furthermore, an annoying ground buzz that arose when I rolled off a volume pot (not mentioned earlier in this thread) went away.

dklsplace:
I didn't recall that thread! I'm glad you found a good fix, but I don't understand if your solution and mine are the same.

Have a good Sunday, guys!

--Carlo

 

[capnjuan]

Thank you Don and good luck Carlo! cj