Three bad pots in a row?

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Matthew Seto
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Three bad pots in a row?

Post by Matthew Seto »

Hello all... Not sure if this is just bad luck or if I'm doing something dumb. I wired a guitar and it had a noisy volume pot. I lived with it for a while since it was fine as long as you didn't move it. It just made noise while it was being turned. Something came up and I needed it fixed on short notice, so I went to the local music store and grabbed a no-name pot off the shelf. It had the same problem. I was not too surprised as I did not expect this to be a quality part. So I ordered a new part and replaced it a third time and it still has the same problem! At this point, I think I must have something goofed up in the electronics...

So the obvious first thought is that it is not properly grounded. I'm pretty sure that it is based on 3 things- 1) the tone pots are fine (all grounded to the same location) 2) it sounds fine as long as it is not moved and 3) if I remove the ground wire, then it is obviously not grounded, i.e. just touching it makes a lot of noise.

Any ideas? I'd like to believe it is a bad pot, but with three in a row, this does not look good for my wiring skills... :oops:

Thanks for the help...
Jason Rodgers
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Re: Three bad pots in a row?

Post by Jason Rodgers »

Can you post a schematic or drawing of your setup?
-Ruining perfectly good wood, one day at a time.
David King
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Re: Three bad pots in a row?

Post by David King »

Pots are pretty simple devices. Could you have overheated the pot when soldering to it? Most of the time this happens when soldering a ground to the back of the pot.
Tone pots aren't in the signal chain so they are unlikely to make noise.
The main cause of pot noise is DC offset but that requires a battery which you didn't mention.
Matthew Seto
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Re: Three bad pots in a row?

Post by Matthew Seto »

I'll get a schematic up shortly...

They are active pickups, so there is a battery involved. Also, there's a very good chance that I overheated the first one. I tried to take better care on the second, and on the third I used alligator clips to try to eliminate overheating as a cause. I've also grabbed a bottle of electronics lube/cleaner, and tried spraying it out. Will research DC offset problems/solutions, but any further guidance is welcome!
David King
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Re: Three bad pots in a row?

Post by David King »

Matthew,
Don't solder grounds to the back of pots, that was already a bad idea 60 years ago when pots were built a lot tougher than they are today. Best practice is to solder the case ground wire to the lock washer that goes under the pot. Alligator clips aren't as useful as a bigger soldering iron which can get the job done faster with less incidental heating of the interior parts. Watch some soldering videos that explore the basic concepts of tinning and efficient heat transfer. Soldering should take place nearly instantaneously with almost no heat migration. It's very easy to do with clean surfaces and good technique.
Daryl Kosinski
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Re: Three bad pots in a row?

Post by Daryl Kosinski »

I agree with David. I have worked for 39 years repairing industrial electronics and instrumentation. Many thousands of pieces of equipment have crossed my bench, not once do I recall seeing a ground wire soldered to a pot case. It seems only musical instrument electronics uses this bad practice. I have seen a large ground lug big enough to fit a pot shaft. http://www.newark.com/abbatron-hh-smith ... 0000028007
Matthew Seto
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Re: Three bad pots in a row?

Post by Matthew Seto »

Here's the schematic. That's not a good representation of the switch- it's a 3-position switch that turns on one PU, or the other, or both. I've been tweaking it, it's a work in progress, but this is where it's at now:
guitar_schem.png
guitar_schem.png (10.62 KiB) Viewed 15360 times
Matthew Seto
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Re: Three bad pots in a row?

Post by Matthew Seto »

David King wrote:Best practice is to solder the case ground wire to the lock washer that goes under the pot.
That's an awesome idea, I think it's exactly what I'm looking for. Seems obvious after you mention it. Daryl, the lugs you mention look handy, I'll probably make one until I get to a point where I think I might use a bag of a hundred.

Thanks for the help, guys! Hopefully, fourth try's the charm...
David King
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Re: Three bad pots in a row?

Post by David King »

Matthew,
If you have a VOM or DMM see if there's a dc voltage across the volume pot. You probably just need a DC blocking capacitor or two between the pickups and the output jack. I'm pretty sure that's why your volume is noisy. The caps should have been built into the pickups but we don't know that for sure.
Matthew Seto
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Re: Three bad pots in a row?

Post by Matthew Seto »

David;

I grabbed my multi-meter and checked the battery first, just for reference: 8.77V. I plugged the guitar in to the amp and checked the volume pot. There is most certainly a DC voltage across it. I read a voltage drop across the volume pot that varies from 0 to 9, depending on the volume setting.

So if I understand this properly, we either want the full voltage drop across the pickups (which would be the case if they had a cap built in), or the full voltage drop downstream of the volume pot, which would be the case if we add a cap next to the jack (closest to where the amp would have a DC blocking cap). In the former case, the pot is at 0 potential, in the latter case it's at 9V potential, and no voltage drop in either case. Am I understanding correctly?

Do we prefer one of those cases over the other? Better to add a cap just behind each PU, or one just ahead of the jack?

Thank you for the help;
Matthew.
David King
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Re: Three bad pots in a row?

Post by David King »

Matthew,
I think the best would be to put the blocking caps as close to the pickups as possible. Selecting the right value is important as the pickup is an inductor with a variable resistor. Adding a capacitor turns it into a tuned circuit and if the cap is too small it will cut into the audible frequencies, if it's too large a value it won't block all the DC. I'd start with 10mfd electrolytics but make sure you have them in series and get the polarity right.

What buffer circuit did you use in the pickups? Is it the old J-FET Alembic Stratoblaster? https://www.google.com/search?client=sa ... 8&oe=UTF-8

Guitars are supposed to be all AC signal with no DC offset. A DC voltage drop across the pot implies that you are slowly heating up the carbon track in the pot and that's going to generate some noise and use up the battery a lot faster. A problem with designing unity gain preamps for pickups is that unity gain circuits are rather unstable and can go into oscillation. You really need to look for a circuit design that can avoid this. Look at some of these for ideas: https://www.google.com/search?client=sa ... 8&oe=UTF-8
Matthew Seto
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Re: Three bad pots in a row?

Post by Matthew Seto »

I did not make the PUs myself (which is why one would make the buffer circuit? or else I've got more serious issues than I thought...). They are Schaller S6's.

I've got no electrolytic caps in my meager stash. If there's still a Radio Shack open around here I may be able to get some tomorrow, otherwise it'll be a few days while they are shipped... I'll get them installed and report back.
David King
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Re: Three bad pots in a row?

Post by David King »

Matthew,
As far as I can tell the only strat style active pickups Schaller makes are the 273 and 275. The S6 single coils has alnico magnets but it's definitely a passive pickup. What happens when you simply disconnect the battery?
This is a more common error than one might imagine. People see a pickup with a hot and ground and a drain wire and instinctively assume it's an active pickup. No harm done in all likelihood but it would explain something.

David
Matthew Seto
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Re: Three bad pots in a row?

Post by Matthew Seto »

OK, sorry for the slow update...

I got a set of caps and installed them and this significantly reduced the noise. I went ahead and replaced the pot as well because with all the trial and error, it was pretty abused. I soldered the ground to the shaft washer as mentioned above.

Also, I yanked the battery and it did fine without it. I was wondering why I ever thought they were active in the first place, went back and checked my notes- the vendor that sold them to me calls them active in their wiring lit (thought it just says "...active pickups, like Schallers,...etc.", does not mention model names).

So now everything is nice and quiet! Thank you David for the help!
David King
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Re: Three bad pots in a row?

Post by David King »

Matthew,
Thanks for getting back to this. I was really wondering what might be going on. Now you can pull the caps out too as you should have no need for them. They may be blocking some of the low end which might not be a bad thing in a guitar but you won't know until you try it both ways.
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