Potentiometers always have static scratching noise

[Fuzzdemon]

Hi all just having a problem for some time now with potentiometers (even good ones) always give a static scratching noise when you are physically turning them, but the other pots ive added in other areas of the board work ok. I will post a picture soon of the 2 places I have the pots soldered to. If I replace the volume pot with a micro PCB pot it works ok but for some reason the mini pots for external mounting are always noisy.
Thanks for any help.



These are the type of pots im using...

[Mark Hammer]

Alpha pots, like the one shown, have rarely let me down personally. However, it has also been my experience that the smaller the diameter of the pot, the more abrasive the wiper tends to be. Keep in mind that pots can only work because the wiper is pressed up against the resistive strip. The implication is that sometimes pots will scrape off small particles of the resistive strip, and they can accumulate, as a form of "dirt" inside the pot, leading to tiny momentary discontinuities as the wiper is rotated.

There are chemical treatments that can repair or avoid this, but the simplest and cheapest alternative is to gently pry up the tabs of the back cover of the pot to remove it and expose the resistive strip. You can then use a cotton-tipped applicator to wipe any residue off the resistive strip.

Additionally, the connection between the solder lugs and the resistive strip is maintained by the little rivets you see in the picture. Every once in a while, a pot may arrive with one or two lugs not providing as snug a fit and connection as one might like. While the back of the pot is off, you can get in there and give the rivets a little "pinch" with your needlenose pliers to secure the connection, before you replace the back cover on the pot.

I have found both of these procedures to help reduce potentially noise potentiometers. And, best of all, unless you have to actually go out and buy the tools, it's free!

[Fuzzdemon]

Additionally, the connection between the solder lugs and the resistive strip is maintained by the little rivets you see in the picture. Every once in a while, a pot may arrive with one or two lugs not providing as snug a fit and connection as one might like. While the back of the pot is off, you can get in there and give the rivets a little "pinch" with your needlenose pliers to secure the connection, before you replace the back cover on the pot.

Ive only ever experience the above part of your comment but in this case its not loose connections or dirt, ive had this issue from just these 2 parts of the circuit and another 1 which I changed to a switch instead but I really dont want to do that with these 2 pots, no room apart from anything else.

btw I wasnt very clear in my first post I mean ive tried lots of different pots that I know are good and clean, its almost as if this part of the board is too sensitive for pots and needs switches.

[nickb]

Can't say anything without a schematic. Can you post it?

[Fuzzdemon]

Can't say anything without a schematic. Can you post it?

This is the schematic, I envy you because I dont fully understand them yet, and it doesnt help when my resistor/cap/transistor location numbers dont match what I see in schematics.



This is closer to mine, I have Q1, Q2 and Q3 transistors but I cant cant find a schematic that has the Q3 on it. The Q3 is where I have my scratchy volume pot wired.

[nickb]

Sorry, but i'ts crucial to have the right schematic with the correct component refs but good try What I was thinking was you might have DC across the troublesome pot and by looking at the sch I could see if that might the case. What you could do is put a voltmeter across the pot and measure the DCV to see if it's an issue.

[Fuzzdemon]

Sorry, but i'ts crucial to have the right schematic with the correct component refs but good try What I was thinking was you might have DC across the troublesome pot and by looking at the sch I could see if that might the case. What you could do is put a voltmeter across the pot and measure the DCV to see if it's an issue.

lol I will try and find the right one, its weird mine is so illusive and I have a few wah's like this. Im not sure how to do the voltmeter thing yet, I dont know what volts to expect, should it be plugged in when testing? do I measure the inner and outer lugs or both outer? I know how to measure everything except volts not needed to yet.

I suspect the Q0 transistor in that schematic is the same as my Q3, im certain actually, kind of.

[eschertron]

Are the pots you've soldered in replacing other pots, or replacing fixed resistors? I'm thinking the same thing as NickB, some of the resistors in that circuit possibly have enough voltage across them to cause a pot to be noisy. A schem would keep us from guessing.
There may be workarounds for some pots with DC, not so much for others. If your desire is to find a sweet spot by tweaking and then leave it there, then the noisy pot is not an issue and can be left in that spot or swapped out for an equivalent-value fixed resistor.

[Fuzzdemon]

Sorry, but i'ts crucial to have the right schematic with the correct component refs but good try What I was thinking was you might have DC across the troublesome pot and by looking at the sch I could see if that might the case. What you could do is put a voltmeter across the pot and measure the DCV to see if it's an issue.

ok I found the right schematic, looks like I could be wrong about the Q3 being the same as Q0. The Q3 is on the far right side.



btw ive removed all the buffer components on this board so it looks like this

[Fuzzdemon]

Are the pots you've soldered in replacing other pots, or replacing fixed resistors? I'm thinking the same thing as NickB, some of the resistors in that circuit possibly have enough voltage across them to cause a pot to be noisy. A schem would keep us from guessing.
There may be workarounds for some pots with DC, not so much for others. If your desire is to find a sweet spot by tweaking and then leave it there, then the noisy pot is not an issue and can be left in that spot or swapped out for an equivalent-value fixed resistor.

Yes the pots are replacing fixed resistors.

So its a voltage issue then probably, at least im closer and its something ive overlooked, and no I really want pots on the volume control at least and preferably the R9 resistor. I hope that schem can help you decide what the problem is, im happy to do whatever it takes to fix, I dont want to drill more holes and use switches instead of pots, I think id rather live with the scratching but that really sucks.

Wait, I should say only 1 pot is replacing a fixed resistor, the other is replacing a fixed transistor (the volume pot)

I must say I was kind of hoping there would be a quick fix for this, maybe just solder something onto the pot to clean up the signal etc, I guess its not that simple.

[nickb]

ok I found the right schematic, looks like I could be wrong about the Q3 being the same as Q0. The Q3 is on the far right side.

[ATTACH=CONFIG]42974[/ATTACH]

Oooo well done

For measuring just go across the outer lugs. Anything greater than 0.1V or so is not good.

[Fuzzdemon]

Oooo well done

For measuring just go across the outer lugs. Anything greater than 0.1V or so is not good.

I set the MM to volts and touched the outer lugs of the pot and its showing me a different reading depending on which way the pots dial is turned, as if im measuring ohms. Its going from 0.0v to 0.262v.

[Leo_Gnardo]

I set the MM to volts and touched the outer lugs of the pot and its showing me a different reading depending on which way the pots dial is turned, as if im measuring ohms. Its going from 0.0v to 0.262v.

Then it's to be expected, you're changing a DC level and that makes for scratchy noises as you change settings.

After that it's what eschertron said in post #8 ^^^.

Note in some effects, crackle is to be expected, and labeled as such.

[Fuzzdemon]

Then it's to be expected, you're changing a DC level and that makes for scratchy noises as you change settings.

After that it's what eschertron said in post #8 ^^^.

Note in some effects, crackle is to be expected, and labeled as such.

Thats a relief to see that pic, it looks like it could be unavoidable in some cases or he just cant be bothered correcting it, I find it too annoying to live with because it gets louder with gain and volume.

I actually had a "super hard on" once and I played with the knob and it gradually got better and better.

[Leo_Gnardo]

Thats a relief to see that pic, it looks like it could be unavoidable in some cases or he just cant be bothered correcting it, I find it too annoying to live with because it gets louder with gain and volume.

Regardless of annoying scratch, Mr ZVex and his sister (who painted the boxes) must have made a fortune by now. Not at all a tough circuit to build, and they sell for gold dust money.

I actually had a "super hard on" once and I played with the knob and it gradually got better and better.

A little more information than we need...