Scratchy pot?

[devnulljp]

Quick question about a scratchy pot on an old amp.
Before I pull it apart, is there any point?
Is it likely that just squirting contact cleaner into a tone pot will cure scratchiness or is it better to just replace the thing?
It's an old solid state amp...
Thanks

[greco]

I have read several threads/articles on this topic in a variety of forums/tutorials.

My conclusion...you will likely end up changing the pot eventually, as the cleaners are often only a short term fix.

Others may (and likely will) disagree.

Peace

Dave

[bagpipe]

My thoughts exactly. Any time I've used the spray-in cleaners, its only been e temporary fix. I've always ended up having to replace the pot.

I have read several threads/articles on this topic in a variety of forums/tutorials.

My conclusion...you will likely end up changing the pot eventually, as the cleaners are often only a short term fix.

Others may (and likely will) disagree.

Peace

Dave

[devnulljp]

Thanks for the input. You know I knew that, deep down, but was just hoping someone would prove me wrong.
Thanks

[greco]

If you are going to replace the pot(s), I would consider NOS Allen-Bradley brand pots. These are a bit more money, but are very well made IMHO.

I await the learned comments of others.

Peace

Dave

[mhammer]

There is this wonderful stuff called Stabilant, from a company in none other than Thorn Hill, Ontario. http://www.stabilant.com (read the app notes). I was turned onto it by a buddy who has a busy studio that lays tracks to 2" tape and has far too many faders that can ill afford to be noisy.

Stabilant is not a contact cleaner, but rather a contact enhancer. It is not supposed to remove stuff from the surface of conductive surfaces, but rather add a conductive layer to them. You can apply it after you have sprayed contact cleaner, though applying contact cleaner after Stabilant is counterproductive. It is called Stabilant because it is a chemically stable compound that you apply once for the life of the product and it remains stable, meaning it doesn't dry out or break down. You can apply/use it anywhere that you have conductive surfaces. I have used it on all my pots (any time I get new pots for projects, I always pry the tabs and pop the back off, coat the resistive strip with Stabilant, and re-assemble the pot before using it), on RCA connectors on the back of units, inside stomp-switches, and find it can sometimes work miracles with old chips sitting in economy-class IC sockets where the tarnished IC pins and socket pins do not make perfect contact with each other (a teeny dab on each socket pin does her nicely).

Think of it as filling in that last micron of gap between A and B. So, if you have a well-used pot where the surface of the resistive strip has been worn from the wiper moving back and forth, Stabilant can fill in that gap and act almost like a liquid solder joint. Since the layer created cannot be THAT thick, there will obviously be some limits to what it can de-scratchify. I would not expect it to redeem a 20 year-old wah pot, for instance, if the user plays "Tales of Brave Ulysses" and "White Room" in a Cream cover band, 4 nights a week for the last decade. However, if applied to a new or moderately used pot, it can certainly provide prophylaxis against wear and tear because it also has lubricant properties The app notes on the web site note other applications, like edge connectors (bring your N64 cartridge port, or the remote that came with your TV, back to life!). If I have a stompswitch that is misbehaving (because too much heat during soldering has caused the small dab of grease inside to flow and coat the contacts), I take the switch apart, clean the grease off the contacts and apply a small dab of Stabilant to the contacts before re-assembling. Works wonders.

In Ottawa, I buy the stuff at a place called Reset Electronics in small 0.5ml vials for 50 cents each, though I gather they do not make these any more. Where you would get it outside of Ottawa, I have no idea, but since the company is Canadian you can probably get it easier here than our American cousins can. One half-ml vial can usually do me for 30 or more of the small Alpha pots, but apparently you can also thin it out with alchohol to make it stretch even farther. The larger repair shop-sized containers will run you a hefty price, but 50ml would last any user for a long long time (by my estimate, that would treat at least 300 pots). If you can find the little 50-cent vials, they come with a small plastic applicator that lets you dab a sesame-seed sized droplet on a conductive surface and spread it around. And even though it does NOT cure cancer, restore hair, improve gas mileage, julienne fries, make women fall instantly in lust with you (though they may tolerate your sick hobbies more if there is less crackly noise associated with it), or make your penis larger, I really cannot recommend this stuff highly enough.

Certainly, take steps to clean the gunk off pots that may have accumulated. But DO note that some of that gunk is not dust, but the residue of what has been worn off the pot's conductive surface by the rubbing of the wiper. When there is less resistive element in the pot than there used to be, THAT's when you need Stabilant to make up the difference.

[Wild Bill]

If you are going to replace the pot(s), I would consider NOS Allen-Bradley brand pots. These are a bit more money, but are very well made IMHO.

I await the learned comments of others.

Peace

Dave

My experience has been a little different. If a pot has likely never been cleaned then a shot of cleaner quiets it down and it will be quiet for another bunch of years! If the pot is really old and has seen a LOT of use then it may be too worn for cleaner to help. Unless it's a sealed unit a pot WILL pick up dust over the years. That's much less a serious problem than actually having the resistance element worn down.

Be careful about the Allen-Bradley pots. They haven't been made for some years and you have to pay attention to the taper. By this I mean, if you're using the pot for a speed control you use what's called a linear taper. A 10% advance in rotation means a 10% change in the wiper ohm value, 80% means 80% and so on. Volume controls and many tone controls need an audio, or logarithmic taper. The human ear needs a bigger and bigger amount of change to sound smooth as the volume increases. The change starts off slow and then really kicks in above 30-40%.

If you use a linear in an audio or log application what happens is that all your adjustment is between 0 and 3. After that there's little or no change.

So what's the problem? 90% or more of the pots you find are linear taper. That's where all the sales volume lies. Log tapers are much rarer. Worse yet, most salesguys at electronic stores have no idea of the difference and may never even have heard of an audio taper. They will cheerfully sell you a linear taper, thinking that's the only kind there is.

As a sidebar, a lot of low power solid state practice amps in the stores today seem to be using linear pots. I'm too cynical to believe that's just 'cuz they can buy them cheaper. I believe it's because in the store when some young kid tries it out he's not likely to be allowed to turn it up past 3. So naturally he thinks if it's that loud on 3 when he gets it home and turns it up even more it will be even louder! He buys the amp, gets it home and then finds out that 3 is as loud as it gets. You sell more amps that way.

:food-smiley-004:

[mhammer]

The taper to use will depend on the function it provides. Though there are plenty of exceptions, generally, when the control simply provides "more" (more volume, more speed) you will want a log/audio taper, and occasionally what is called a reverse-log or "C" taper. When the control provides more and less (textbook case is a blend control or cut-boost tone control) such that you need to know where the "middle" is, a linear taper pot will be called for.

Some people are very fussy about adhering to the stated taper. For my part, it is all a question of dialability. The manufacturer selects a taper based on assumptions about what it is you will want to be able to dial in with precision. For example, if your interests are in the range of tremolo speeds between 3 and 5hz, and you have no interest in anything slower than that, chances are pretty good the original pot taper will be wrong for you and either a different taper or some custom-tailoring is called for. The original MXR Distortion+ called for a 1M reverse-log taper. Why? Because the first 900k of resistance change is as useless as tits on a bull, and the taper is there to quickly get you past the part of the Gain control's range where you only have a bit of clean-ish boost, and into the dirtier range. On mine, I simply went straight for the last 100k of resistance, used a linear pot, and I have all the dialability of dirt I need. If I want a clean booster, I have other pedals for that.

I think Bill is partly right about the deceptive ways in which pot taper can be used to misrepresent a product. But my sense is that an equal proportion of the time, pot taper choices are not really choices at all, but simply the lack of a choice, as in "This calls for a 50k pot. We have a bunch in inventory. Let's use them."

The reason why even sealed pots can acquire "dust" is precisely for the reasons I noted earlier. The wiper makes contact with the resistive strip by applying pressure. Since the wiper rubs against the resistive strip, over time it produces its own dirt by scraping off tiny particles of the resistive strip. This is precisely why I recommend Stabilant. While pots CAN and do get scratchy because dirt has invaded the pot through openings, simply removing the dirt with contact cleaner will not always completely remedy the scratchiness if it is also a result of reduced contact continuity between wiper and resistive strip due to gradual erosion of the strip. One probably needs to take a pot apart to have a better sense of what is going on in there.

[greco]

WOW...did "we" ever open up a can (or POT) of worms here !!

Very informative thread.

Thanks devnulljp

Peace

Dave

[Wild Bill]

WOW...did "we" ever open up a can (or POT) of worms here !!

Very informative thread.

Thanks devnulljp

Peace

Dave

This is what I was hoping for when I first joined this board, Dave! The pool of talent keeps growing. We've had a number of very technical guys chip in to help folks. We're quite lucky to have Mr. Hammer on board! He's VERY knowledgeable, particularly with pedals!

If I recall he's got his own webpage, with tons of useful stuff! Maybe he could be good enough to post a link...:smile:

:food-smiley-004:

[devnulljp]

Yes, this is great. More than I was expecting, and all good to know.
Thanks all for chipping in.

[mhammer]

With that kind of introduction, how could I refuse? :smilie_flagge17: It hasn't been updated in a bit, due to the reorganization of the old AMPAGE website (Steve Morrison, the moderator/webmaster there graciously gave me some space and an interface to post stuff), but here you go:

http://hammer.ampage.org

Some stuff needs updating, some needs taking down, and some is dead links, but there is still plenty of useful material.

[greco]

mhammer...really enjoyed your site (link in above post)

Thanks for sharing all that information/experience/wisdom, etc.

Bookmarked it !!

Peace

Dave