We all know the problem. After a few years the pots on our amp get scratchy. By that, I mean you get this scratchy noise every time you rotate them. This is particularly bad when you leave the amp standing around for a few months (years or even decades!)
What Causes Scratchy Pots?
You’ve probably guessed that it’s just dirt build up, and you are right. However, I bet you’ve never taken a pot apart and looked at the el-cheapo construction of the thing? You probably know it’s a variable resistor. Turn it one way and you get no resistance (short circuit). Turn it full the other way and you get maximum resistance—1k, 10k, 100k or whatever the value of the pot. Mid way you get mid resistance. You can see this is exactly what you want from a volume control, for example.
So how do they get that variable resistance? Simple. They PRINT a carbon track onto a circular piece of mica. Connect each end of this track to the outer two terminals of your pot. That’s a fixed resistor of course. But now you make up a ‘wiper’ which you connect to the middle pin of the pot. This ‘wiper’ is a flimsy, 0.1p piece of sprung metal. As you rotate the pot, the wiper slides over the circular printed carbon track. Voila! One variable resistor. Yours for 10p, Guv. (That’s manufactured cost of course. £3 to you! Kerching!)
Trouble, is, none of this is what you’d call robust. It’s open to the elements (not sealed) so dirt, dust and moisture can coat the printed carbon track. Now the wiper has to bump and scrape its way over the dirt and this is what causes the noise.
How to Cure Scratchy Pots
Ok you’re now an expert in how they make ‘carbon track’ pots and why they get noisy.
Now on to how to clean ‘em.
The solution, IN TEHORY is very simple, Spray some suitable solvent over the carbon track and … job done.
In practice though, there is a big problem. It’s VERY hard to get the nozzle of your spray into the tiny opening of the pot body which gives access to the track. Often the pots are mounted ‘upside down’ so the opening is hidden. Many’s the time I’ve turned a chassis upside down and tried to get inside it with the nozzle to clean the pots. It can be very tricky. Plus of course, you have to take the chassis out to clean the pots which is a mission in itself half the time.
Before I go on to give you a tip on how you can avoid this about 50% of the time, let’s talk about which cleaner to use.
There is only one. Caig Deoxit D5. Get it on eBay. Not cheap but it’s the dog’s whatsits. You could use another ‘electrical contact cleaner’ but this stuff is good.
DON’T USE WD40!
Okay so here’s that tip I promised you. Try it first as there is nothing to lose. Put the amp on its back so the pots are pointing at the ceiling. Remove the knobs on any pots you want to clean. Spray the DeOxit on the shaft close to where it enters the chassis with the hope and prayer that enough runs down the shaft and into the pot body. Turn the pot backwards and forwards a few times. 50% if the time this will work. 50% of the time you have to take the chassis out and try to get the nozzle into the pot.
Very occasionally (on older amps) a pot will not respond to treatment and you’ll have to change the pot. Good luck with that! I often have to tell my customers that a ‘simple’ pot change or input jack socket change actually involves the complete dismantling of the amp and removal of the main board to get to the solder contacts. It’s one of my least favourite jobs because it’s a LOT of work for not much reward (“There, there, your scratchy pot is all better now. That’ll be £60 + VAT please….”)