OK you have got an amplifier and it is all sound and the electronics are all working, but then when you try to turn up the volume, or fiddle with the bass control, what happens is you get a whole load of popping and scratching and crackling when you turn the knob. Sounds terrible, but then it settles down - or maybe it doesn't settle-down and the sound drops out and comes back in again unpredictably. The amp is getting old now, and has recently been brought back into the house after spending some time in the garden shed.
So we all know the story, and the cause of it is dirty or oxidised tracks inside the potentiometers on the volume control. Generally speaking, the build-up of dirt or oxidisation will occur around the point where the sweeper arm inside the control rests on the track. So if you leave all the controls in the same position when you switch-off the amp and put it into storage, then thats where the scratchiness will occur, and you will get it at that full volume. If you turn down the controls to zero, then the scratchiness, when it strikes, will only effect the control at low volume. Although, it has to be said that the older the amp then the higher the probablilty that the pots will degenerate, and this will be right across the entire sweep, so you will probably have to clean the pots anyway at some eventual time.
How do you clean the potentiometers? Well you have to take the covers off and get inside the amp. The potentiometers are those circular things at the back of the control knobs. Of course, you knew that already. Most of the amplifiers I have serviced have potentiometers enclosed in a metal casing with three pins coming out of the side in a slot in the casing. To clean the pot you need to apply a solvent-based cleanser to the track and finish with a light lubricant. The easiest way to do this is to spray a short burst of a cleansing/lubricating agent into the pot through the slot in the side and revolve the sweeper arm back and forth for a short while. Specialist electronic supply shops sell switch/contact cleaners in aerosol cans which have a thin bendable nozzle for getting into difficult corners. The one you are most likely to come across is made by Servisol with a product name Super 10. Other switch cleaners have their uses but you must ensure that once the solvent has evapourated, then the product contains a lubricant that is left behind on the track. The lubricant is needed firstly for mechanical protection - you don't want the metal sweeper arm inside the pot scratching away at a dry track, and secondly you get better electrical contact with a lubricant. Some people find WD40 works for them, and this is one of the famous 4000 uses listed in their advertising, but it should be borne in mind that WD40 is formulated for general cleansing and lubricating use, meanwhile a specialist electronic cleaner has been specifically manufactured for that sole purpose. My personal preference is for the electronic cleanser and this is because it results in a cleaner job. WD40, in my experience seems to leave behind a hell of a lot of greasy residue that is only going to attract more dirt. However, in its favour WD40 is readily available in most hardware stores and supermakets whereas the other you need to hunt around for it.
For real difficult jobs the only way to clean a potentiometer is remove it from the amp and dismantle the outer casing. This may also require that you use a soldering iron to remove it from the printed circuit board or any attached wires. The outer casing on most potentiometers is held in place by small bendible tabs so it is not too tricky to remove. Once the innards are exposed you will be able to spot the problem immediately and apply the solvent directly using a cotton bud.
Cleaning the potentiometers is a job that comes to most amplifiers when they reach a certain age, but worth doing as it can bring new life to seemingly clapped-out equipment. You should only need to do the job sparodically - if it is something you are having to do every couple of months then that would be a sure sign of deterioration on the pots, and you should then consider having them replaced by a service engineer.
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