By far the most common repair we get enquiries for is when an amp has is either old or has been stored in dusty or damp conditions.
What causes it?
Oxidation and dirt builds up on the volume and tone controls (potentiometers or pots for short) carbon tracks. This causes dirt to get in the way of the electrical connection and will either sound like an old TV on a static channel when rapidly turned or the volume will jump all over the place and not be in correlation the the actual knob position.
In the first instance we use a spray called switch cleaner or contact cleaner. This is a blend of chemicals that clean the tracks and provide a protective conductive layer on the carbon track. Sometimes if the pot is slightly dirty this will do the trick first time and you'll be ok for a few years before more dirt builds up. If on the other hand after the cleaner has dried there are either erratic volume changes or the crackles are still there we simply change the pot. It's not worth the time in trying to clean the track or spray it any more.
Often when people contact us they say they've tried to spray the pot but it's not done anything. Quite often we find that they've sprayed the metal outside or not been able to get the nozzle right into the insides and the cleaner has been wasted. It's quite hard to get the nozzle right in there as often the access slot is facing the PCB down. We've found the easiest way is to bend the plastic nozzle into a hook shape. This enables you to spray back towards you and hits all the vital insides! That is a workshop secret though! keep that between us!!!