It sounds like the ages old noisy pot problem, potentiometers are basically a strip of a carbon like material, forming a resistive track, "painted" in a piece of phenolic board with a sliding contact that rides the resistive track from end to end, that resistive track, can be straight or circular, then you have the straight slider pot or the regular turning knob pot.
When the resistive track gets dirt or dust from use, the contact point with the slider becomes erratic and then you get those simptoms, noise when you adjust the pot or little spots were it even loses contact with the resistive track.
A.- There are several ways of fixing that, if the problem isn't too bad you may try working the pot back and forth several (actually like 20 or 30 times) doing this the slider usually cleans its own path and returns to normal, at least for a while,
B.- Then there is the next level, which is a can of electric contact cleaner, to (try) to fix the pot you have to : 1st.- DISCONNECT THE POWER,
Then open the amp, and locate the dirty pot, then you will see where the 3 contacts connect the thing to the amp circuit, just above the contacts there is usually (in case the pot is not a sealed unit) a slot were you will spray the cleaner and work the pot back and forth to loosen the dirt and leave a clean track,
C.- in case that you find a high quality sealed pot the only solution will be to replace the pot with a new one. there has been cases were the old pot can be removed and opened very carefully, then cleaned with high quality isopropyl alcohol, WARNING : do not use cheap isopropyl alcohol, I mean the 70% type, in which the other 30 % is water, your pot may not like it, try to use at least 90 %, or 99% even better.
The only problem with this could be that you may need to be proficient with a soldering iron, and that to reach some pots some REALLY serious disassembly may be required.
The disassembly procedure can be applied to almost any type of potentiometer, there is a catch, the alcohol will dilute and probably remove the original lubricant in the pot axle, so after this procedure you may need to lubricate the pot, applying a q-tip soaked but not dripping with a little sae 40 W motor oil, were the axle goes into the pot at the exterior of the case, it will absorb enough oil by capillarity to get the needed lubrication, DON'T overdo the oil thing, be very sparse and wipe the excess.
If you decide to go the replacement way, let me tell you that NONE the amplifier manufacturers, use custom made potentiometers in their amps, its actually the other way around, they use off-the-shelf standard components to keep costs down, so being an electronic engineer for many years, I can help you to locate the right component, or a standard sized similar replacement component.
All that is needed is all the numbers you can gather from the part itself, AND usually a few good pictures of the part. sometimes if there are no references in the part, it can be measured with a ohm meter, and the size to get a mechanical fit in the space available.
Let me know.
