Just bought an old 80's Bass 12 off fleabay - really grubby and looked like it hadn't been used in ages. When I switch it on there is a loud humming sound. Pots very crackly, So I have stripped it down, given the cabinet a good going over, given the board a visual inspection to see if there is anything black and charred hanging off it - all looks in order. Pots have had a good drenching with contact cleaner to dislodge any grime.
A more detailed description of the hum:
Amp switched on, no lead attached. volume and gain at zero, amp humming through speaker. Gradually rotate the gain control, the hum quiettens down (but still there), then goes back up to the original volume with gain at 10.
With guitar attached, same thing happens but with the gain at zero there is a sound like shortwave radio interference.
Does anyone have any ideas what it might be? It's a pretty basic looking board, so rather than taking it to an amp tech, I'm wondering if I can sort any component replacement myself to save a few quid (usual disclaimer about high voltages/ possible death etc.) At a guess, I wonder if it needs the caps replacing - it's over 30 years old, so that may be a possibility. While I'm at it I may replace the pots as the previous owner has squeezed the split shaft on each of them and in some cases slightly bent the spindle.
I appreciate that it is difficult to diagnose a problem without seeing the amp, but it sounds like it may be something that is relatively easy to fix. Any suggestions would be much appreciated.