There are repairs we do all the time for amplifiers, these include sorting out crackles from dry joints on PCBs, excessive humming, broken jack inputs, output valve rebiasing and replacing broken valves, crackling or not working pots and controls.
More complicated repairs are when there has been a fire in the amplifier and many parts need changing including transformers, rebuilding of PCB damaged tracks, faultfinding of class AB output amplifiers blow FET and MOSFET stages.
Most of the faults we see are as a result of general wear and tear, with the majority of faults being more noticeable when the amps are running hot in both valve and transistor.
Often hums are as a result of storage capacitors drying out, think of them like water reservoirs smoothing the supply voltage. Capacitors used in storage have a sandwich of conductors and insulators which are soaked in a liquid called an electrolyte. Over many years of use, heat and wear the liquid dries out and renders the capacitor useless.
Other sources of humming can be broken screens, and missing earth connections either on mains or signal.
Crackling and jumping volume on pots
Crackling pots can be annoying and sounds like static when turning, but in worse case situations you can find that a volume dial is either off or 100% on causing all kinds of problems with in particular master volumes!
In the first case we will soak the pots with some switch cleaner which cleans the oxide, dust and grime from the resistance track. In 90% of the cases this solves the fault and allows you to go for many years with a quiet control. The other 10% we will change with a like part.