The problem is almost certainly either an earthing problem or a smoothing capacitor problem.

Electrolytic capacitors 'dry out' in old age / lots of use. To recognise the likely offending components look for large cylindrical components. The markings are likely to be something like 4700uf 50v. This describes the capacitance (value) and the maximum working voltage. As this is a 100w amplifier and fairly old to boot, it's not likely the supply voltage was particularly high (perhaps 50v maximum). It's vital that the replacements are at least the same voltage, and ideally the capacitance should be the same or greater. As somebody else mentioned earlier, these components are polarity sensitive....connecting the wrong way round will result in a big bang! Electrolytic capacitors, particularly large smoothing capacitors should really be changed every 10 years or so.

Regarding the possibility of an earth problem: the suggestion of a squirt of switch / contact cleaner down the jack sockets is a very good one, although if the sockets are very corroded you could replace them easily and cheaply. What is probably happening is that one contact (the 'signal' contact) is ok, the earth contact is probably poor. The effect of this is for the amplifier to have an aerial connected to it's input. This aerial picks up anything that's flying around (eg mains hum) and then amplifies it. Ever tried touching the tit on a jack plug connected to an amplifier? It's just the same.

If you're stuck, email me at eclipse.electronics@btinternet.com

Cheers,

John