A faulty power supply will create a hum (usually caused by weak filter/electrolytic capcitor(s) or bad regulator components being unable to product a 'clean' voltage supply). In other words the AC power isn't being properly converted to a clean DC supply inside your receiver - and therefore you are hearing the 'hum' of the AC through your speakers.

There's a simple test, to make sure it isn't being caused by something else - if the hum is still there no matter if your listening to AM, FM, audio from a tape or CD or phono then it's very likely a power supply problem.

Often the hum will be present even with the volume turned down (if the power supply filtering is really bad), but usually the hum will get worse when you turn up the volume since the the hum will be increased by the (left and right) amplifer(s). Check to see if the hum is coming from both speakers - as it likley will if the power supoply is faulty.

In addn poor grounds (ie in your audio cables, ect) will allow stray "AC" to be coupled into your receiever. Disconnect the audio cables temporarily to check...

In the event that it's a bad power supply it can be relatively easily/inexpensively repaired by a competent technician (replacement capacitor is ~$5 with ~~ an hr or so work) - assuming the stereo is worth spending money on.

Hope this helps.