problem with Pioneer SX-727

Hi there! I have just got a mint condition SX727 and all seems to work great and sound nice.
However if I use either of the switches "low filter" OR "audio muting, low -20db" it sounds like it shorts out the speakers and the sound shuts off, then the amp restarts (i hear a relay click about 3 seconds after this just like when you first turn it on).
I'm picking there must be a shorted capacitor? I would appreciate any help.

Thanks very much.

Sounds like the unit is going into protection mode. Someone over at AudioKarma might have a clue. You should always check old used amps/receivers for DC offset before you hook up to your good speakers. With no input source connected, put a multimeter on the mV scale, connect the leads to the positive and negative speaker outputs and observe the reading. You want to see DC offset in the ideal range of +/-0-15mV, and no higher than about 50mV.

There is probably DC in the circuitry where these switches operate. Possibly a coupling capacitor (passes audio signal AC but blocks DC) may have failed. As you switch in and out, the DC offset applies itself to the downstream amp stages which amplify it. A protection circuit detects it and cuts off the speakers. Meanwhile the input circuit in the amp is probably 'seeing' the DC and self-correcting for it; when its servocircuit has brought it down to within a volt or so of zero, the protect circuit reconnects the speakers.

Hi, thanks for the answers now I have the service manual the problem looks like what you guys say there is just a little resistor / capacitor circuit and no doubt the cap is shorted = dc from the resistor straight to the amp. I will put this on the list of stuff to do like replacing a couple of lamps and maybe re-capping. For now just enjoying it and will give it a good internal clean etc one day.
There is also some hum (only noticeable when no music) and often some FM radio that comes thru faintly on one channel when using tape1 input. Probably contamination on the main power / selector switch?

Thanks again.