Yamaha RX V675 sound issues

[jjne]

I have an old Yamaha RX V675 receiver which is starting to develop issues with the sound, although it doesn't quite match up with what folk normally report with these.

From cold, the sound cuts in and out. When this happens the amp is losing sync with the optical input (speaker graphic disappears from the front panel). But this is not resolved by putting the unit into pure direct mode. After a couple of minutes this resolves, although there are occasional clicks from the speakers after that.

At first I thought it was either a speaker relay or the DSP, but I don't think it's either. All sound is lost and the amplifier is losing sync at the same time which I think would rule out a failing relay (as I'd expect that to only affect one speaker) and the fact that bypassing the DSP doesn't seem to make a difference I think rules out that chip.

It doesn't appear to be the optical cable or the source (TV) as the same cable plugged into a soundbar is fine, and once the Yamaha warms up it's also OK.

So I'm thinking it's further up the chain in the amplifier - are there any known faults on the input stage on these? I think the next step is to connect this via analogue and see if that makes a difference.

[jjne]

The analogue inputs are the same. Should be noted as well that if it's switched off and on, sometimes repeatedly, the problem does clear.

[CapLeaker]

get yourself the SM and look at the block diagram. Open the unit up and check with an oscilloscope where you are losing the audio. Keep it simple and start with the analog side.
Also check if all voltages are present and within spec. Don't forget to check the caps.