My good ole' mini stereo amplifier started to behave extremely weird
recently. Beyond not being able to use it, I'm really puzzled with the
symptoms:

There's a strong reverb effect and the vocals are canceled and slightly
to the right - seems like a phase shift. I put on a Beatles song to
better test it:
- The vocals are canceled even when I hold my ear against one speaker,
or when I flip the polarity for one of the speakers.
- I hear both channels in each speaker, and each channel on both
speakers.
- With balance full left, the left speaker sounds slightly louder, with
full right it sounds centered.
- I tried the two available inputs ("DAT" and "AUX"). Same thing.

I opened up the unit, and as far as I can tell, everything looks fine.
Nothing burnt or melted, no excessive dust (and they were smart enough
that no board is facing up). I vacuumed it just to be sure, didn't
change anything. You can see snapshots at
http://www.flickr.com/photos/3127847...7594269637399/

The unit is a Sony MHC-1600. Its amp, if that means anything, is
STK4132II. The only thing I did to it was to move it to another room
for one night.

So my only conclusion is that the unit spontaneously crosses (mixes)
both channels, with inversed phase, before splitting it back to the
stereo amp. How the hell can that happen, not to mention how to fix
it?!