Exactly as ‘Conen’ says, the compressor had a mind of its own and produced a mono signal whatever went into it. As well as that, the compressor seemed sluggish and not working properly.
The first fault was a dry joint on the PC board in the ‘difference’ channel, this locked the amplifiers into a ‘mono’ mode.
The second fault was a failed transistor in the LED drive circuit to the optical compressor, this allowed a meter indication of compression…. but it hardly did anything.
So with the PC board fixed and the drive transistor replaced, it’s back to working as good as new!