This Beocenter really is the gift that keeps on giving.

However, I've now encountered a problem to which I can see a solution, but it's a bodge.  I don't know if the problem pre-existed because the tape player was non-functional.

What happens is that on cassette play only, there is significant mains hum in the right channel.  The problem appears to be provoked by the metal cover of the pre-amplifier that effectively bonds the internal floating signal and chassis grounds with the external casework of the unit.  When the metal cover is removed, the hum disappears.  There is continuity (0.5R) between the internal and external cover mounting points without the metal cover in place.  When the tape section is energized, I can measure 2.8 mV between internal metal and external case, cover off.

I could refit the cover using insulating fasteners and disrupt the ground loop (if that is what it is) but if anyone has any other ideas, that would be great.  The cassette section does not use the same modular mounting on an insulated base but is bolted direct to the metal case - as is the tape power supply. 

My guess is that some stray voltage is quite likely with this arrangement.  The signal ground from tape section (Dolby board) to pre-amp is good.

Picture shows a couple of clips in place to replicate the hum.

Grateful for any guidance, as always.