Power amp diagnostics / earthing advice: help needed

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LAV

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I got hold of what otherwise seems to be a mint Rotel RB-1080 power amp, late model with bi-wiring binding posts. No previous repair work evident. Driving this with a Marantz AVR, using the unbalanced inputs.

Amp seems to be working fine but there is audible hum in the left channel. When switching the input selector to balanced mode, the hum disappears.

During the testing process I felt a tingle when touching the amp and the interlink coming from the AVR at the same time. Out came the Fluke: I am measuring close to / around 62 Volts DC / 75 V AC between the amp earth and the audio ground connection! How come?

Some more background:

* By design the AVR does not have a mains earth pin on its mains socket.

* The Rotel RB-1080 does have a mains earth pin its input socket.

* No measurable DC offset at the speaker outputs.

* Mains wall socket wiring checks out OK

* Hum is present even with no input wiring connected, but only in the left channel. If I short the input is is quiet.

Q: please advise on where this very signficant potential difference (between pwr amp & AVR) could be coming from? Could this be anything to do with the hum?

I measure about 100 Ohms between audio gnd. and mains earth (?)

** Edit: Right?the plot thickens. If I disconnect the mains earth from the RB-1080, the hum disappears, as does the funny potential difference between the audio GNDs of the Rotel and the AVR?

(Also, please help me to find a schematic, one from the RB-991 might also do. As a matter of interest, my Rotel RB-1070 does not have a mains earth pin, and I measure continuity / very low resistance between audio earth and mains earth)
 
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