In your Beogram, apart from the muting relay, there are no electronics in the signal path so any sound related problem will have to be with the cartridge, the muting relay, leads and plugs.
When un-muted (playing with the arm down), you should be able to check the continuity directly from the tonearm cartridge socket pins to the DIN connector.
When muted, both signal outputs are grounded. The relay mutes when not powered.
What amplifier are you using ?
Martin