Thanks Nick I'll do that for sure.   Yes when I turn it off I am not even touching the TT so any vibration would have to be some kind of mechanical switch in the TT.   Which is what the mute relay is I gather.

It just cut out again but this time I tried to jiggle all the wires coming out the back to see if it would kick on.     Nope,  it seems to be electrical in nature and not mechanical.    A 1 second on/off and its fixed.     So much for the loose wire theory.    Probably has something to do with that mute relay.   Like it builds up a charge in it and powering it off resets it somehow.  Strange for sure.

When it comes to old electronics, nothing is starnge 😁

Put a voltmeter across the muting relay coil and see if the voltage changes during play.

Martin

It occurs to me that the muting relay (should) cycle whenever a record ends but Rajod states it takes ~5hrs for the defect to happen. 

Does the relay work when the tonearm goes to the parked position?

How about putting a Flukemeter in record mode across the problematic output and seeing what changes after several hours? Or, checking the values of both channels when normal, and then again once the problem appears. This should include the signal pairs as well as the signal to chassis ground values.Knowing how and what changes would be good diagnostic information and may provide a clue. 

Yes the mute relay works in the parked position.    However one time when in parked position I could hear static coming out of the speakers.  Which would point to the mute relay not quite muting in the parked position.    But that has not happened in months.   

Is that a electrical switch or mechanical?    Cleanable or just through it out type part?

The muting relay has mechanical contacts, operated by a solenoid.
If the controlling electronics fail, the voltage to the relay coil could be creeping up or down over hours, causing
the relay to "slowly" activate its contacts.
It's a theory, at least.
You can also desolder the relay and remove it completely. The Beogram will play without it though you will then
also hear a little noise when the needle lands and lifts but it's not particularly loud or annoying for a test.

Martin

How about I just spray contact cleaner on them to start?

I am not sure how that relay works but does it not mute both left and right channels at the same time?   It only seems to affect the left channel and only by around 50 percent in apparent drop in loudness.     Wonder how its affecting just one channel and not both?   Its a mystery.

The bronze contacts inside the relay are separate.
One of them could even be bent or partly stuck.

Contact cleaner is good when you can NOT get closed mechanical contacts to conduct current.
It will not prevent closed mechanical contacts from being conductive.
It does not isolate, it losens oxidation (and doesn't remove anything, it merely losens the junk and allows it to
shift around and in the best of cases it doesn't add too much to the mess).

Martin