And the relevant bit of the circuit:

I think this relay on the Dolby board is on the signal path between the playback amplifier and the Dolby processor.

I removed it and find that one of the normally closed pole pairs is intermittent or high ohms.

It may not be the whole story but it needs replacing so if anyone has a decent used one they are willing to part with, please PM me.  I've not had much success trying to repair them.

They are now obsolete but various sellers in China stock the Panasonic NF4EB-24v which I think is the same. 

I will replace the 4 pole c/o relay with 2 Omron 12v relays with the coils in series.

The faulty component in the signal path turns out to be the NE545B Dolby processor.  The signal was going in but not emerging.

My relay replacement for the old National relay is in place and now provides a reliable signal path.

The breakout pcb was made for me in China (5 USD for 10 plus shipping 🙂 ) and arrived at my door in the UK less than nine days after ordering.  A couple of Omron G6K dpdt relays provide the four way switching function.  I mounted them in such a way as to observe coil polarity as the Omrons have a polarised coil.

I replaced the Dolby IC and normal service is now resumed at least as far as playback is concerned.

Which is not to say there aren't a host of further problems that need fixing which will keep me busy through lockdown.

Merry Christmas,all.

One problem which caused a certain amount of head scratching is that sometimes the tapehead bridge only retracts to the Pause condition (semi-retracted) when Stop is invoked after Play - rather than the Stop condition (fully retracted).  In the Pause condition, the cassette cannot be ejected and I had to cycle through Play and Stop again to get to a condition where it could be ejected.

Performing mechanical adjustments on this deck requires B&O adjustment tools which I would guess are now as rare as hens' teeth.  My (hopeful) suspicion with this fault was that it was an intermittent failure of the analogue control circuitry rather than a mechanical misadjustment of the solenoid which drives the tapehead bridge.  Without the necessary experience and tools, my mechanical 'adjustments' would be unlikely to end in convergence.

The obvious first step was to check the pull-in switch board (PC27) which drives the two solenoids.  Voltages in Play mode seemed to be ok, but I pulled the board (a bit of a faff) to check components.  Everything seemed to be in spec, but as a precaution I changed the 1N4148 diode and the 18v Zener that clips off any peaks in the unstabilised 30v supply.

I have no idea if changing these components could have made a difference, or whether reflowing the board has helped, but the original intermittent problem after a few days has not reappeared.  Yet, that is.

If anyone can throw light on this that would be great.