Is anyone nice there over the Pond willing to lend Andrew a Beolink 1000, just to be sure?

It sounds as though the second MCP (and possibly the first one) needs a new infra red opto coupler on the volume control. When you operate the volume control the microswitch which you have cleaned tells the MCP that you are turning the control and the direction in which you are turning it, the actual control pulses are generated by an alternating black and white printed strip around the outside of the wheel passing in front of the opto coupler. I think what happens is that  dust collects around the wheel and causes a static charge which eventually kills the opto coupler! The part number for the opto coupler is Siemens SFH900-2 (SFH900-1 also works fine) and they're very hard to find! The good news is that they're not hard to replace and if you can't find one I've got one. :-) If you PM me with your address I can send you one. 

I see what you are saying Beoselecta. Maybe before I have one sent to me I should try to swap the one off the bad MCP to the good one. The good one did have some dust and debris in it that I cleaned out with a dust can. Also, while it wasn't perfect, the first one could at times get the volume indicator to adjust.

The Opto Coupler, is it the three solder joints that you are pointing at in the picture? Or is it the chip right next to it?

----edit----

After a google search of the Siemens PN, I see the item you are referring to.

Hi Andrew, sorry about the slightly vague photo, I couldn't figure out how to post two pictures! Here's a picture of the other side of the board (yes, it's the three solder points I'm pointing at). While it's certainly worth trying the one from the other MCP I've got a feeling you might find that it's shot as well. From your description of how you could sometimes get it to work, it sounds like it's failing too! They do go bad quite regularly, I've just finished refurbishing four MCP's and three of them needed new opto couplers!

Here's another picture from the front this time.

Ah, I see. Is it like a hall effect sensor then?

Andrew wrote:

Ah, I see. Is it like a hall effect sensor then?

It is one! Also used in the Beomaster 8000 for volume and tuning, and on the Beogram 8000 to count motor turns. All very nice! 🙂

It's not in fact a hall sensor, it is an infra red LED and detector side by side in the same package and it "sees" the reflected light from the white areas around the wheel. A hall sensor is a magnetic field detector and works on a different principle altogether, although they can both be used to achieve similar results.:-)

Yes, of course, like the ones in old cassette decks that were used for auto-stop - Beocord 5000 anyone?

Beocord 5000? It's been a long time since the last one on the bench, but isn't that a round rotating magnet and
a reed relay next to the tape counter?

Martin