If everything works except for the volume it could be a dirty microswitch or dust on the optocoupler, both are fixable.

Master Control Panel 5500 volume wheel diagnose:

Make sure good batteries are in.
Rotate the volume wheel.

- If the MCP lights up but volume is not changed, the wheel switches are fine and the opto dead.
It's a three-pin device reading the black/white pattern printed onto
the side of the (inner part) of the volume wheel. The opto and its light output is known to degrade.
I have a few original, factory new optos in the drawer.

- If the MCP does not light up, as in no reaction at all, the switches are bad and will need cleaning or replacing (or something else is wrong).

Martin

I had one which looked like the opto was dead but when I dismantled it there was too much dust in the housing for the light to get thru'.

I cleaned it out, re-assembled and it worked fine! 

Well, I am totally hopeless at things like this! Is it relatively straight forward to dismantle + clean etc? Or should I take it someone - and if so, does anyone have any idea who?!

Thanks!

I'll try the suggestions re lights etc and report back.

Right, I have checked it. It just doesn't do anything. It used to work intermittently, then more rarely. Now - nothing. Rotating it has now effect at all. And I imagine that it is gunged up - I have a log burner in the same roomwhich kicks out a LOT of fine dust / ash, and the MCP has been in the same room for YEARS.

Do I just undo the scrws underneath to dismantle it? If I manage to get it apart, is there a recommended  cleaning fluid or something? Could I use the same stuff that I clean my apple keyboard with?

Hi, when I bought my Beosystem 5500 I had the same problem with MCP 5500. What I did: I dismantled the rear lid, it was full of dust inside!!! Cleaned very well the rotary volume wheel (I was surprised, looks like a ball-bearing), cleaned very well three pins device with very soft brush and with some air-flow spray (the same as for the cleaning of computers keyboard). Then I put small spray of WD-40 (a cleaning and lubrication liquid) onto pins and the "ball-bearing" and cleaned from dust everything else inside the MCP 5500. If you do this-be careful-clean unused lubricating liquid, it's very "oily"! Then I put new bateries...and MCP works as new :) Good luck if this will solve your problem. If not-maybe you need more expert help from other people in this forum, I'm just a beginner :)

Thanks so much for this! beobeetle, I did as you suggest - got a can of compressed air and blew dust away. I will get some WD 40 - the volume works again perfectly, but the wheel now squeaks a feels slightly "graunchy". I think some dust must have gone into wher it shouldn't so I'll lubricate it a little. 

But - I can now alter the volume again!

(I aws stunned by the inside of the MCP - doesn't it look COMPLICATED!)

Good job !
But don't use WD40 in anything B&O. Keep it in the garage, it's good for losening rusty bolts on old cars.
You would know why if you'd seen what I have seen in turntables etc.

Take the things apart and clean them. WD40 does not remove dirt, it just shifts
it about and may even add to the mess.
If anything needs lubricating, I suggest using a good acid-free oil, f.e. sewing machine oil
or a thin silicone grease.

Martin

Dillen, Thank you for your remark! I'll keep it in mind. Unfortunatelly I had no idea what to use :), so I choose this one...and you can imagine how difficult was to clean unused liquid (but I did it :)

So andrewl, use what Dillen sugests. Good luck!