I agree the most likely cause is a faulty balance slider, they do age and go faulty. Either the carbon track deteriorates or the plastic carrage mechanism fractures. Typical probs are when sound comes and goes, scratchy with blank spots, steps in volume. In my case the actual slider/wiper mechanism had broken leading to major surgery. I did a web search for a replacement slider, alas non found so I very carefully rebuilt the existing one which thankfully is still working fine. I did wonder if to contact a B&O dealer for spares but the thought of the encounter put me off.

To get access the slider you need to remove the rear vented cover and then the two screws near the lid hinges. Half open the lid and gently wiggle the whole cover loose. It is held by plastic tabs that mesh with the sloping front.

This may seem like a silly suggestion but make sure the 1900 is the prob and not the leads/sockts to and from the speakers, it may be worth doing a quick check with headphones to make sure the fault is in the 1900.

Hi Bart. I think it might be a good idea to send a PM to Martin (known as Dillen on here) If he hasn't got one, I'm sure he will point you in the right direction.
Also try  Frede (Known as Classic) he has the patience of Job when it comes to repairing small parts of B&O equipment.
Good luck!

Andy T.

Solder out the pot and take it apart.
Be careful, especially with the bronze parts and their tiny coal tips (avoid touching the tips or they will most likely fall out and become lost).
Clean the black plastic "sliding bridge" using isopropanol (tapehead cleaner) and glue the bronze slider back into place using superglue.
Give it an extra tiny drop and leave it to settle completely (overnight). No glue up the bronze arms and none on the tips either, only around, under and on the base of the bronze part.

Now would be a good time to clean the tracks in the bottom of the potentiometer. Give it a bit of contact cleaner, leave it to soak for a couple of minutes and then wipe it dry using cotton buds.
Clean and dry is good.
Don't just spray contact cleaner in and leave it there.
It will losen the oxidation and shift it around alright but it takes human action to take it out.

Put the pot back together and solder it in.
If one pot is bad, then the other two are probably going too, check them all (don't mix them up, they are of different values!).
Finally, while in there, take a look at the radial type capacitors at the main board, check if they look cracked or like they have started to leak something.
If so, grab a cap kit and replace the lot. There are
about 60 of them in there and you'd be amazed at
the difference. A lamp kit is also available.

Martin

My Beomaster works again! I'm really amazed by the strength of the superglue. When closing the pots the metal sliders are pressed down quite a lot, which takes so much force that I really thought the glue wouldn't hold. But it's been playing songs now for two days without breaking (knock on wood), so I suppose the fix worked out allright! Thanks for all your help! I'm enjoying my Beomaster again.

 PS. I also checked out all the caps that I could see with the top part removed and they all looked fine. 

A picture of the broken slider:

The left one is broken the right one is still OK. As you can see I had to remove a lot of grease before I could start with the superglue. The plastic tab that has broken is really small, it's about 3.5 mm long and 0.5 mm wide.

 I've forgot to take a picture after the fix.