A friend of mine has a Bm1900 which has decided to have a mind of its own. After its been playing for 30-45min its starts going louder on its own then back to the normal level then louder again then back to normal level. Does it at untimed intervervals. All the caps have been replaced on this unit.

 Any clues?

Fingerprints or dirt inside the glas?

No glass on a BM1900 but the impedance thought is not bad.
It can be the volume control system itself that has taken
a too high impedance, maybe a transistor with a thermic problem,
presumably near the input from the touchfield.
Note that there are two different versions of
volume control PCB used in the BM1900/2400 (16 steps and 128 steps).
New version (128 steps) is mounted from serial nos:
BM1900 1826011
BM2400 1828014
Some older units may have been converted but that's rare to see.
Check (or simply replace, they are cheap) transistors TR23/24/25/26
Check that the little bronze forks from the touchfields each
catch their respective PCB pin and doesn't touch anything else.

In the newer versions a current leakage problem has been noted
in production regarding 3IC2/3IC3 but most often this results
in a non-responsive or slow-acting volume control or it will only run either up or down. It's worth checking anyways that resistor 3R62 is 330Kohms, replace if different.
Finally, make sure that the correct type and wattage of panel lamps are used, this is critical !

Martin

Thanks Martin

Very precise intructions as usual. I don't think its the lights as I replaced them  while ago with the wattage you said, was somewhere under a 1w. I'll replaced all those components that you have indicated. I'm sure it'll will be one of them.

1 Watt lights?

 Shouldn't be there 14V 0,04A lights installed?

... otherwise it becomes really hot.

B&O Service manaul says 12v 1w, but Martin gave another type which works better. Can't remember what the rating was

I think i used 14V 0,04A mini lights. A 14V light used with 12V has a much longer lifetime ... about 3-4 times more and a minimum of heat... and the symbols are much better to read, no bright spot. 

Reichelt electronic has these lights. 

The correct lamps are 12-15V 1W. Unfortunately some service manuals give wrong specs.

A couple of different glass envelopes can be found but all are interchangeable since they are soldered in rather than plugged. 

A lamp kit is available, containing a full set of correct lamps and mounting instructions.

Martin

I only had a look at the BM1900 today. I haven't changed anything yet but have noticed exactly whats happening on closer inspection.

The problem is with the radio. The stereo light doesn't come on (it is not burnt) and I'm not sure if the radio is playing in stereo. My friend says it isn't. The balance lights shine equal brightness but dim and do not change ever. Also when selecting tape or phono the balance lights stay on BUT when using the tape function (cd player playing through it) there is no problem with the volume controls. Only goes funny whilst using the radio. So i'm not sure which area I should concentrate on.

 Any more clues?

P.s. moving the stereo on or off make no difference. Sometimes when tuning in a channel I see a quick flash of the stereo light

You will need a fairly strong signal (antenna) to get the stereo light on.
The discriminator light circuitry is strongly dependent on correct lamp wattage.
With the wrong lamps mounted it won't move. It also needs a fairly good signal to show anything.

As everywhere else, first thing would be to replace capacitors and
check that the lamps are correct, also the volume indicator ones.
Bad caps can cause the strangest and most unpredictable faults.
First when you are sure all this is OK you can start measuring around - if the fault is still present.

Martin