Check the IF-transistors.
They are known to develop tin whiskers and short from the die to the casing.

Martin

Thanks for you response Martin!

I'm a bit of a novice when it comes to repairs. What is the best way to check the IF-transistors?

DC readings or continuity from emitter/collector/base respectively to ground, would be the easiest.

Martin

How do i do this easiest? As I said before, I'm not that technical and would be very sorry if I accidentally did something wrong. Just to be clear, am I supposed to screw it open?

(sorry for the late response, I was very busy)

Yes, of course.
You cannot measure, diagnose or repair anything from the outside.

Martin

When I unscrew the back of the Beomaster. Where do I look for and how do I recognize if a part is broken and should be replaced?

You don't unscrew the back of the Beomaster.
You take the four dial knobs off, losen the screws at the bottom of the cabinet (either two or four) and push the chassis out of the cabinet.
You can push on the four spindles where the knobs sat.

If your Beomaster has built-in speakers, their internal connecting plug must be disconnected when the chassis is apprx 8-10cm out.

With all due respect - and don't get me wrong, but from what you write, I don't think this repair sounds like a job for you.

Martin

As a matter of fact I have already removed the four large buttons once. This was to remove the protective plastic that was still on it from the factory. So I already have experience with this, and I am capable to do this again.

Then carefully sliding the chassis out should also not be a problem. My problem, however, is that I have no experience yet and don't know what to look for once I have pushed the chassis out. If you could describe that to me, then I would most sertainly be able to do it. As for the speakers, they are built-in so I would have to disconnect them. 

I don't plan on opening the radio until I know what to look for and what to do. When this indeed turns out to be beyond me, I know how to find my way to a B & O dealer or refurbisher. Then I will then contact them to see if they can do something about it.

You will be looking for bad transistors in the IF-stages.
Check for continuity/shorts, particularly from the screening pin to the emitter, collector and base pins.
You can also measure the DC voltages on the pins and compare to the readings in the service manual.
If bad, they must be replaced. Don't use the original type again - they will all be bad by now, even if from unused old stock.

Keep in mind, that Beomaster 900 has positive ground/chassis.

Martin