Replace the idle current trimmers and set the idle current as shown in the servicemanual.
If the hum continues, check the filter capacitors and rectifier, both for the amplifiers and for the low-voltage power supply.

Martin

I had something like this some time ago, Peter kindly loaned me his BM4401 to compare with my BM4400 units....at one point while listening to Peters 4401 I noticed a disturbing hum from both speakers, this wasn't there the day before or I would have noticed, I looked at everything like yourself and drew a blank...by chance before I powered down everything I switched off the nearby standard lamp I have in the room and the hum stopped! this is an upright lamp with a transformer feeding several halogen bulbs, bit naff really but gives good light onto my work area, this was somehow causing noise that the BM4401 was picking up....yet my BM4400 was unaffected.

Just a thought

Thanks Martin and Craig! I appreciate your input!

I did try switching off all the electrical devices in the room but it did not seem to make a difference. I believe by now all the bulbs we have are LEDs, so I turned them all off, as well as the internet router; made sure the dishwasher, washing machine and such are all turned off. However, this did not seem to make the hum go away. I very much wish there was some easy fix to this, so perhaps I'll try once again tonight. 

If this doesn't help then tomorrow I'll take it back to the shop where they did the renovation. I don't know how to do any of it myself (the most I soldered in my life was jack plugs for my guitar cables, and they never lasted too long), however it sounds like it could be a relatively easy fix...? Perhaps this would also be a good opportunity to replace the faulty balance potentiometer with the one Martin kindly sent me. 

Piotr

I have had a similar problem with my Beogram 3400, type 5726.  I discovered by accident that when moving the cords (not power cord) that go from the turntable to the receiver, the sound would increase or stop depending on how or where I moved the cord.  Then I noted  when I actually sort of bent the cord at the plugs, the humming stopped and I was able to play records with no problem.  So I intend to replace the plugs with new ones and see how that works.  FWIW, the turntable is properly grounded.

we'll see

Ed

Just an update, even though it has been a while... Shortly after I wrote the above posts about the hum the front right channel went out, so I took the Beomaster back in for service. It just came back, it took six months (the vintage audio people around here seem to be overwhelmed with the amount of work they get). They replaced the balance potentiometer with the one I got from Martin and also changed the filter caps - they were the only capacitors that were not replaced the previous time.

Thanks to the new potentiometer channel #3 is back.

However, the hum is still there. It appears after about 1-1.5 hours, as before. I did a small experiment and placed a computer fan on top of the Beomaster, where the vents are, that was sucking all the heat out. After a moment the hum became more quiet, to a point were it almost disappeared. I think this suggests e.g. a cracked solder joint somewhere, that expands under heat and causes a ground loop somewhere inside, or something of similar sort. In any case, before I find someone better than the previous guys to fix this, the quick solution I will try is to place small cooling fans behind the receiver to see if this helps to control the temperature and keep the hum to a minimum. 

Again, thanks for the suggestions! 

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Don't adapt the Beomaster to the fault.
Repair the fault.

Check those idle currents!
One or more may run astray when the Beomaster warms up.
If one of the cooling fins at the back gets warmer than the others after playing for a while - that'll tell you the channel with the problem. 

Martin

Thanks for the support, Martin. I know what you're suggesting is the proper way to do it, but it will require some patience and a skilled tech.

What I can say is that I gave the previous techs the complete manual and schematics (BM2000+BM3400) and they have set the idle current according to the specifications. The cooling fins are at normal temperature the whole time, don't even get warm to touch (they did in fact become hot quickly before the service).

I imagine that, for instance, there might be a buildup of heat in certain areas inside, that causes some cold solder joint to expand/contract - I had a similar situation in a Sansui receiver, where the FM scale lights would alternate going off and back on as heat fluctuated - the solder joints for the lightbulbs were known to crack. 

The problem is not a solder joint.
If all idle currents are fine - also after an hours playing, replace the low-voltage rectifier.
It's a known troublemaker. It usually affects the tuner section first but can also cause problems in the preamp.

Martin