Some time it took but here are the results. I took out the speed changer mechanism and cleaned it thoroughly. With some PRF 7-78 (contact cleaner and lubricator) I was able to get the fine-speed-adjust mechanism moving: it was just jammed. Unfortunately the knob's "plug" for it was already cracked (by trying to turn it with force) - it can be still adjusted if you pull of the knob and use pliers! I adjusted it with the stroboscope stripes and decided to leave it as is (knob in place but no fine-adjusting by it). I don't use that adjustment too often, anyway.
The 33/45/78 rpm selector is now working perfectly. Once I had already assembled the whole thing just to note that it was running only in 45 and 78 rpm modes (because of my mistake in assembling the mechanism) but after fixing that there has been no trouble with speeds at all.
I adjusted the arm with some help from this thread: http://forum.beoworld.org/forums/post/31410.aspx Also as I have a B&O stylus pressure scale (as seen in here: http://www.beoworld.org/prod_details.asp?pid=1254) I checked and adjusted the pressure to be something around 2 grams - as told.
Then the excaiting moment: testing it with a record! Yes - it works - but only one channel present! Quick check with oscilloscope showed that there was signal in both channels. After resoldering the DIN plug still the same - one channel only when amp connected. Then I got it: the cable itself has a dead-spot and it worked only in certain angles. I will replace the whole cable - it's anyway yellowed and dirty. After taping the cable to a working position (for testing) - finally both channels output and everything works! It sounds quite good indeed. I had only one dusty record and it sounded quite crackling thus. Also I noted some distortion, especially with louder passages - could this indicate worn stylus? I will experiment with the stylus pressures and better records tomorrow.
It took around 6 hours but it was worth it - seeing a living BeoGram is always worth working. :)
Bang & Olufsen - The art of controlling sound, picture and light