I've found out what was causing the problems in the left/right channels. It was the balance potentiometer i had repaired earlier. I hadn't had pinched the potentiometer assembly back together tight enough, so there was some loose movement in it.
Does anyone have a picture of a wall mounting bracket for a BM 1900? I'd like to place the BM to my livingroom wall, but i'm not sure how to do it yet... I was thinkg i'd hang the BM directly to the wall so it would hang exactly vertical. But i've understood the brackets (atleast for a BM 4500) will position the BM slightly canted. If i hang the BM vertical will the lid stay slightly open all the time?