I got a practical example why you really should disconnected the BM from mains before doing cabling issues in a MCL setup. The result was some bangs, maybe three and a dead BM 6500. The main fuse was blown but that did not resolve tho whole issue. When I connect the BM to mains the BASS, TAPE 2 and TIMER OFF are lighted and a small spark is seen from a relay unit under plastic cover. These are only signs of life in the BM in addition to the normal humming from the transformer.

Is there anything I can do myself, should I give up and take the unit to repair shop or or is the unit irreperable?

The unit is not irreperable but to many B&O dealer ships they are because of age.

A few might give it a look but they most likely do not have the knowledge to fix it.

Get in contact with Dillen here. He can help you out all the way.