First Fixes

I opened the BM and I hooked up board 6 (5/10/±15 power supply) to external regulated DC supplies. I also disconnected all cables from the board to isolate it from the rest of the BM8000. The pic shows the connections directly to the main reservoir caps (C31/33/34).


I limited the current in each of the supplies to 250 mA. Then I turned them on simultaneously via a switchable power strip. Immediately the -24V arm went to zero volts (and 250 mA), indicating a short circuit. Inspection of the reservoir caps immediately showed that C34 was short circuited...nothing unexpected in these BMs!

I replaced all three reservoir caps, and C21/39/40 for good measure (they were those red cans that also like to malfunction after 30 years). There was a little snag when I tried to replace the big ones: They were glued to the circuit board with silicone. Heating up everything with my wife's blow drier softened the silicone, enabling removal w/o jeopardizing the cheapo easily crackable circuit board (one really wonders why they did not use fiberglass boards in these quite expensive receivers...). The pictures show the silicone blobs and the board after replacing the caps with new ones.


I ran the board again with all other connections not connected, and the currents now are ~30-40 mA in the ±23V arms, while the 10.4V supply showed ~10 mA (note that the grounds of the 5V and the ±23V systems are not connected when all plugs are disconnected from the board, i.e. a connection must be made between both grounds to run both systems with the external supplies at the same time-the picture only shows a ground connection to the ±23V system).