There is no real formal service manual for these units, just a schematic, and some adjustment data. While the BX3 and BX3-MkII share some of the same design, there are some differences, the primary being the amp boards. The BX3 has 4 amp boards, 1 for each channel. The BX3-MkII has 2, with 2 channels on each board. But the protection circuit seems to be the same, if all is well, then you are able to hear the speaker relays click when you press the corresponding button on the front panel. If you don't hear a click, it's in protection. If you are looking at the board that the fans connect to, on the BX3 that is the BF-40 board, on the MkII it's the B008A board. In either case, that is not the board the protection circuitry is on. The protection circuit is on the other back panel board. On the BX3, it is the BF-10 board, on the MkII, it is the B007A board.
I currently have both on my bench at the moment, so I have had some time to compare the schematics.
If you isolate the amp boards from the protection circuit, you can (hopefully) determine where the issue is. The amps do not need to be connected for the protection circuit to work, so if you isolate them, and you can hear the speaker relays click when you push the buttons, then the issue is with 1 or more of the amp boards, and not the protection circuit. If they are all isolated, and you still don't hear the speaker relays click when you push the buttons, the issue is with the protection circuit.
You can isolate the amps in 1 of 2 ways. Either by disconnecting the power to the amp boards, or the output wire from the amp boards. I am at work at the moment, and not able to look at the units, so this is from memory, and the schematics (my memory isn't that good
).
If you have the BX3 (not the MkII) then you have 4 amp boards. The MkII has a fuse for each rail for each channel on the Z01A board on the bottom of the unit. The BX3 does not have this board, or the fuses, On both units, each amp board has its own (large) blue and red power wires that all connect to the Z10A board in the case of the MkII, or in the case of the hacked up BX3 I have on the bench, pretty much directly to the large PS filter caps. These red and blue wires connect to the amp boards via push on connections, if you disconnect all of them from amp boards (Make careful note of where they go, or better yet, take pictures) then without power, the amps are isolated.
The second method is to disconnect the output wires from the amp boards, They are (I believe) Red, Blue, Yellow, and Gray. I don't remember (and the schematic does not say) what color goes to what channel. There are 2 wires of the same color close to each other on the boards, the smaller goes to the meters I think, or it is the input (
Edit: the small colored wires go to the meter board), in either case the larger is the output, Again, it is a push on connection.
If they are all isolated, and you still don't hear the speaker relays click when you push the buttons, the issue is with the protection circuit. If all 4 relays click when the associated button is pushed, then you have a problem on 1 ore more of the amp boards. Connect them 1 at a time till it does not work, that channel is at fault.
In either case
Turn the power off before you disconnect/connect anything! If you don't, you take the very high risk of releasing the magic smoke.
Try this, and let us know what you find.
