did you try replacing the fuses on the rear panel and the internal fuses? (unplug the unit first)
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I have the same exact amp. It is a great amp.
Things to check...
The fuse on the back is a ceramic slow blow 12A 125/250v (1-1/4" x 1/4"). You will need a meter to tell if this type of fuse is blown or not.
All four internal rail and speaker fuses are identical. fast blow AGC 6A 125/250v. These are glass fuses and you can visually see if they are blown. You might need to remove them in order to see them clearly.
The rocker switch on the front panel can go bad. Use a meter to determine if the switch has gone bad. If it has, the exact same one can be purchased from Mouser Electronics, part 979-1555.312
Assuming it's not the front rocker switch, then there might be a bigger problem. I owned a EX-442 Sonata, and twice over a period of about 3 years, several of the output transistors failed. This in turn blew out the internal rail fuses. B&K thought the probable cause was short circuiting. I'm very careful of that sort of thing and was reasonably sure this did not happen. Although I have no logical explanation for the failure. I asked the designer that when the rail fuses blew, why this didn't protect the output transistors. I said the rail fuses seemed sort of redundant if they didn't protect the output transistors. He seemed a little irritated with me for saying this,and gave me some technical explanation of which I don't remember as this was back in the early 90's. Still seemed a bit strange that the fuses failed to protect the unit especially when it happened twice. I only know of this happening to my unit and no one elses. So not sure if this was/is a common problem.
As written above, I just discovered that the maid pushed in my trigger switch on the ST-2140. Now I don't have to buy a replacement amp. One more thing to be grateful for this Thanksgiving Day. Before this event I was completely oblivious to the trigger switch, but I am not a high tech guy in any way. B&K and B&W Matrix 804 are a very good marriage.