I'm trying to repair a Bowers & Wilkins Amp675. It's an active base speaker.
The mains power comes into the small brown circuit board, which is mounted on the green circuit board. It is a low-power switch-mode supply, which powers the amplifier board.
The amplifier board is the one at right angles to the other ones.
The small brown circuit board also has a relay which switches power to the main switch-mode power supply. The main switch-mode supply is under the aluminium plate. Power from that goes to the green amplifier board. The speaker connections are on the right of the green board.
When the control switch is turned on, as signal from the amplifier board goes to the small power supply board, which turns on the relay for a few seconds.
There are two wires from the amplifier board back to the small power supply board. They are labelled "OK1" and "OK2". The relay will only stay on for more than the few seconds if neither of those wires are pulled low.
On the one that I am trying to repair, the OK2 wire is pulled low when the amplifier is turned on, so the system gets shut down after those few seconds.
I can't see any faults. The amplifier board is a multi-layer one, so hard work to trace the circuit. I can't find a circuit diagram anywhere.
Any suggestions on how to go about repairing this?