Hitachi TC 400

OK, the green and the pink wires are the base and the emitter of the power transistor.

Let's assume that the output stage is class A, and the collector is connected to (??where) The red wire coming from the transistor is connected to the collector, which is apparently isolated from the heatsink -- where is it connected?

BTW, the transistor appears to be a fairly wimpy 6.5W NPN transistor.

Only 1 connection to the collector seems odd.

The photo we're missing is one looking down on the board over the heatsink -- a top front view.

 

Always worth posting extra information in a new post, otherwise the thread doesn't get marked as active and nobody will look...

It looks like the red wire has become detached too now?

To me it looks like the wire still connected is connected to ground. The other end of the speaker will almost certainly be connected to a capacitor that is, in turn, connected to either the collector or the emitter of a transistor (and possibly that power transistor)

trace where the wires from the transistor go (one is base, the other is emitter, the collector is connected to the transistor's case) and see where they connect on the board. I'd be looking for one which leads to a relatively small pad or track that has a resistor and a capacitor connected to it. the other end of the capacitor should currently be connected to nothing. That's where the speaker goes.

Hopefully it's that simple.