Pioneer SA-1040 stereo amplifier - 2 out of 4 channels not working

[metalgod_dave]

Hello all, I'm hoping someone can help me with this.

I have an old school Pioneer SA-1040 4 channel stereo amplifier with only 2 of the 4 channels operational.

As in, the system in on, audio is playing, 4 speakers are connected to their respective terminals, and only 2 are working.

I've tried swapping speakers around on different terminals and also different speaker settings.

There is no visual sign of burned or failed components that I can see.

I'm assuming it's the channel transistors themselves, but I could be wrong.

The amp consists of one main board, there are no slave boards or modules.

They're not the best but I've uploaded a few pictures......front, main board, channels.

Any and all comments welcome.
Thanks.

[Enzo]

I am confused when you say you have four channels in a stereo amp. Stereo means two channels. Could it be that you have left and right channels, and then for each you have Speaker A and Speaker B connections? In other words you could have two different speakers connected to the left, one for A and one for B, and two more on the right? ACcording to the service manual, that is what you have.

If that is so, WHICH two speakers do not work? Both on side left or both on side right? Or maybe both left and right work on the A connections but neither work on B, or vice versa? This is important. If the left speaker (or right) does not work on both A and B speaker terminals, then that channel of the amp is faulty. But if left and right work on A or B, then both channe4ls are working, and you have an issue with the connections.

You are aware there are switches on the front for both A and B speaker terminals? You have to have both buttons turned on for both sets of speakers to work.

[soundguruman]

You probably have a bad speaker switch or speaker relay.
It's a 2 channel amp, not 4 channel.

[oc disorder]

From looking at the schematic diagram (not the block diagram) it appears to me that four speakers have to be connected for both sets of speaker terminal outputs (A & B) to work.

Firstly check both the left and right channels are ok (this could be checked with headphones and using the balance control).
Then check Speaker A output Left and Right with button A on front panel pressed in.
This should also work.

Pressing in Speaker B button on the front panel should then stop all output if no speakers are connected to Speaker B output.

Conversely if speakers are connected to speaker outputs B and only button B is depressed
sound should appear at the speakers until button A is pressed .

The reason is..... The speakers from the respective outputs are placed in series by a clever switching arrangement.

This arrangement is to protect the amplifier from driving a too bigger load when all 4 speakers are connected.