If the output stage relay never activates how can the microprocessor
detect any psu issues?

Alan


Because the speaker relay drive is only one of many microprocessor
functions. The micro (generally) looks at all this stuff before it ever
energizes the relay.
I do protection related troubleshooting all the time. Different
manufacturers do it differently, but generally there are sensing lines
tied to the power supply, the output stages, and often to thermal
sensing devices (thermistors, posistors) mounted on the heat sinks.
There is associated circuitry between the micro and the monitored
stages; resistors, gate diodes, buffer transistors, etc.

Mark Z.

Yeah I thought that may be the case. It is now getting difficult to do
much without the service manual.

However, I would have thought that a lot of the sensing was done by
analogue hardware within the hybrid o/p ic?


Alan