"Alan Liefting" wrote in message
...


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



I don't do that much Technics brand, but yes, there were some Technics
models where the relay drive was from the output IC. This is by far the
exception however, and would not account for other necessary protection
functions - power supply balance, fast shut-off, power-down interrupt, etc.

I agree a service manual is probably necessary unless something pretty
obvious can be found, a bad fixed voltage regulator or some such.

Many if not most receivers have a fast shutoff feature which if
malfunctioning, will never allow the relay to fire. This usually consists of
a single diode off the power supply, lightly filtered by a small
electrolytic cap, anywhere from .47uF to maybe 10uF. These small caps
sometimes are dried out (open) and the constant DC the protect circuit is
looking for becomes a half-wave. The circuit keeps the relay held off. I've
seen this a fair number of times.

Mark Z.