"Alan Liefting" wrote in message
...
Mark Zacharias wrote:
"Alan Liefting" wrote in message
...
Got a Technics SA-EH600 home theater system with a fault. The display
comes up when first powered up from the mains but when the power/standby
button is pushed it says "GOOD-BYE".

The relay that activates the +/- supplies for the output stage never
activates. Did a basic check for shorts and opens but nothing was was
obvious.

Any ideas?


Alan Liefting


Blown channel. DC offset. Power supply issues. The usual. One can jumper
the power relay contacts and bring the unit up on a variac, then look for
DC offsets or excess current draw. Alternatively, one can use a
double-ended POLARIZED power cord plugged into the switched AC outlet on
the rear, and connect that to the variac. If there is a second switched
outlet on the receiver, plug it's own power cord into that one - this
would ensure that the microprocessor and smaller sub-supplies are
energized.
Might be a good idea to put a smaller main fuse in there or substitute a
100 watt lamp for the fuse. Then if there's a hard short, the lamp will
alert you by glowing bright. A dull glow after initial charging of the
power supply caps is normal.


Mark Z.

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.