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

"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.

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

"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.

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

"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.

"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

Bad joints on the regulator transistors are very common on these. I've also
had fusible resistors go open for no apparent reason. If you still need
schematics, mail me off-group from a valid address that can receive
attachments, and I'll see if I can sort you something out.

Arfa