Technics SA-EH600

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

Reply to
Alan Liefting
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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.

Reply to
Mark Zacharias

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

Alan

Reply to
Alan Liefting

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.

Reply to
Mark Zacharias

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

Reply to
Alan Liefting

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.

Reply to
Mark Zacharias

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

Reply to
Arfa Daily

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