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Technics SA-EH600

Discussion in 'Electronic Repair' started by Alan Liefting, Jan 14, 2009.

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  1. 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
     

  2. 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.
     
  3. If the output stage relay never activates how can the microprocessor
    detect any psu issues?

    Alan
     

  4. 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.
     
  5. 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
     


  6. 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.
     
  7. Arfa Daily

    Arfa Daily Guest

    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
     
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