Hafler DH-500 in Protection

Discussion in 'Solid State' started by moodydan, Jan 12, 2018.

  1. moodydan

    moodydan I love the punk rock. Subscriber

    Messages:
    615
    Location:
    Versailles. KY
    Not really familiar Hafler equipment and this one has me stumped. I've looked at several other threads about this same topic, but I'm not having much luck. This DH-500 came to me this way - powers on, switch light is solid, but does not come out of protection. I've replaced capacitors and transistors on both channels of power amp, and replaced caps, tranisistors, and diodes on protection board (PC-9A). Also replaced Protection Relay with NOS relay.

    Transistors Used:
    2N5550 --> ZTX694B
    2N5401 --> ZTX795A

    Diodes were straight ahead replacements (1N5240B, 1N4003, and 1N4148). Caps were also value for value.

    I measured DC at one side of the relay and got ~20mV; I jumped the relay and have good output for prolonged listening without excessive heating, but I'd rather keep the relay in circuit.

    Do you folks have any advice?

    Thanks,
    Dan
     

     

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  2. moodydan

    moodydan I love the punk rock. Subscriber

    Messages:
    615
    Location:
    Versailles. KY
    For anyone interested, R105 (27kΩ, 1/2 watt) on protection board was dead. The guys at DIYaudio helped me track it down!
     
  3. slimecity

    slimecity Super Member

    Messages:
    2,520
    Location:
    New Zealand
    I think first you need to ascertain whether the amp is in protect due to high DC offset or not.

    If its high DC offset - then the protect circuit is working properly and doing it job - and you need to find why you have high offset coming from one or both power amp circuits.

    If you have normal DC offset - then there is a fault in your protect circuit.

    Measure DC offset on the amplifier output side of the relay.
     
  4. moodydan

    moodydan I love the punk rock. Subscriber

    Messages:
    615
    Location:
    Versailles. KY
    DC is fine. I had a bad resistor in the protection circuit.
     
  5. slimecity

    slimecity Super Member

    Messages:
    2,520
    Location:
    New Zealand
    sorry I didnt read your earlier post fully.

    Do you have a schematic of the protect circuit and can you post it here?
     
  6. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    25,710
    Location:
    uk.. the middle bit
    next time study how the protection works . you may have guessed this already .
     

     

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  7. moodydan

    moodydan I love the punk rock. Subscriber

    Messages:
    615
    Location:
    Versailles. KY
    It would have been nice. All I had was the service manual commonly available online, which has no helpful voltages listed and the protection board shown in it (PC-9C) is not the same version as mine (9A). So I was kind of blowing in the wind.
     

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