Hafler DH-500 in Protection

moodydan

I love the punk rock.
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
 
For anyone interested, R105 (27kΩ, 1/2 watt) on protection board was dead. The guys at DIYaudio helped me track it down!
 
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.
 
sorry I didnt read your earlier post fully.

Do you have a schematic of the protect circuit and can you post it here?
 
sorry I didnt read your earlier post fully.

Do you have a schematic of the protect circuit and can you post it here?
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.
 
I know this thread is dead and buried but here’s the schematic, I too have a similar problem but I’m also in the process of completely rebuilding my 500 from the ground up with more adequate wiring.
 

Attachments

  • DH-500_amp_man.pdf
    838 KB · Views: 24
Test points as in the board connections to the output stages?

#3- power supply +90VDC

#4-not sure, +1 volt or so

#5,6, and 8 are the output of the amplifier so ideally 0VDC but could be a few mv + or -.

#7- ground

#9-not sure, -1 volt or so

#10- power supply -90VDC

Craig
 
I know this thread is dead and buried but here’s the schematic, I too have a similar problem but I’m also in the process of completely rebuilding my 500 from the ground up with more adequate wiring.
Maybe it's archived but i'm not able to open the schematic. Hopefully it is for the earlier PC9a protection circuit not the later PC9C?
 
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