Hafler DH500 Channel Out Help Needed

[KingWzrd]

Hello everyone, I have a DH-500 that recently had the right channel stop putting out sound. I've read a few posts about ensuring the relay clicks when the amp it switched on (it does) and all fuses inside and the back are not blown. The light is normal on the front of the unit and the fan is spinning like usual.

I've read the manual that states, "If the relay will not close (no signal at the output, with an input signal) check for excessive DC offset (over 1.8 volts) from either channel at locations 7 or 8 on PC-9-the small relay circuit board. This indicates a defective amplifier channel, requiring competent service. If there is no indication of excessive offset, the fault is in the relay or
on the PC-9 circuit board."

For fun, I checked the voltage at positioms 7 and 8 on the PC-9, and they were -.7mV and 332mV. I don't really know what these values indicate but they are within the range stated in the manual and the relay closed, so this leads me to believe its not the PC9 or the relay.

In the photo. You can see there is a burn spot under the sand resistor, but there is the same burn on the other channel as well. I made sure the solder connection was good and it seems to be.

Edit** I just found this posting, and my problem appears similar to them: https://audiokarma.org/forums/index.php?threads/hafler-dh-500-channel-out-help.117548

How should I go about testing each component of the amp board? Do I unsolder all the wires on the left and right to remove it from the amplifier entirely before testing components? Or do I test them while hooked inside the amp?

Thank you for your time and help!

 

[ivandezande]

Well something had previously blown up on both channels, super weird.
I replace all the small signal transistors on every DH-500 I see. I've had the bias transistor stop working, the DC offset be over a volt, and no signal at all. The heatsunk predrivers and drivers can stay.

If fuses are all good, relay clicks - Start there. And yes, check those dropping resistors that were mentioned.

Does the channel bias? You're sure it's the amp and not the preamp?

 

[loudnoises]

Depending on when yours was made, there is a resistor in the p500 on those boards that is a known failure point. Consensus was it was just a bad batch though. If i remember correctly, it may have been r693? I'll have to go look tonight. It was in the upper right corner, may have been one of the two next to the film cap there. Had one go out on me years ago and it's been happy since.


One of these iirc.

 

[KingWzrd]

Hello everyone, I have a DH-500 that recently had the right channel stop putting out sound. I've read a few posts about ensuring the relay clicks when the amp it switched on (it does) and all fuses inside and the back are not blown. The light is normal on the front of the unit and the fan is spinning like usual.

I've read the manual that states, "If the relay will not close (no signal at the output, with an input signal) check for excessive DC offset (over 1.8 volts) from either channel at locations 7 or 8 on PC-9-the small relay circuit board. This indicates a defective amplifier channel, requiring competent service. If there is no indication of excessive offset, the fault is in the relay or
on the PC-9 circuit board."

For fun, I checked the voltage at positioms 7 and 8 on the PC-9, and they were -.7mV and 332mV. I don't really know what these values indicate but they are within the range stated in the manual and the relay closed, so this leads me to believe the problem is in the PC-9 board. Am I on the right track? If so, what could

My problem appears similar to these: https://audiokarma.org/forums/index.php?threads/hafler-dh-500-channel-out-help.117548/

Depending on when yours was made, there is a resistor in the p500 on those boards that is a known failure point. Consensus was it was just a bad batch though. If i remember correctly, it may have been r693? I'll have to go look tonight. It was in the upper right corner, may have been one of the two next to the film cap there. Had one go out on me years ago and it's been happy since.

View attachment 3277627
One of these iirc.

Really appreciate the help! I'm not experienced doing diagnostics, so I thought I'd remove the board from the amp before probing around and testing values. Is that necessary?

@ivandezande Can you describe what you mean by "does it bias"?

 

[ivandezande]

Really appreciate the help! I'm not experienced doing diagnostics, so I thought I'd remove the board from the amp before probing around and testing values. Is that necessary?

@ivandezande Can you describe what you mean by "does it bias"?

Get it to a tech…..this is not the amp to be working on if you don’t know what bias is.
Lotta current, lotta volts.

 

[loudnoises]

I didn't need to remove it from the amp, may have dislodged the board but didn't touch any wires. It was a number of years ago.

As has been said, this isn't really an amp to cut your teeth on. 85v rails and one color wire isn't the best combo. It is worth your attention though, lots of mods and parts are still available - with the exception of the outputs, those are a challenge and you'll likely never find a matched octet, let alone 16.