Hafler DH-500 problems... ugh! DH500

nifaron · 8th November 2008, 10:13 PM

I have a Hafler DH-500 that is giving me trouble. It’s worked flawlessly until a couple weeks ago when I turned it on and got nothing. The red light is on and solid (not flashing) and I noticed that the relay didn’t click. Assuming that it was the relay I picked up a NOS relay on ebay and installed it. Nothing. I put jumpers across the relay terminals to couple the amp with the outputs. The left channel works and sounds fine, but I get just a very low/quiet hum from the right channel. Any ideas? I’m at a loss… Thanks!

megajocke · 9th November 2008, 12:43 PM

I'd suspect a missing power supply rail, maybe just for the input circuit. There are two resistors filtering it, R31-32 in this manual:
http://www.hafler.com/techsupport/pd...00_amp_man.pdf

I'd check those.

nifaron · 9th November 2008, 11:30 PM

Quote:

Originally posted by megajocke
I'd suspect a missing power supply rail, maybe just for the input circuit. There are two resistors filtering it, R31-32 in this manual:
http://www.hafler.com/techsupport/pd...00_amp_man.pdf

I'd check those.

Thanks for the reply! I checked R31 and R32 on both the working and non working channels. They were all 51 ohms -/+ 2 ohms - the schematic calls for 47 ohms, so I assume that is okay.

Any other thoughts?

Thanks!
Aaron

Mooly · 10th November 2008, 11:49 AM

Any DC offset present on the faulty channel. Measure before the relay.

nifaron · 11th November 2008, 12:04 AM

Quote:

Originally posted by Mooly
Any DC offset present on the faulty channel. Measure before the relay.

Yes! 20vdc! Ouch!

Mooly · 11th November 2008, 06:35 AM

Get it fixed then

Do you know where to begin ?

nifaron · 11th November 2008, 01:53 PM

Quote:

Originally posted by Mooly
Get it fixed then Do you know where to begin ?

No, but I bet you can tell me...

Mooly · 11th November 2008, 06:25 PM

Will have a closer look at the circcuit tomorrow, but this may help for now.
Looking at the circuit on page 16 of the pdf has the fuse f401 blown. If it has leave it as it is. Do not connect the speaker.
The 3 pairs of FET outputs. Do any of them get hotter than the others. Normally I would not suggest running a faulty amp up but this one doesn't seem to protest from what you say.
It's possible one or more of the FET's is faulty.
From ground measure the voltage on the two diodes D15 and D16 ( the left hand ends

technical description ) should be near to the supply voltage ie 85 - 90 volts.
Check R33 that connects to Q12 and Q13. Not burnt ? Whats the voltage across it, that's tags 4 and 9 to the outputs ?

Edit- Make sure the outputs are not getting hot. I recommend using a 100 watt filament bulb in series with the mains when fault finding. Can save a lot of grief.

8th November 2008, 10:13 PM	#1
nifaron diyAudio Member Join Date: Nov 2008	Hafler DH-500 problems... ugh! DH500 I have a Hafler DH-500 that is giving me trouble. It’s worked flawlessly until a couple weeks ago when I turned it on and got nothing. The red light is on and solid (not flashing) and I noticed that the relay didn’t click. Assuming that it was the relay I picked up a NOS relay on ebay and installed it. Nothing. I put jumpers across the relay terminals to couple the amp with the outputs. The left channel works and sounds fine, but I get just a very low/quiet hum from the right channel. Any ideas? I’m at a loss… Thanks!

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9th November 2008, 12:43 PM	#2
megajocke diyAudio Member Join Date: Jan 2003 Location: Solna	I'd suspect a missing power supply rail, maybe just for the input circuit. There are two resistors filtering it, R31-32 in this manual: http://www.hafler.com/techsupport/pd...00_amp_man.pdf I'd check those.

10th November 2008, 11:49 AM	#4
Mooly diyAudio Member Join Date: Sep 2007	Any DC offset present on the faulty channel. Measure before the relay.

11th November 2008, 06:35 AM	#6
Mooly diyAudio Member Join Date: Sep 2007	Get it fixed then Do you know where to begin ?

11th November 2008, 06:25 PM	#8
Mooly diyAudio Member Join Date: Sep 2007	Will have a closer look at the circcuit tomorrow, but this may help for now. Looking at the circuit on page 16 of the pdf has the fuse f401 blown. If it has leave it as it is. Do not connect the speaker. The 3 pairs of FET outputs. Do any of them get hotter than the others. Normally I would not suggest running a faulty amp up but this one doesn't seem to protest from what you say. It's possible one or more of the FET's is faulty. From ground measure the voltage on the two diodes D15 and D16 ( the left hand ends technical description ) should be near to the supply voltage ie 85 - 90 volts. Check R33 that connects to Q12 and Q13. Not burnt ? Whats the voltage across it, that's tags 4 and 9 to the outputs ? Edit- Make sure the outputs are not getting hot. I recommend using a 100 watt filament bulb in series with the mains when fault finding. Can save a lot of grief.

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