Hafler DH-500 problems... ugh! DH500 - diyAudio
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Old 8th November 2008, 11:13 PM   #1
nifaron is offline nifaron  United States
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Default Hafler DH-500 problems... ugh! DH500

I have a Hafler DH-500 that is giving me trouble. Itís 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 didnít 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? Iím at a lossÖ Thanks!
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Old 9th November 2008, 01:43 PM   #2
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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.
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Old 10th November 2008, 12:30 AM   #3
nifaron is offline nifaron  United States
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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
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Old 10th November 2008, 12:49 PM   #4
Mooly is online now Mooly  United Kingdom
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Any DC offset present on the faulty channel. Measure before the relay.
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Old 11th November 2008, 01:04 AM   #5
nifaron is offline nifaron  United States
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Quote:
Originally posted by Mooly
Any DC offset present on the faulty channel. Measure before the relay.
Yes! 20vdc! Ouch!
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Old 11th November 2008, 07:35 AM   #6
Mooly is online now Mooly  United Kingdom
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Get it fixed then Do you know where to begin ?
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Old 11th November 2008, 02:53 PM   #7
nifaron is offline nifaron  United States
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Quote:
Originally posted by Mooly
Get it fixed then Do you know where to begin ?
No, but I bet you can tell me...
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Old 11th November 2008, 07:25 PM   #8
Mooly is online now Mooly  United Kingdom
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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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