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wangro81 9th October 2006 08:49 PM

Hafler DH-500 Protect Circuit Problems
I have recently been running into trouble with my Hafler DH-500. The fan on the amp seems to rev after just a few minutes of operation. A few minutes later, the protect circuit kicks in and the amp shuts down for several minutes before it turns back on and repeats this behavior. I thought that it might have been a dirty input signal at first, but have tried running it through several sources with the same results.

I usually have the amp at low to moderate levels so I really don't think that I'm pushing it to the point that the protect circuit should kick in so quickly. Any ideas on what I'm faced with here?


mrshow4u 9th October 2006 08:55 PM

....Hmmm, maybe a bad thermo-switch on the heatsinks, or a bad connection to the thermo-switches, ....or real hi temp activation

AndrewT 9th October 2006 09:01 PM

how about opening up and checking some temperatures.
Take care with voltages in the 500, I believe they are quite high.

BobEllis 9th October 2006 09:50 PM

My DH500 didn't like one particular set of speaker cables and took to oscillating. Not badly enough to set the protection off, but the fans kicked on high almost instantaneously. Try it out with no speaker cables and the inputs shorted. Same behavior?

Another problem with any fan cooled amp is dust . The buildup of dust bunnies in the heat sink tunnel acts as an insulator, so it will get hot in a hurry. Are they plugged up? Compressed air and pipe cleaners are your friends. ;)

Andrew is right - Rails are around 85V, so watch out.

Total bias per channel is supposed to be 450 mA per channel. (pull a rail fuse and clip your ammeter in.) I don't have my schematic handy, but for some reason I think it is one of those where bias pot wiper failure runs the bias up. The amp uses dirt cheap open pots. Dust could cause a problem there. If it has, I recommend replacing the bias and offset adjust pots.

BTW, the protection you seem to be experiencing is a thermal switch on the heat sinks that cuts off AC power when overheated. THIS MEANS THERE IS MAINS VOLTAGE ON THE TOP OF THE AMP MODULE. The switch terminals are NOT insulated on my amp. Exercise extreme caution poking around this amp. The fan is also run off mains power (through a couple of resistors for speed control)

djk 9th October 2006 11:02 PM

I had a bad bias transistor in one DH500 that caused it to overheat in about 5 minutes.

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