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Old 4th October 2009, 02:49 PM   #1
Pyre is offline Pyre  Canada
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Default Hafler DH-200 Bias Problem

Hello,

I have a Hafler DH-200 that I am fixing for a friend. Its a factory mono block by the looks of it. The one side of the amp was heating up and blowing the fuse. I figured just about everything on that side needed to be replaced so I bought a set of MOSFETs and a kit with all the transistors and diodes. I tested everything on the board and replaced any resistor that was out of spec. I also replaced the two 100uf caps. I did not have any non polarized caps so I was unable to replace the other two. My problem is that after all this the amp still has the problem. I tested the bias and turned down all the way its 700ma+ which I cant imagine is good for anything and probably what cooked everything in the first place. What would cause the bias to be so high? Something must be blown but I cant figure out what. Any help would be greatly appreciated.
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Old 4th October 2009, 06:02 PM   #2
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Transister Q9 is most likely at fault. It is a 2N2222.
Fred
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Old 15th October 2009, 12:35 PM   #3
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Thank you Fred, You were completely correct. I did not think so at first as I replaced it with a brand new 2N2222 that came with the transistor kit I bought. That is why I was so confused. I replaced everything on that board that did not test within spec. When I was at the parts store yesterday I bought a 10 bag of PN2222A's and soldered one in when I got home. Put it all together and it biased right up. Not sure why that 2N2222 did not work before but the amp works now and I can get some sleep at night and give them back to my friend.

Any idea why someone would have replaced all the grid stop resistors R40-R43 from 220R to 470R? Is it so it can handle more bias without oscillating?
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Old 15th October 2009, 06:42 PM   #4
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One trick I learned with bias circuits is to put a zener across them to catch out of range voltages can that damage things.

All my amp designs have used this and not had an output stage pop yet.
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Old 15th October 2009, 08:35 PM   #5
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You must be a tube guy. Grid stopping resisters ?
To answer your question they should be 470 Ohm but may have changed during production run. Also to improve the sound put a 20K resister across c9 on pc-6,
This will lower open loop gain and flatten responce so the -3 point will be 20K.
This lowers TIM distortion. I was a Service Tech @ Hafler in the early 80s when they where located in NJ.
Fred
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