Hafler DH-200 Bias Problem

Pyre · 4th October 2009, 02:49 PM

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.

freddymac406 · 4th October 2009, 06:02 PM

Transister Q9 is most likely at fault. It is a 2N2222.
Fred

Pyre · 15th October 2009, 12:35 PM

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?

nigelwright7557 · 15th October 2009, 06:42 PM

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.

freddymac406 · 15th October 2009, 08:35 PM

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

4th October 2009, 02:49 PM	#1
Pyre diyAudio Member Join Date: May 2006	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.

15th October 2009, 06:42 PM	#4
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	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. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

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4th October 2009, 06:02 PM	#2
freddymac406 diyAudio Member Join Date: Jan 2005 Location: S.C.	Transister Q9 is most likely at fault. It is a 2N2222. Fred

15th October 2009, 12:35 PM	#3
Pyre diyAudio Member Join Date: May 2006	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?

15th October 2009, 08:35 PM	#5
freddymac406 diyAudio Member Join Date: Jan 2005 Location: S.C.	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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