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Old 18th June 2017, 02:50 AM   #31
llwhtt is offline llwhtt  United States
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In the December 2006 issue of Audio Express there's an article about the DH-500 and it's failure mechanisms. The amp in the article also had the base leg of Q13 missing, D13 and D14 were shorted, and R9 was bad. Some of the output FETs had failed OPEN!!!!! Anyway see if you find the article if you can not maybe I'll try to scan it and send it to you. PM me with your email address.

Craig
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Old 18th June 2017, 04:07 AM   #32
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Quote:
Originally Posted by llwhtt View Post
In the December 2006 issue of Audio Express there's an article about the DH-500 and it's failure mechanisms. The amp in the article also had the base leg of Q13 missing, D13 and D14 were shorted, and R9 was bad. Some of the output FETs had failed OPEN!!!!! Anyway see if you find the article if you can not maybe I'll try to scan it and send it to you. PM me with your email address.

Craig
Go figure. I'll see what I can dig up. Hopefully I'll get a chance this weekend to remove the output section and see what happens. I have also heard that some of the 47K resistors can be suspect even if they measure OK. I might try and replace those.

I do have a lead on some original outputs at decent prices so worst case scenario I won't have to spend an area and a leg to get working mosfets.
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Old 18th June 2017, 02:13 PM   #33
llwhtt is offline llwhtt  United States
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The author gives a couple of subs for outputs but that was over 10 years ago so no telling what still available. MOSFETS seem to come and go much faster than bipolars. The TO-3 case doesn't help matters. The author used TO-3P, four per rail as there not as stout as the TO-3.

Craig
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Old 19th June 2017, 02:44 AM   #34
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So I went ahead and desoldered the wires to pin 4 and 9. Brought the unit up on the variac with the dim bulb in place. Stayed a bit brighter than before and the relay never clicked on. Did some measurements and everything was all over the place. Pin 4 and 9 were both negative voltage but was downtrending quite fast. The Tops of Q11 and Q8 (suppose to be -/+ 1.2V, respectively) were also both negative and downtrending (~ -2V). I also measured pins 5, 6 , 7, and 8; and all were around -13V except for 7 which was still zero. I was going to let it settle down but I realized the Q8 was getting very hot so I stopped.

Not really sure if this is helping me narrow down the problem. Would installing the resistors across 4-6 and 6-9 help stabilize? Is there any specific resistors I should use? Should I have removed the rail voltage supply to the output mosfets also?

My next step at this point will be remove the rail voltage to the outputs completely and see what that does.

If any of the above makes sense I would appreciate some insight. Thanks
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Old 19th June 2017, 02:47 AM   #35
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Also going to finish replacing all the leftover diodes just to rule those out (i.e. D1-6).
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Old 19th June 2017, 03:11 AM   #36
kct is offline kct  Canada
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Would installing the resistors across 4-6 and 6-9 help stabilize?
Yes, as without there is no feedback and the amp floats towards one rail side.

Just two equal resistors, lets say 470 Ohm to 1k should suffice.

Yes, you should also disconnect the Fets from the rails to be safe.


Now with the two resistors temporarily installed the amp should balance out to zero(ish) and you should measure pretty close the equalized voltages in this circuit. If this is the case investigate the Fets and the gate resistors.
If not there is still a problem in the circuit elsewhere.
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Old 19th June 2017, 10:40 PM   #37
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Now we are getting somewhere!!

I removed voltage to output stage and placed 1K resistors across points 4-6 and 6-9. I now have -50mV at the shorted C6 cap. The relay kicks on and I could cautiously play some very low music through the one channel. DC offset is essentially zero. I don't have any voltage at points 4 or 9 but I am thinking that is to be expected given no outputs.

Looks like my problem is at the output stage. I checked the resistors and they are within spec. So this most likely points to a bad mosfet. I do not know of any way I can further test these outputs. I was thinking I could remove all of them and install one pair at a time to see at least which pair is the culprit. Could I narrow it down in regards to the positive or negative side given I had an overall positive voltage at C6 initially?

That will be the plan unless someone can give me a better way of testing.

Thanks again for the help. There is definitely light at the end of the tunnel!

Last edited by mjstriker5; 19th June 2017 at 10:43 PM.
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Old 19th June 2017, 11:15 PM   #38
phase is offline phase  United States
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I found matching sets of the outputs from buyer qua-co on the bay, fantasia audio is his store.
The exicon parts are great.
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Old 19th June 2017, 11:41 PM   #39
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Ya, I have talked to that person about the amp already as I was thinking about upgrading to his driver boards at some point. Have a source for some possible original outputs if I only need a couple. If not I will be ordering some of the exicon mosfets.
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Old 19th June 2017, 11:50 PM   #40
djk is offline djk
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Driving hard into low impedance loads burns out the gates of the outputs in Hafler amplifiers.

At that point you need a complete rebuild, outputs, drivers, zeners, etc.

The outputs will test OK from Drain to Source, but have a low impedance on Gate to Source.

http://www.exicon.info/PDFs/ecf10n20.pdf

http://www.exicon.info/PDFs/ecf10p20.pdf
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