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Old 12th August 2002, 05:29 PM   #11
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Join Date: Jul 2002
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Well. I have checked resistors in the neighborhood of the Bias Mosfet, Q11, and can't find anything amiss on either channel. Ditto for orientation of devices. However, with Vdc ( I stated this wrong in an earlier post, describing it as Vac--I'm learning slowly, anyway!) across R7 and R8 at about .800 each, and a nice waveform trace on the output, and with my too-high +DRV to -DRV voltage of 17+ Vdc, I get NO Vgs on Q11, and No Vds on it either. Same for both channels. I supposed I should conclude that both Q11's are toasted? I guess I will have to pull them out of the circuit and check. I built this thing stock with OCSL boards and made no omissions except R81 and the lag comp Cap. Oh yeah, no fan or Q for it either on power supply board.

This has been a real puzzle. I have built a few things since my first dynaco kit in '76, and my Leach amp has been putting along (however boringly) since '78 without incident. But I can't seem to get over this hump. I must have killed too many braincells along the way.
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Old 12th August 2002, 07:21 PM   #12
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Time to replace the bias transistors.
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Old 13th August 2002, 11:31 PM   #13
PassFan is offline PassFan  United States
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You are aware of some misprints on the cards from OCSL as well as the layout in the original article. I vagely remember something about this and it was addressed in the questions section an a later issue. I'm out of town working right now and don't have access to my old notes. If you caught that then nevermind but if not you may have some resistors loaded wrong.
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Old 13th August 2002, 11:57 PM   #14
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Location: Seattle
Thank you PassFan for the additional thought. Yes. When I bought the OCSL boards there was a memo included in their shipment; the memo from Mr. Pass listed a few errors that had crept into the board layout drawing. I thought I accounted for them all, but it is worth a comparison to the schematic one more time, and I will do that.

Thanks again for the help from everyone at DIYaudio. This forum helps keep the OMG! (panic) level down.

Larry Wright,
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Old 26th August 2002, 10:35 PM   #15
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Well I still don't see where I got a problem with R's on both channels. And yesterday I unsoldered one of the Q11 bias mosfets and checked it in the original testing circuit described in the article. At 15 vdc I get a Vgs measurement (for an IRC 510) of 3.49 volts--right in the range for all the other 510s I measured.

Before this, I tried to check junctions on the device with the diode test function on my trusty old Beckman MM. (Forces 5ma of current through). Got weird results: expected voltage drop in one direction, and OL in the other, then after switching leads back and forth, a very low reading in place of the OL reading (which would be bad--shorted innards or something). Switching leads again, however, I get first the expected voltage drop in one direction, then back to OL. Almost like the device was 'latching' in one state or another. Maybe you can't test a mosfet in this manner??? In any case, back into the published test setup, and the device measures normally every time (and right in line with two other 510's that have never been in the circuit)... Got my copy of The Art of Electronics Friday and gonna ed-u-cate mahself, but still don't got no background music to read by!!

Larry Wright
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Old 18th February 2011, 01:35 AM   #16
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Location: Illinois
I'm struggling with the same problem. Have checked Q11..VGS 4V and replaced R80 even though the one I pulled out measured spot on. I'm still measuring 18V across +/-DRV. The amp has been in sevrice for 15 years but recently went out on me. At this point I'm suspecting it may be a bad zener (either Z3 or 4). Don't have any spares in the box so before I go that route thought I check here.
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