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Old 28th February 2011, 10:41 PM   #41
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"Connecting one side at a time I can dial everything in. Connect em both and it's a crap shoot."

Ah, that comment brings back memories..... There is a line in the A75 article that says something to the effect of "connect one side, adjust it, completely disconnect it and hook up the other side and adjust". I spent over a month trying to adjust the pots on my A75 with both sides connected. When I would get close to turning up the bias, it would start to oscillate as I was turning the pots. Finally, I did the adjustments, one side at a time. Then I connected up both and turned it on -- and voila, as long as I had the inputs connected to something, it worked fine. Turn it on with the inputs not connected and it would oscillate. Perhaps this is part of the problem you are seeing?

JJ
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Old 28th February 2011, 11:51 PM   #42
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Doh! That explains why it was so much easier to adjust the board before installing it.
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Old 3rd March 2011, 02:22 AM   #43
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Help again. I've got the DC dialed in both sides, no oscillations and everything seems to be a go. 0-10 mv dc offset on both channels and about 6.5v across +/- DRV. When I hook up the scope and the signal generator with a 1khz tone from my computer the DC on one side goes +3V and on the other side -3V. not what I was expecting. I'm running unbalanced with the -In on the pcd tied to the star grd along the with the grd on the input jack. Nothing new or different that they way it's been the last 15 years. I've been over the connections again and again and have no idea what's up now. I'd post some pics if you think that might help. Not sure how but I'll figure it out.
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Old 3rd March 2011, 02:27 AM   #44
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DC on your computer output? If so use a 1 uf cap to block the DC.
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Old 3rd March 2011, 02:38 AM   #45
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I suppose that's possible. I'll give it a check and call it a day. 5am rolls around pretty early and I'm no young pup any more. Thanks again Bob. I think I've learned more about this amp now than when I built it. If it blows again I'm not sure I've got the patience to do another repair job. Stay tuned.....

Dan
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Old 3rd March 2011, 09:45 PM   #46
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I checked for DC on my computer and that isn't the problem. It was a long shot since with the computer connected before feeding a test tone I didn't measure any VDC. When I switched up the grounding I moved the Reg PSU grounds to the star grd which may be the cause. Gonna move them back to the main cap plate grd tonight and see if that makes a difference.
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Old 6th March 2011, 01:51 AM   #47
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update for today. Frontend checked out, +/- 10mv offset on both sides and a good looking trace on the scope. Everything back together...output and unreg PS connected. But now I have +3vdc on one side and -3vdc on the other on the outputs. Not sure what the problem is now. The output is clean. When I measured the +/- DRV to ground before hooking up the output stage the + measured about 5V and the -DRV at 0. Didn't seem right but the DC dialed in fine. R7/8 at 1V/.8V respectively.

Any thoughts anyone?
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Old 11th March 2011, 04:30 PM   #48
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ok. just about to put this back together. Everything looks like a go.

Last edited by Dmatuszak; 11th March 2011 at 04:36 PM.
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Old 11th March 2011, 06:38 PM   #49
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Old 11th March 2011, 09:16 PM   #50
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The heatsinks getting nice and toastly, but the fans keep the temps down. If I did my math right 300mv across the ouput resistors should give me class a up to about 65W with an 8ohm load.

8 ohm bias fot 100% Class A
Rail VDC35
Load Impedance 8
=4.375 A
Output Devices 6
Bias/device = (ma)300
=1.8 A
Watt Bias rms 63.0

I'm using 6 each of the IRFP140/9140's per channel. These are rated at about 180W each if I recall the specs.

Can't wait to plug back in and listen to some music.

Last edited by Dmatuszak; 11th March 2011 at 09:19 PM.
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