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Old 2nd July 2004, 04:38 AM   #1
Duo is offline Duo  Canada
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Default Citation Sixteen

Well, here is a very nice looking amplifier from Harman/Kardon.

I have one here that has a problem which is the same in both channels. When powered up, both channels are swung hard to the positive supply rail even though none of the outputs appear to be bad. The transistors on the driver boards are good as well.

I'm wondering if this is a good chance at being the chip and if so, is it a common problem?
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Old 2nd July 2004, 08:25 PM   #2
Duo is offline Duo  Canada
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Sorry, it's the negative rail that the amp is stuck on...

Does anyone have any clues?

I'm beginning to believe that it's the ua739 opamp in the input stage that is causing the problem.

I measured about 2.5V between the inverting and non-inverting inputs, so yeah, it's not working properly in that respect.
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Old 2nd July 2004, 10:14 PM   #3
Duo is offline Duo  Canada
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Has anyone here even seen a Citation Sixteen??
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Old 3rd July 2004, 12:07 AM   #4
anatech is offline anatech  Canada
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Hi Duo,
The chip may not be able to correct for the fault and be just fine. Since both channels are affected, look in the power supply or common bias circuit first. I have worked on these eons ago, quite frankly, I forget the configuration.
Above all, keep an open mind.
-Chris
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Old 3rd July 2004, 12:10 AM   #5
Duo is offline Duo  Canada
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Thanks for the reply.

Well, I have looked at the power supplies and they are doing about +-68V which appears to be just fine. There's no unnecessary current draw.

I'm not sure what you mean by the common bias circuit? On the input? Gainstage? Output? Which part?
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Old 3rd July 2004, 01:09 AM   #6
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check the + supply for the opamp
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Old 3rd July 2004, 01:27 AM   #7
anatech is offline anatech  Canada
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Yup, that's one place.
If we had the schematic, we could help you in more detail. Look for anything that is shared by both channels. Op amp supplies, bias supply. Do you have any bias current in the output stage?
-Chris
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Old 3rd July 2004, 01:40 AM   #8
Duo is offline Duo  Canada
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Well, I checked the opamp power. It is fine. +-10V on each opamp.

There is nothing shared between either channel besides the chassis, power cord, power switch, and ground. Very separated design, I like that.

Stupid of me that I didn't think to test the output current.
There is no output current flow at all. Well, I'll certainly be looking into that.
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Old 3rd July 2004, 01:44 AM   #9
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Worked on a few of them.. Even have a schmatic for it. Not here but at the shop.

There is a bunch of info available for mods on this amp also.. I believe Nelson Pass wrote an article on making this amp into a mosfet output...

I'll locate schmatic.. and my notes on repairing amp.
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Old 3rd July 2004, 01:45 AM   #10
Duo is offline Duo  Canada
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Ah, thankyou very much.

Yeah, I've already seen some things I'd like to modify in this amplifier. It has real potential
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