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HK Citation 16A Dc offset issue
HK Citation 16A Dc offset issue
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Old 25th June 2018, 02:53 PM   #31
stereohifi is offline stereohifi  Canada
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Default DC offset now below 10mV

Replaced R19 from 1Mohms to 470Kohms, now DC offset is below the prescribe 10mV from HK Citation 16A manual.
Mostly this pcboard has shifted in specs, until I can obtain a matched pair of drivers Q13-Q12
Some pictures of the amp and pcboard, there is no front plate for now, and yes audio wires are to be replaced...
Attached Images
File Type: jpg P1010078.jpg (464.7 KB, 35 views)
File Type: jpg P1010077.jpg (496.6 KB, 34 views)

Last edited by stereohifi; 25th June 2018 at 03:00 PM.
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Old 25th June 2018, 02:55 PM   #32
stereohifi is offline stereohifi  Canada
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HK Citation 16A Amp pics, DC offset now after 30mins cooking at 25mV....
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File Type: jpg P1010075.jpg (464.6 KB, 30 views)
File Type: jpg P1010076.jpg (519.5 KB, 26 views)
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Old 25th June 2018, 03:00 PM   #33
stereohifi is offline stereohifi  Canada
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PC boards
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File Type: jpg P1010078.jpg (464.7 KB, 26 views)
File Type: jpg P1010077.jpg (496.6 KB, 5 views)
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Old 25th June 2018, 03:24 PM   #34
stereohifi is offline stereohifi  Canada
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Default DC offset on speakers posts.

Well to compare both channels now , both varies a bit and changes from below 10mV to 25mV of course I'm having some music on them just to make running and stop terminate inputs and measures, kind of normal I think.
But what fuzz me, it's both VR1 are set to near or at -10Vdc for correcting offset.
I wonder if new or recent amps needs to have such a negative volts correction?
Of course depending on design. but this amp is from 1976 era ....
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Old 25th June 2018, 05:24 PM   #35
Mooly is offline Mooly  United Kingdom
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HK Citation 16A Dc offset issue
Thanks for the piccys, it always good to see what is being worked on. They look clean and in good order.

Although the offset correction (-10vdc) sounds a lot, if you think about it you'll see its actually a very tiny correction in the scheme of things. What is important is the current injected into the R16,17,18 node and 1meg and 10 volts gives a current of just 10uA. So its pretty low. It is trying to develop a correction voltage across the combined resistance of R18 and R3.
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Old 27th June 2018, 04:48 PM   #36
stereohifi is offline stereohifi  Canada
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Default Conclusion dc offset HK Citation 16A

Got a new pair of PNP -NPN MPSU10 and MPSU60 transistors.

Replaced Q13 as low hfe readings below his complementary Q12 at 36 instead of 50.
Installed new Q13 MPSU60 transistor in place, and this push dc offset in the positive region. Then installed the Q12 NPN MPSU10 and got a better balance.
Like R19 play a role in compensation of current in the stage for correcting DC offset went from 470Kohms to 330Kohms.
Now I can trim down DC offset to zero and have some play in positive and negative region with VR1.
I did apply the same logic to R19 on the other good pcboard. as this one had the VR1 adjustment pot all the way to -10vdc to properly trim down DC offset.
Installed a 470Kohms and now DC offset is fine to below 10mV, with room for adjusting trimpot VR1.
Now I had a long term test and this HK Citation runs clean and real HiFi
Now summer is here might use it outdoor for backward party!



Many thanks to Mooly for the help
Regards.
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Old 27th June 2018, 05:11 PM   #37
Mooly is offline Mooly  United Kingdom
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HK Citation 16A Dc offset issue
That's excellent, well done Its definitely been one of the weirder 'issues' I have come across.
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