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HK Citation 16A Dc offset issue
HK Citation 16A Dc offset issue
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Old 21st June 2018, 05:42 PM   #11
stereohifi is offline stereohifi  Canada
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Question DC offset

Hi, after waiting for some parts, did all of the zener diodes replacements 10v and 5,1 v
Did also replaced 1N914 diodes.
Did tested R4 for 22kohms OK.
Tooked all pc board transistors for hfe tests
Q2-3-4-5-6-7-9 tested ok with hfe equal or better than 70 with Ib : 1mA
Q8-10-11 tested OK with hfe equal or better than 170 with Ib: 1mA
Q12-15-17 tested OK with hfe 48 and better and close.
?? Q13 have a lower hfe at 36 still with Ib : 1mA a MPSU60 PNP transistor ??
Q19-20 did measured Ok with hfe :190 with Ib: 1mA


Q21-22 are are hfe of 66 and 70 with Ib of 10mA


So basically Q13 have a lower hfe , may this pull down the DC offset to a constant 0,55v with VR1 all the way to -9,85 volts?
Considering his complementary transistor Q12 at a hfe of 50



Again all solder was reflowed and all resistances tests and measure OK.
All caps are new.
Lower voltage is +9,9v and - 9,85v
+65v and -65v for Vcc.


I did interchanged the good pc board driver on the other channel output transistor and issue remains on pc board either output transistors I can have 10mV DC offset, but not with this pc board .
thanks.

Last edited by stereohifi; 21st June 2018 at 05:45 PM.
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Old 21st June 2018, 07:20 PM   #12
Mooly is offline Mooly  United Kingdom
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HK Citation 16A Dc offset issue
This 'ere offset voltage then...

Has the amp some previous unknown history. In other words, what was the original fault ?

Are you certain the DC offset is just that, pure and constant DC, or is the amp oscillating at high frequency ?
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Old 22nd June 2018, 02:34 PM   #13
stereohifi is offline stereohifi  Canada
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Default DC offset

HI thanks for answering Mooly, yes there is a 0,55volts dc at the speakers output with inputs to ground. Other channel is below 10mV DC. Both channel bias are fine and solid at 50mV DC.

It is a strait baseline on the scope no hf noise other than very low power supply normal residuals . and even if I did not yet replaced the big ps caps.
It did try both inputs and probes and have a 100MHz scope here, so I can go quite high HF but only yo see a clean normal DC value....

All zeners are replaced, but it seems that CR5 is a bit low at 4,8 v ?
Will go to try replacing Q13, as it is a long weekend here and won't be able to have the MSPU10 parts ....
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Old 22nd June 2018, 03:08 PM   #14
stereohifi is offline stereohifi  Canada
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P.S. scope reading is 0,55vDC with a flat baseline no HF garbage.
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Old 22nd June 2018, 05:59 PM   #15
Mooly is offline Mooly  United Kingdom
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HK Citation 16A Dc offset issue
Here is what I would do...

First question. Has the amp got an unknown history ? I ask that because you mustn't assume anything when it comes to faultfinding.


Does the voltage on the wiper of the preset (one end of R19) swing correctly from minus 10 to plus 10 volts ?

Has any unauthorised work been done with regard to bridging and bridge mode ? What voltage do you see on each side of R13. I would assume the point marked 'E7' is at zero volts but check.

CR5 you mention isn't an absolute voltage. There should be 5.1 volts or thereabouts across the Zener, and you should see -10 on the anode of the diode which is derived from CR4.
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Old 22nd June 2018, 06:33 PM   #16
stereohifi is offline stereohifi  Canada
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Amp is being used in hifi set-up, normal dust, no overheated pc traces,



E7 reads 0volt.
R19 measures 1 Megohms, and swing +9,9v and -9,85v.


Bridge wiring is not used, only E4 wire on both channel goes to ground.ie: power supply center tap.


Voltage on R13 across on bad pcb; 0,19v and from ground to each side of
R13: 0,58v and 0,54v

Voltage on R13 good pcb: 0,009v and 0,046v.


Again, using the bad pcb to the other channel output stage does still have this behaviour with a 0,55v dc at speakers posts.



Thanks.
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Old 22nd June 2018, 06:35 PM   #17
stereohifi is offline stereohifi  Canada
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VR1 adjustment is at -9.85v to obtain 0,55v dc offset , if i operated this VR1 pot towards positive voltage, DC offset increases. Goes to over a - 2volts dc.
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Old 22nd June 2018, 07:42 PM   #18
Mooly is offline Mooly  United Kingdom
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HK Citation 16A Dc offset issue
I'll give it some thought tomorrow all being well.

A typo crept into my reply above, I meant R3, not R13 but you guessed that I think.

Quick things to consider:

1/ Check the DC voltage at the junction R1 and R2. Lets be sure that no stray DC voltage is appearing at the input causing a 'valid' offset condition. Compare the voltage with the good channel.

2/ The caps around the feedback network, C4 and C5 could imbalance the DC conditions if one were short or leaky. The AC performance would still be OK. That's definitely one to check.

3/ C3 is needed for stability but I'm not 100% sure what the effect would be if leaky.

4/ Check the voltage across R5 (560 ohm) is around 650 millivolts give or take. Compare with the good channel.

5/ Compare the voltages across the current mirror resistors, that's R6 and R7, R8 and R9, R24 and R25. Each pair should be similar. Compare to the good channel.

I'll look closer tomorrow, but have a look at the above first.
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Old 23rd June 2018, 02:21 PM   #19
stereohifi is offline stereohifi  Canada
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A special thank you for Mooly helping here. Regards


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Old 23rd June 2018, 05:36 PM   #20
Mooly is offline Mooly  United Kingdom
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HK Citation 16A Dc offset issue
Did you get a chance to try/look at any of the above ?
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