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HK Citation 16A Dc offset issue
HK Citation 16A Dc offset issue
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Old 23rd May 2018, 08:45 PM   #1
stereohifi is offline stereohifi  Canada
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Join Date: Nov 2012
Location: Repentigny, QC, CND
Smile HK Citation 16A Dc offset issue

Hello, Citation 16A with black face plate so it's a 16A. Works on both channel, lots power could acheive bias on both channel for 50mV across 0,47ohms output transistors emitters. Issue with one of the channel with 0,56volts DC offset.
Other channel is fine down to 10mV.
New caps on both boards, original tested resistors and transistors and zeners.
New 15T Bourns tripots on both channels. Solder reflowed.
Here's the voltages readings I get from trouble channel;
Q7 collector; 0,22v
Q12 collector; 64v
Q12 emitter; -0,6v
CR5 anode ; -4v
Q8 emitter; 0,7v
Q15 emitter; 64v
Q15 collector; 1v
Q18 collector; -0,7v
Q18 emitter; -2,2v


all voltages are a bit more negative value than supposed as per schematics and the good channel.
Is Q1 needs more investigation?
All small ceramic caps are there and tested OK, very high resistance.
R26 and R10 are new 3,9Kohms 3 watts type.
+ and - 10v are OK.
Does a leaky transistor could do that? Or leaky ceramic??
Thanks.
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Old 23rd May 2018, 09:00 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by stereohifi View Post
Issue with one of the channel with 0,56volts DC offset.
The offset pot VR1 should be able to take care of this.
What range of offset does VR1 give when adjusting it?

Last edited by rayma; 23rd May 2018 at 09:04 PM.
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Old 23rd May 2018, 09:07 PM   #3
stereohifi is offline stereohifi  Canada
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Location: Repentigny, QC, CND
Yes, VR1 should drop it down, but I cannot reach a lower value with pot adjustment?? This is a brand new tested 1 Mohms 15T Bourns trimpot.

Of course I did compare all voltage on the good channel, but I don't have a bunch of new transistors and other parts in hand and I like to rely on more experimented members with the Citation 16A here....
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Old 23rd May 2018, 09:15 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by stereohifi View Post
Yes, VR1 should drop it down, but I cannot reach a lower value with pot adjustment?
So the half volt of offset is the minimum possible, with VR1 set all the way to one end?
Something is wrong, then. The input stage is a dual matched transistor in a single package,
perhaps it has a problem, if all the capacitors were replaced.

Last edited by rayma; 23rd May 2018 at 09:21 PM.
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Old 23rd May 2018, 09:28 PM   #5
stereohifi is offline stereohifi  Canada
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It might be? but Q1 is a dual NPN differential pair of transistors that did not suspected at first.
It's a 2N2916.
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Old 23rd May 2018, 09:32 PM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by stereohifi View Post
It might be? but Q1 is a dual NPN differential pair of transistors that did not suspected at first. It's a 2N2916.
It's possible.
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Old 23rd May 2018, 09:45 PM   #7
stereohifi is offline stereohifi  Canada
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Will order some, but nothing from digikey or mouser I can find, it's obsolete of course Ebay does have some, will wait ...
Thanks
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Old 23rd May 2018, 11:00 PM   #8
llwhtt is offline llwhtt  United States
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Mouser has a few duals that should work fine, a bit pricey though, MAT01 for example. I didn't check the pin configuration. The other option would to get some singles with similar speacs and start matching them.

Craig
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Old 13th June 2018, 05:19 PM   #9
stereohifi is offline stereohifi  Canada
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Default Bias issue persist???

Hello, just got new or NOS 2N2916 dual NPN tranasistors for replacing Q1.
Still with a new device , issue persist, Lower DC offset I can set is 0,56volts.
Here's Q1 voltages readings, both Emitters, Base and Collectors are the same.
Q1

E= -0,9v
B= -0,23v
C= +7,6v
Case is 0V...
All caps are replaced now, none of the original ceramics are on the board.
CR5 and CR6 are new also.
Both CR3 and CR4 are ok, at +/- 9,4 v
C1 and C5 are new replaced.
again all resistors measures fine to specs. no components overheats.
Plan on replacing CR1 and CR2....
will be back
Thanks.
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Old 13th June 2018, 06:39 PM   #10
stereohifi is offline stereohifi  Canada
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Join Date: Nov 2012
Location: Repentigny, QC, CND
Default DC offset issue persist

Quote:
Originally Posted by stereohifi View Post
Hello, just got new or NOS 2N2916 dual NPN tranasistors for replacing Q1.
Still with a new device , issue persist, Lower DC offset I can set is 0,56volts.
Here's Q1 voltages readings, both Emitters, Base and Collectors are the same.
Q1

E= -0,9v
B= -0,23v
C= +7,6v
Case is 0V...
All caps are replaced now, none of the original ceramics are on the board.
CR5 and CR6 are new also.
Both CR3 and CR4 are ok, at +/- 9,4 v
C1 and C5 are new replaced.
again all resistors measures fine to specs. no components overheats.
Plan on replacing CR1 and CR2....
will be back
Thanks.
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