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Old 18th September 2004, 06:57 AM   #1
maxw is offline maxw  United Kingdom
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Default DC offset problem

I have put together my BrianGT Gainclone and I am currently testing it and have 42mV of DC offset in one channel and 80mV in the other.

About my setup:
I have connected AC ground to both PCBs CHG.
It is on a desk, not in a case.
Input is the short redish fat cable that is not connected to anything because I see that Input Ground is connected to CHG on the PCB anyway.
I am using a 10 Ohm resistor to measure the DC offset.
I am very sure all my soldering is good and suspect I have mis read something about grounding?

I have read all the grounding posts I could find and the NIGC userguide and this http://www.decdun.fsnet.co.uk/gaincloneFAQ.html page.

Here is a pic of my setup:

Click the image to open in full size.

Some one tell me what I have done wrong
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Old 18th September 2004, 07:01 AM   #2
maxw is offline maxw  United Kingdom
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here is a closeup of my assembled PCB if it helps any.
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Old 18th September 2004, 07:02 AM   #3
Franz G is offline Franz G  Switzerland
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First picture: I cannot see a ground connection between the rectifierboard and the central ground. Missing? Or is there no ground at Brians rectifier boards, just diodes on it?

Question: the blue cables are the ground? They are to long, as here is current flowing! Suggestion: You better use a very thick and short blank cupperwire between the two boards. In the middle of this wire you can connect the PSU and the AC ground.

Second picture: between wich pins did you connect the feedback resistor?

Franz
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Old 18th September 2004, 07:19 AM   #4
maxw is offline maxw  United Kingdom
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Quote:
Originally posted by Franz G
First picture: I cannot see a ground connection between the rectifierboard and the central ground. Missing?
I didn't know you needed one! I cant see one in the pics in the userguide. What connection does it come from?

Quote:
Originally posted by Franz G
Question: the blue cables are the ground? They are to long, as here is current flowing! Suggestion: You better use a very thick and short blank cupperwire between the two boards. In the middle of this wire you can connect the PSU and the AC ground.
[/B]
The blue cables are CHG. So connect both CHGs and then one cable from this to star ground which also has the chassis (not yet though) and AC ground connected to it?

Quote:
Originally posted by Franz G
Second picture: between wich pins did you connect the feedback resistor?
Franz [/B]
pin 3 and 8, see attached pic.

Thanks alot for your time
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Old 18th September 2004, 07:22 AM   #5
Franz G is offline Franz G  Switzerland
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Sorry, when I am confusing you, about the rectifier board, as I edited my posting to late.

There is no ground at brians rectifier board, of course.

Quote:
The blue cables are CHG. So connect both CHGs and then one cable from this to star ground which also has the chassis (not yet though) and AC ground connected to it?
Yes, this is a way to go. The middle of the "new" connection should be the "star center".

Franz
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Old 18th September 2004, 07:30 AM   #6
Franz G is offline Franz G  Switzerland
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When I look again your first picture: Just shorten the blue cables as much as possible, connect them and try...

Franz
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Old 18th September 2004, 07:56 AM   #7
maxw is offline maxw  United Kingdom
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Quote:
Originally posted by Franz G
When I look again your first picture: Just shorten the blue cables as much as possible, connect them and try...

Franz
Done and its the same (see attachment pic)
Would this rule out a grounding problem?
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Old 18th September 2004, 08:01 AM   #8
Franz G is offline Franz G  Switzerland
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Oops, now you do have a small ground loop! Sorry, I was misleading again.

Remove the (thin) cupper wire between the boards, as the blue cables are doing the job.
Franz
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Old 18th September 2004, 08:10 AM   #9
maxw is offline maxw  United Kingdom
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Quote:
Originally posted by Franz G
Oops, now you do have a small ground loop! Sorry, I was misleading again.

Remove the (thin) cupper wire between the boards, as the blue cables are doing the job.
Franz
Done and still the same offset
Thanks for your patience
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Old 18th September 2004, 08:12 AM   #10
Franz G is offline Franz G  Switzerland
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Max

Wait, till our US friends wake up! I leave now, to drink my breakfast coffee!

Franz
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