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Old 17th October 2006, 01:16 PM   #1
eddog is offline eddog  United States
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Default Curious voltage difference between V+ and V-

Hi, I have a Brian GT 3875 amp that I have been running with a dual secondary torroid on my work bench for about 1 year. Well the torroid had to go into another project so I am having to use a center-tapped transformer of questionable design that I had laying around. I've changed the power supply as per the instructions(Jumpers between diodes as pictured in the assembly manual) and when no load(amp boards)are connected I get a good 19.5 both - and +. However when I connect the amp boards the V+ stays at 19.5 V but the V- dropes to - 16.5. The DC offset becomes high at .5V also. These same amp boards when connected with the dual secondary torroid had DC offsets less than 10mv. Suggestions appreciated.
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Old 17th October 2006, 01:27 PM   #2
boholm is offline boholm  Denmark
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The centertap of the transformer - did you put that to GND?
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Old 17th October 2006, 01:50 PM   #3
eddog is offline eddog  United States
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I have the centertap connected to the PG+ of the rectifier board which is also effectively PG- due to the jumper on the board. I then run wires from PG- of the rectifier board to the PG- connection of both amp boards. There is no reason to run the PG+ wires to the amp boards correct?(same potential as PG-)
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Old 18th October 2006, 08:31 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
sounds like you have inadvertently made another change.

Have another look and try to identify the changes before to after.
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Old 18th October 2006, 11:02 AM   #5
Nordic is offline Nordic  South Africa
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Aren't the rectifier diodes set up diffirently on Brians boards depending on wheter you have a split secondary and when you have 2 seperate secondaries?
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Old 18th October 2006, 01:34 PM   #6
eddog is offline eddog  United States
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Yep, you remove diodes D2,D3,D6 and D7 then jumper between the remaining diodes to form a Bridge rectifier. I am pretty sure that I have it wired correctly. Would a leaky cap after the bridge allow some AC to get through. Maybe my V- side cap is not healthy.
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Old 18th October 2006, 02:24 PM   #7
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Sounds like one diode is'n working and you have only half way rectification at the low level side. That would explain the drop when loaded.
Do you have a scope, to see that the ripple at the lower voltage side is higher & half-wave?

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Old 18th October 2006, 07:28 PM   #8
eddog is offline eddog  United States
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Default Voltage issue repaired

Well the power supply was alright, I disconnected my source input and my voltage issue disappeared. Turns out my source has some DC on it's output as well one side had some small amout of AC also. I have put a 1uf cap on the signal inputs and the problem has gone away. Thanks for the suggestions they always cause me to put more effort into my problem solving.

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