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Old 9th January 2013, 10:39 PM   #1
tony Ma is offline tony Ma  United States
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Default Bias, not same output etc .... Pls help

Dear All,

I have B&K St-140 that I replace the electrolytic capacitor with elma silmic.

One side (R channel total dead) so I replace with new transistor J50 and K135.

Just FYI: I measure secondary wiring is 46V x 2 and the DC input to PCB is 65V

There are 3 cable going to PCB. RED 65V, Green 0V, Black (-)65V. (I put the multimeter to DC with black tip to amp chassis)

Last night I found this thread

Amplifier Distortion, DC-Offset, and You! - AudioKarma.org Home Audio Stereo Discussion Forums

I did measure mine with no load meaning just amp itself and I set my Multimeter to DC with + to RED speaker out and - to black speaker out

When it start R = (-) 89mv and L = (-)87mv

After 20 minute R = (-)87mv and L = (-)94mv

My questions are

1. Why I got negative reading?
2. How to set bias? which Pot? there are 2 pots here (I only have Multimeter)

Can some one guide me step by step please??

tks
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Old 9th January 2013, 10:42 PM   #2
tony Ma is offline tony Ma  United States
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pls see picture
Attached Images
File Type: jpg Front B&K st-140 other.jpg (561.2 KB, 53 views)
File Type: jpg Front B&K ST-140.jpg (585.0 KB, 49 views)
File Type: jpg Original capacitor.jpg (346.1 KB, 50 views)
File Type: jpg Back B&K ST-140.jpg (722.3 KB, 49 views)
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Old 10th January 2013, 11:46 AM   #3
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You mention DC offset and also Bias ?????
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Old 10th January 2013, 12:49 PM   #4
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P1 sets offset voltage.
P2 sets the bias current of the output stage.Setting P2 is dangerous for the amp if you don't know exactly what you are doing

http://6streetbridge.blogspot.fr/201...amplifier.html

Edit: +/- 65V looks high for only one pair of output devices

Last edited by bobodioulasso; 10th January 2013 at 12:53 PM.
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Old 10th January 2013, 12:51 PM   #5
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I see no presets in the piccy just blank spaces.. Unless they are hiding underneath the pcb
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Old 10th January 2013, 12:57 PM   #6
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Can be seen on 4th pic.
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Old 10th January 2013, 01:08 PM   #7
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Quote:
Originally Posted by bobodioulasso View Post
Can be seen on 4th pic.
And now i see .. thankyou .. Hi ho hi ho its of to spec savers i go
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