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Old 2nd December 2012, 03:45 PM   #331
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I am inclined to think the reason for the positive bias is a disbalance from the transconductance stage which the servo fails to fully correct. With the servo switched off there is a positive 1 - 3v appearing at the output of the transconductance stage. No idea why as my bipolars are carefully selected. Maybe i have made a mistake somewhere...
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Old 2nd December 2012, 03:59 PM   #332
pdul is offline pdul  Denmark
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It sounds exactly like the problem i have too. the wandering offset on output, and riaa, but input and psu is ok. Finally someone else have the same problem
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Old 2nd December 2012, 04:06 PM   #333
pdul is offline pdul  Denmark
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Could it be the fets playing tricks?
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Old 2nd December 2012, 04:08 PM   #334
pdul is offline pdul  Denmark
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My problem is the same in both channels
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Old 2nd December 2012, 04:13 PM   #335
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Fixed the issue with the insufficient voltage across the output 2SK170. Indeed Joachim was correct - top FET needed replacement.

As for the randomly moving dc levels i still have no idea. And it is very consistent between channels.
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Old 2nd December 2012, 04:14 PM   #336
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Quote:
Originally Posted by pdul View Post
Could it be the fets playing tricks?
Which fets would those be? The problem exists ahead of the output buffer... and is not related to the servo.
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Old 2nd December 2012, 04:23 PM   #337
pdul is offline pdul  Denmark
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What else? The bipolars are well matched
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Old 2nd December 2012, 04:40 PM   #338
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I had initially an offset of around 25 to 40mV. That went down after a while. I assume that is forming of the electrolytics in the input stage ( 2 x 6800uF ). They have some leakage current that goes down with time in use.
If after several days in use the DC offset at the output does not go down there are instructions in the building manual how to raise the "authority" of the servo.
Yes, i ment searching for J310 that leave enough Vds ( Uds ) over the 2SK170.
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Old 2nd December 2012, 04:44 PM   #339
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There is also a little discussion on the other Paradise thread if the OP134 in the group buy are fakes. No conclusion yet but if that is the case then the servo opamps have to be substituted with a genuine one.
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Old 2nd December 2012, 04:47 PM   #340
pdul is offline pdul  Denmark
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It is not the stable offset from the buffer, that is the problem, it is the wandering offset at the riaa note +/-100mv. Jumps around constantly as if it were sampling a bigger curve.
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