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Old 20th March 2013, 04:53 AM   #51
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I'm not familiar with that scope so I wanted to confirm, especially since you only mentioned the 0.45v of offset. It could have been that you were using a probe that didn't match the scope so it would display the wrong vertical amp settings. This is a common problem.

Do the voltages change on the op-amps when you drive it hard enough to force it to 15v of offset?

Does the subsonic filter make a difference (in reducing the offset)?
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Old 20th March 2013, 06:27 AM   #52
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Probably nothing, but curious the next time you're in there taking measurments for Perry, the voltage/trace on pin 2 of U304.
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Old 20th March 2013, 12:40 PM   #53
ultra is offline ultra  Greece
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ok, set gain high so the amp has around -15VDC of offset (the offset is negative)
here is what i get at the op-amps
(+/-6V is present on pins 4 and 8)

U300
pin 1 0.182
pin 2 0.177
pin 3 0.003
pin 5 0.182
pin 6 0.170
pin 7 0.170

U301
pin 1 2.364
pin 2 -2.465
pin 3 0.004
pin 5 -0.005
pin 6 2.364
pin 7 -1.377

U302
pin 1 -2.232
pin 2 0.068
pin 3 0.005
pin 5 -0.012
pin 6 -2.20
pin 7 2.645

the subsonic does not affect the offset

pin 2 of U304 has around -0.17 at all times.

Last edited by ultra; 20th March 2013 at 12:44 PM.
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Old 20th March 2013, 06:45 PM   #54
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Was the output badly clipped when you were taking these readings?

Were any of the outputs of the op-amps U301 and 302 badly clipped when taking these readings?
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Old 20th March 2013, 11:09 PM   #55
ultra is offline ultra  Greece
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yes, the output was badly clipped.

pin 1 and 7 of U300 was also distorted, looked like a trapezoid.

pin 1 of U301 and U302 was also very distorted as well

pin 7 of U301 and U302 had a bit blurry square wave output.
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Old 21st March 2013, 04:44 AM   #56
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This part of the circuit seems to be working as it should but it may require a few more tests using two probes to monitor two points in the circuit simultaneously.

Post the waveform that you have on the center leg of Q430 and the 3rd leg of Q431. Set the scope to DC coupling, 50v/div and set the timebase so that you have 3-4 complete cycles.
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Old 21st March 2013, 11:49 AM   #57
ultra is offline ultra  Greece
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my scope at the current set up is only rated for 20V/div so i have set it to 5V/div and both probes are set to x10.
first pic is at lower gain, and the second one is at higher (with more than 20VDC offset at the output).
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File Type: jpg IMGP4542.JPG (810.6 KB, 23 views)
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Old 21st March 2013, 08:31 PM   #58
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I need 1 waveform per display with the trace centered on the reference line before probing the test points.
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Old 21st March 2013, 09:56 PM   #59
ultra is offline ultra  Greece
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something like this? (probe is set to x10)

i noticed that T400 is a bit warm while T401 is cold...hmm
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File Type: jpg 3rd leg of Q431.JPG (935.9 KB, 12 views)
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Old 21st March 2013, 11:34 PM   #60
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That's not what I would expect to see. Was the trace centered on the reference line before you probed the test points?
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