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Old 26th December 2011, 09:34 PM   #1
diyAudio Member
Join Date: Jan 2004
Default Help with waveform on rails - looks unregulated


While troubleshooting a dead channel on a QED amp I put the scope on the rails and I get what looks like partially rectified waveform, plus 3 phases?

Is my scope broken?
I mean normally the scope works fine. Why 3 phases? Positive end of rectifier bridge shows large sine 1/2 waves. Negative shows smaller ones.

Attached Images
File Type: jpg positive_rail.jpg (19.7 KB, 77 views)
File Type: jpg negative_rail.jpg (26.6 KB, 75 views)
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Old 26th December 2011, 10:56 PM   #2
JoelS is offline JoelS  United States
diyAudio Member
Join Date: Oct 2011
Location: New York New York
that kind of look on the scope has to do with the way the triggering is working. there can only be one voltage at any instant of time, but if you believe what the scope is telling you here then there is 3 different voltages at each instant. play with the trigger level, or use more time per division, this should clean up.

is that supposed to be the regulated voltage? maybe the main filter caps after the bridge are not working, or dead regulator

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Old 27th December 2011, 01:22 AM   #3
diyAudio Member
Join Date: Dec 2005
Location: Buenos Aires
It's a triggering problem. Try setting the trigger source to LINE.
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Old 27th December 2011, 01:28 AM   #4
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Join Date: Jan 2008
Location: Virginia
Yes the trigger is not working.
In the first pic looks like you have one shorted diode in the power supply bridge.
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Old 27th December 2011, 01:10 PM   #5
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Thanks !
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