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Nanao MS2931 stuck in protection
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Question
soyl
Hello,
I'm trying to fix this monitor that doesn't display any picture and was supposed to have shorted components in the deflection area due to potting compound leakage from the linearity coil:
I've removed the coil, the HOT and the flyback to clean up that mess but the HOT turns out to be OK:
http://img12.imagevenue.com/loc453/th_343454500_008_122_453lo.JPG
http://img188.imagevenue.com/loc456/th_834232041_004_122_456lo.JPG
http://img21.imagevenue.com/loc422/th_840638573_003_122_422lo.JPG
So I did a light bulb test to see if the problem is in the deflection or in the power supply and the bulb lights up for a couple of seconds and the voltage is the expected 180V but then it shuts down and goes in protection:
http://img12.imagevenue.com/loc543/th_345021819_0112_122_543lo.JPG
According to the manual, if voltage at pin 7 of the control IC U901 (M62281) goes above 4.0V the power circuit operation stops and in fact I have 5.3V there:
http://img154.imagevenue.com/loc156/th_540817947_015_122_156lo.JPG
I've tested pretty much everything in the primary side: the mosfet Q901 is good, the diodes D901, D902, D903, D904, ZD901, ZD902 are all OK. C910 is OK, photocoupler PC901 is OK and I didn't find any bad resistors including the SMD ones. The diodes on the outputs of the power transformer (D921, D922, D923, D924) are also good.
What could be the culprit? Bad control IC (U901)? Bad b+ detection IC (U923)? Or could it still be something in the deflection?
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