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Vizio l32 makes high pitch noise then off then repeats

 
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PostPosted: Wed May 07, 2008 2:43 am    Post subject: Vizio l32 makes high pitch noise then off then repeats Reply with quote

i have repaired several lcd tvs now and this is the first one i have seen do this.

when you plug it in the light (yellow maybe? green?) comes on for a second and then off, while this is happening there is a very high pitch squeal coming from the back that seems to increase in pitch, then when the light goes dark the sound goes away.
and then it starts all over again and continues until you unplug it.

i have cracked the case on it and looked through the power and control board . i only see one issue on the power supply, the large 450v 150pf cap is just barely extended, have not discharged the cap yet to see if it holds charge or not.
need help on this one, it is for a friend who is way down on their luck and really needs some good fortune

thanks in advance
[email protected]
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Wed May 07, 2008 8:23 am    Post subject: Reply with quote

It sounds like the power supply is tripping for some reason. Either a fault in the PSU itself or a short circuit/overload on any of the output voltages.

Check for shorted rectifier diodes on the secondary side and also check for shorted filter caps. Try running the PSU with the load disconnected and suitable load resistors instead.
If there is a current sense resistor somewhere on the primary side (typically in series with the emitter/source of the switching transistor), make sure that it hasn't changed value. Also check out any snubber networks (R/C/diode) across the switching transistor or transformer primary.
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