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Old 02-25-2012, 04:45 AM   #1
link988
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Default Circuit voltage measurement problem.

Hi

I got an issue with power supply section of my plasma TV. When turned on it seem to get green light like its starting and after like 10 sec goes to blinking red.

I opened up the unit and started pinpointing the culprit so found when TV just switched on at point where it supposed to be 400V to LLC_SUPPLY it goes up to 380V and decreases to about 140V then a relay click and blinking light appears voltages just gradually continues to drop. I traced it to bridge rectifier and got 230V on pin 2 when tv in that 10 sec period and goes to 0 when red light starts blinking. The question is on pin 3 is the Volatge supposed to be 230V as well, as arrows show that it is an input as well? (its probably me not understanding how to measure it maybe? I measured it the same way as pin 2. DMM to AC setting and black to good ground and red probe to pin 2 and pin 3)





Is that AC2 line supposed to be 0V or I am making very stupid mistake when measuring it? Any help or advice please

Thank you.
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Old 02-25-2012, 06:09 AM   #2
retiredcaps
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Default Re: Circuit voltage measurement problem.

Quote:
Originally Posted by link988 View Post
DMM to AC setting and black to good ground and red probe to pin 2 and pin 3)
When measuring voltage at a bridge rectifier, you want to measure DC voltage.

Remember what a bridge rectifier is supposed to do.
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Old 02-25-2012, 03:42 PM   #3
link988
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Default Re: Circuit voltage measurement problem.

Quote:
Originally Posted by retiredcaps View Post
When measuring voltage at a bridge rectifier, you want to measure DC voltage.

Remember what a bridge rectifier is supposed to do.
Thank a lot for reply.

I measured the voltage at pin 4 which supposed to be output pin i think and pin 1 and both show 139 to 137 V variation untill TV starts the blinking light thing which I think is protection mode of some kind as relay clicks and voltage in rectifyer drops.

If input is 232 V AC and output 137 -139 V DC does it mean rectifyer is done for? Or I should look around for defective components around it?

Still curious why does AC1 reads normal 230V and the parallel to it reads 0 V can it be the cause of low output of rectifier?
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Old 02-25-2012, 04:19 PM   #4
b700029
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Default Re: Circuit voltage measurement problem.

Take your measurements relative to the - pin on the rectifier (i.e. put black probe of DMM on it.) The + pin should give approximately 1.4 times your line voltage (~340V for 240V mains, ~170V for 120V mains.)

You can also remove the rectifier and check its 4 diodes separately out-of-circuit.
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Old 02-25-2012, 04:32 PM   #5
link988
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Default Re: Circuit voltage measurement problem.

Quote:
Originally Posted by b700029 View Post
Take your measurements relative to the - pin on the rectifier (i.e. put black probe of DMM on it.) The + pin should give approximately 1.4 times your line voltage (~340V for 240V mains, ~170V for 120V mains.)

You can also remove the rectifier and check its 4 diodes separately out-of-circuit.
Ah ok thanks just did it, and it measures to 298 - 300V and if its 1.4 x V AC then should be 322V aprox so its 20V off is that acceptable?

What could cause the power to spike up to 380V DC operating voltage and then gradually drop to 140V over about 10 sec period?
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Old 02-25-2012, 05:09 PM   #6
link988
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Default Re: Circuit voltage measurement problem.

I found on another forum that graduall drop of voltage is highly likely caused by bad capacitors. So I just had a look around the diagram and its just a guess by could this 2 cpacitors be going bad and thus the whole thing is occuring? (Circled in Red). The caps look all right but that is all I can say about them.



My guess is that TV starts (well no screen no sound but still) green light on then a voltage drop started and when it gets to about 140V the system detects a fault causing it to go into protection mode.
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Old 03-02-2012, 10:35 PM   #7
budm
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Default Re: Circuit voltage measurement problem.

Well that pre-conditoner circuits is PFC Voltage booster to get the TV to run as close as possible as PF of 1, measure the DC the 400vdc caps when the TV is off, then the voltage should kick to close to 400Vdc when PFC circuits kicks in, but if there are problems, the PFC will shutdown, that is why you see DC on the 400Vdc caps goes down. Plasma use a lot of power as it is, and without PFC, it will be worst and consume more power.
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