Mulja tv repair portal
Equipment Repairs => LCD TV forum => Topic started by: Davy on February 18, 2013,08:00:33
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Hi have this Logic TV that's dead after a power cut. Model L22DVDB1
I suspect the switch mode controller IC has latched up, all I can dig out from the markings is C?8842S as far as I can make out, this is a smd device.... one clue is there are two high value resistors feeding in series from the 320V HT yet there is only 1.4V drop across them, these are are marked 914, in my books are 910K Ohms, making the total resistance 1.82M Ohms.
Alot o' Ohms for just 1.4V drop innit, yeah, yeah checked em both by substitution - just in case! Thanks.
Dave
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The switch mode chip is a Chip Rail CR6642S... took a macro photo with an incandescent lamp, came out claer as daylight!!!!
Posted details for future reference,
Dave
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This set is dead after a power cut... nope not the caps, I've changed em as this is what usually happens.
From the 320V HT we have two 910K smd resistors going to the chopper control chip decoupled by a 10nF, when not working I measured 1.4 volts across them, funny thing is touching the IC end with just a test meter probe brought the set to life!!!! This set continues to run merrily until you turn it off, touch the chain end of the chip and up it comes.
Touching any other part of the chopper control with a test lead will turn it ON.
When running this produces a 'start up' voltage of 27V for the chip... so when running there should be an appreciable drop across them and not 1.4V
Appreciate if anyone has data on the Chip Rail CR6642S chip of any suggestions - apart from clodding it in the deepest part of the Pacific!
There just isn't enough start up voltage on the chip to get it working, wonder whats in the chip to cause this to happen?
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If you only have a voltage drop of 1.4 volts that means the start up pin of the IC is being fed with 318.6 volts = BANG! I would look for a print break from the resistor to the IC or if you are measuring 318 volts on the pin of the chip, there must be and O/C inside the chip.
Keith
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My bad Keith.... 1.4V on the IC, thanks for pointing it out - yes it would go bang if there wasn't an open print.
I can touch the IC end of the feed or any part between the rectifiers and the primary side and up it comes. When running there is 27V on the start-up pin from the two series 910K resistors.
Gotta be the chips on-board regulator I'm guessing, I tried lobbing 1 megs across the 910k resistors just to try still no joy, the screen comes up with sound once it's working.
Looked high and low for a supplier and E-mailed a few firms.... I could order a 1000 from China was the nearest I got to sourcing.
The modern tackle are made for lobbing after they break down, surprising how many breaks down just out of warranty! Thanks again Keith.
Dave