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Thread Tools | Display Modes |
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#1 |
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![]() hi im no stranger to these and its almost always the backlight leds
but not this time ive stripped down and tested all leds and they are all good . the psu comes out of sby and sound is present the 12v and 16v supplies are all good but the backlight feeds are only 3volt , this has two chains of leds and two plugs feeding when i unplug the feeds even with peak hold meter theres only 3v the two fets ca5b3d that supply the tx test ok, and i now suspect the surface mounted driver ic msc lx 27901 id ,i wonder does it contain a comparitor cct that monitors the two strings of leds ? and if the two current demands of the strings arnt the same then it switches off the fets ? any ideas please |
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#2 |
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Line Voltage: 230V AC, 50Hz
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![]() What voltage do you have between the legs of either of the 68uF 450V capacitors
The led voltage is given as 200V @ 400mA which appears to be missing. Possible schematic
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Willing to help but I'm no expert. Last edited by dick_barton; 08-04-2020 at 05:33 AM.. |
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#3 |
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![]() full raw dc 378v
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#4 |
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![]() thanks for cct now im off to check volts around the led driver ic
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#5 |
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![]() i guess you have OPEN Zener Diode on LED , with test of just lighting up didn't approve the zener condition .. it should tested with DM on diode mode or you have soldering point on one of the socket of BL string this why 3V on LED BL socket low.
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#6 |
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![]() very interesting is this zener on the psu board or inside with the led string ? as i didnt see any diode on the led strips?
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#7 |
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#8 |
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![]() also with DM on diode mode, if you test each one position it should lid on dimm ( depend on the batteries of the DM ) and if you put the DM probe in opposite direction it should read Zener forward current
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#9 |
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![]() i tested the led strips first with diode test to see if the volts from my meter was enough to light the leds separately then each strip was fed with 27v and all leds lit with even illumination then all the connecting leads were tested for o/c connections
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#10 |
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![]() i can open it up again to check.
but would i not get a feed voltage with strings both unplugged on peak hold meter setting ,even for a second? |
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#11 | |
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![]() Quote:
tear it down and put the LCD at safe place until you have the BL working... as well the screen apart from the T-CON ribbon the set working test the LED as i told you first.. and the socket wiring too. Last edited by Diah; 08-04-2020 at 06:15 AM.. |
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#12 |
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![]() Looks like the led is driven by an high frequency AC signal driven via T902 & T901 and rectified by diodes VD903 to VD906. Have you checked those diodes?
Do you have access to an oscilloscope? |
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#13 |
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![]() yes these diodes have been checked
and i have a scope |
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#14 | |
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![]() Quote:
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#15 | |
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![]() Quote:
if your panel integrated with T-CON, the set will not working if the panel out side the set. also did you check before when you have sound to hear with torch light can see picture on the screen ?? |
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#16 | |
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![]() Quote:
Last edited by jimbofz; 08-04-2020 at 10:28 AM.. Reason: add pic |
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#17 |
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![]() open the metal shield of the T_CON post very clear close photo of it... also you have on it SMT fuse test it black probe on chassis if you have 12V. on both side of the fuse.
BTW: if you confirmed there are 12V on both side of the T_CON fuse.. please put every things in place and turn on the TV, black probe on chassis and read all the voltage as they are listed over here PWM2 -- SW --- 12V --- STB --- 16V --- 3DS --- 3DE --- PWM3 --- Last edited by Diah; 08-04-2020 at 11:01 AM.. |
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#18 | |
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![]() Quote:
there are 8 leds that measure oc when i reverse the test leads on mm using diode test so before i can test the tcon i will try to source some correct leds with the correct reverse resistance and then get back to you thanks for expert help so far |
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#19 | |
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![]() Quote:
all these information can be checked on the led string .. in case no much info on the string label , need to solder out one defect one. to measure it |
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#20 |
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![]() did you test voltage at each led connector on the power board with plugs connected whilst powering the set on ?
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