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Old 08-04-2020, 04:46 AM   #1
jimbofz
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Default no backlight supply volts hisense ub50ec591uwt

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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File Type: jpg IMG_20200804_103722[1].jpg (595.2 KB, 37 views)
File Type: jpg IMG_20200804_103824[1].jpg (496.9 KB, 18 views)
File Type: jpg IMG_20200804_103934[1].jpg (564.6 KB, 17 views)
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Old 08-04-2020, 05:28 AM   #2
dick_barton
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Default Re: no backlight supply volts hisense ub50ec591uwt

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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File Type: pdf RSAG7.820.5687_PS.pdf (57.6 KB, 41 views)
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Last edited by dick_barton; 08-04-2020 at 05:33 AM..
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Old 08-04-2020, 05:44 AM   #3
jimbofz
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Default Re: no backlight supply volts hisense ub50ec591uwt

full raw dc 378v
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Old 08-04-2020, 05:49 AM   #4
jimbofz
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Default Re: no backlight supply volts hisense ub50ec591uwt

thanks for cct now im off to check volts around the led driver ic
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Old 08-04-2020, 05:54 AM   #5
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Default Re: no backlight supply volts hisense ub50ec591uwt

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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Old 08-04-2020, 05:59 AM   #6
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Default Re: no backlight supply volts hisense ub50ec591uwt

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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Old 08-04-2020, 06:01 AM   #7
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Default Re: no backlight supply volts hisense ub50ec591uwt

Quote:
Originally Posted by jimbofz View Post
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?
each LED on the BL string built in Zener Diode for forward current
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Old 08-04-2020, 06:04 AM   #8
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Default Re: no backlight supply volts hisense ub50ec591uwt

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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Old 08-04-2020, 06:05 AM   #9
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Default Re: no backlight supply volts hisense ub50ec591uwt

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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Old 08-04-2020, 06:08 AM   #10
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Default Re: no backlight supply volts hisense ub50ec591uwt

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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Old 08-04-2020, 06:14 AM   #11
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Default Re: no backlight supply volts hisense ub50ec591uwt

Quote:
Originally Posted by jimbofz View Post
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?
there are stage at the bootloader which turn the bL ON.. later we can inspect after you do the most importanr of tsting the lEDs

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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Old 08-04-2020, 06:22 AM   #12
dick_barton
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Default Re: no backlight supply volts hisense ub50ec591uwt

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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Old 08-04-2020, 06:46 AM   #13
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Default Re: no backlight supply volts hisense ub50ec591uwt

yes these diodes have been checked
and i have a scope
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Old 08-04-2020, 06:49 AM   #14
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Default Re: no backlight supply volts hisense ub50ec591uwt

Quote:
Originally Posted by Diah View Post
there are stage at the bootloader which turn the bL ON.. later we can inspect after you do the most importanr of tsting the lEDs

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.
in the process of strip down as you sugested to examine the leds futher i will update thanks
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Old 08-04-2020, 06:52 AM   #15
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Default Re: no backlight supply volts hisense ub50ec591uwt

Quote:
Originally Posted by jimbofz View Post
in the process of strip down as you sugested to examine the leds futher i will update thanks
also please upload photo on whole the back side show how the boards connected..

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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Old 08-04-2020, 10:18 AM   #16
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Default Re: no backlight supply volts hisense ub50ec591uwt

Quote:
Originally Posted by Diah View Post
also please upload photo on whole the back side show how the boards connected..

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 ??
the t con is separate from main board and the test with torch i couldnt see any pic info
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Last edited by jimbofz; 08-04-2020 at 10:28 AM.. Reason: add pic
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Old 08-04-2020, 10:38 AM   #17
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Default Re: no backlight supply volts hisense ub50ec591uwt

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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Old 08-04-2020, 12:37 PM   #18
jimbofz
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Default Re: no backlight supply volts hisense ub50ec591uwt

Quote:
Originally Posted by Diah View Post
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 ---
ok ive tested the leds as per your instructions ,
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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Old 08-04-2020, 01:13 PM   #19
Diah
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Default Re: no backlight supply volts hisense ub50ec591uwt

Quote:
Originally Posted by jimbofz View Post
ok ive tested the leds as per your instructions ,
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
TV BL LEDs are designed by Type(size) and single or double ,,and working voltage then lumen and the last FW currents.

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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Old 08-04-2020, 01:55 PM   #20
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Default Re: no backlight supply volts hisense ub50ec591uwt

did you test voltage at each led connector on the power board with plugs connected whilst powering the set on ?
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