no backlight supply volts hisense ub50ec591uwt

[jimbofz]

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

[dick_barton]

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

[jimbofz]

full raw dc 378v

[jimbofz]

thanks for cct now im off to check volts around the led driver ic

[Diah]

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.

[jimbofz]

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?

[Diah]

Originally Posted by jimbofz

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

[Diah]

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

[jimbofz]

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

[jimbofz]

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?

[Diah]

Originally Posted by jimbofz

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.

[dick_barton]

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?

[jimbofz]

yes these diodes have been checked
and i have a scope

[jimbofz]

Originally Posted by Diah

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

[Diah]

Originally Posted by jimbofz

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 ??

[jimbofz]

Originally Posted by Diah

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

[Diah]

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 ---

[jimbofz]

Originally Posted by Diah

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

[Diah]

Originally Posted by jimbofz

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

[petehall347]

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