Sony Bravia 55 KDL-55EX640 no backlight / flashes once

[triplefour]

got a sony bravia 55 inch LED tv here (first LED tv to fix, and biggest TV ive had yet, also first professional job fixing it for someone else, for money)

when i first plug it in i get the red standby light. when i push power, the red light turns off, and nothing else happens.
when i push power AGAIN, the green light finally comes on.
not every time, but maybe half the time, i see the backlight flash for a millisecond or 2.
if i shine a light at the screen i can see the sony logo, the menu, etc.

when i plug the N64 in i can see picture and hear sound but then there is a menu displayed over it saying the tv is in standby mode enabled or something and then it shuts down on its own.
the sound and picture on the n64 goes out, and after a while it comes back again and then goes out after a second or two. it does this loop a few times and eventually ends up turning off completely. the green light changes to a red light that is blinking. it blinks 6 times, and then stops, then repeats the 6 blinks.

without anything plugged into it i can navigate the menu screens (using flashlight to see) and keep the picture on for longer until it times out for no signal input.

anyway i think i have a backlight problem here, and hopefully nothing else.

i havent taken the cover off yet but will do that soon.

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

ok got the back off... LED tv's dont have inverters, right? they have LED driver boards?
well i cant find that. i could only find the PSU and the Mainboard...
i found a connector going from the PSU board that seems like its probably going to the LED driver.
it has markings that say VLED4+ thru VLED4-

but those wires seem to go directly into the panel itself.

not sure what to do here...

[holmzy]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

check all smt fuses on psu

[budm]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

The LED driver power supplies are on the top left side of the power supply board, you can see 8 grey caps and 8 diodes for 8 strings of LEDs, that is what I think you have.
The connector on the top right has voltage labels for it, what are the voltage on each pins when TV is OFF and when TV is on, make sure to give each pin name and voltage.
You may have either open or shorted LED sting that causes the LED power supply to go into shutdown.
Service manual:
http://elektrotanya.com/?q=showresul...oria=&kat2=all

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

ok it is a 15 pin connector, with one pin (pin 13) not connected
v_off is with the set plugged in, red stby light on
v_on is with the set plugged in, green light on

pin# name v_off v_on
1 3.3vstb 3.3 3.3
2 ACD 2.9 2.9
3 Pwr_on 0 3.3
4 audio12v .5 12.9
5 audio12v_gnd 0 0
6 reg_12v_gnd 0 0
7 reg_12v_gnd 0 0
8 reg_12v .15 12.9
9 reg_12v .15 12.9
10 BL_ERR 0 3.3
11 BL_ON 0 3.3
12 PWM Dimmer 0 1.27
13 NC 0 0
14 TconVcc12v 0 12.9
15 Tcon_on 0 3.26


as for surface mount fuses on the power supply i dont see any but the one main fuse, and its fine.


voltatges, to my limited understanding, seem correct?

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

great. it formatted away all my spaces

[budm]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

OK, looks like you have all the correct signal, the only signal I do not know is the BL_ERROR, not sure if HI (>2VDC) is good, or <1V is good. I did not download the service manual yet to look at it in details.

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

yeah i would like to know that too but i cant find any service manual out there..only the useless "user manual"
where is the good resource for all the service manuals?


edit: nevermind i found it

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

not sure what to make of what they are saying in the manual about 6 blinks because the conditions under which i have to put the tv to make the 6 blinks are different. they are talking like just when u turn it on, it flashes for a second then turns off and goes to 6 blinks. mine doesnt do that it turns on to green and the screen flashes. the only way i get 6 blinks is if i have my n64 plugged into it to actually give it a signal... then it does weird things... ill try it all again and see if i can get a better idea of whats happening
and it seems like it only flashes once after its been unplugged for a while and the first time i try it

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

yes its really strange. if i have the n64 plugged in and switch to the channel its on, then it plays the sound for a second (and if i hold a light i can see the image for a second... ok maybe 2 seconds) but on top of the image is a menu that says something about idle mode and that its going to turn off...i cant read it because its really dark and its too quick. the picture and sound go off but the green light stays on. after about 5 seconds the image and sound comes back, and disappears again. it goes through this loop maybe 3 times before it shuts down with the 6 blinks error code...


the service manual is pretty useless. it doesnt mention what BL_ERR should be, and the pages where it does mention it, its giving it a different pin number than i have on my board so i dont even know what board they are talking about

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

well im stumped....little help anyone?

[budm]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

Since you do have 6X code which indicate as backlights error. You can see if one of the wires to the LED string inside the panel is shorted to the metal panel or not, remove the LED connector from the board and then check each one of the 8 pins against the chassis to see if it shows any resistance.

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

ok i see infinite resistance on all pins to the chassis (a screw holding the psu board in)

[budm]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

OK, so the wires are not shorted to the chassis, next thing can be that one of the string has open circuit or one of the power supply for one of the string is not working.
What you can do is to monitor each one of those pin during the power on sequence to see if they all act the same way with regarding to the voltage readings on each pins, set the meter 200VDC scale and check each pin during power on cycle, may be one or two will show some abnormality.

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

what should i use for ground ref during this test?

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

ok i just went ahead and used the chassis screw on the psu for ground ref and i found that on pin 1 of CN3 and pin 1 of CN4 i get usually much higher voltages than the others.

behavior at power on time is like this:
0v when red stby light
0v when green light goes on
few more seconds pass
voltage present at same moment of backlight flash
voltage quickly drops to zero as im reading it
voltage remains on the other pins
reading the voltage on the remaining pins eventually zeroes them all out as well.
waiting a while does not bring the voltage back
only turning off and repeating the cycle brings the voltages back
they voltages vary with each test but pin1 of each CN3 and CN4 seem to have the highest voltage at around 60v but ive seen it as low as 40 and as high as 100
the voltages on the other pins are usually between 15 and 30v but they vary a lot from test to test.
probably my meter cant get an accurate reading since they immediately drop as i measure them.

heres the breakdown of the test where i held the red probe to the pin as i started up.

CN3
pin#---voltage
1---74v
2---33
3---24
4---21
CN4
1---42
2---17
3---15
4---26

and i did another test where i didnt measure anything as it was starting up, i waited till after the flash and then measured the voltages present as they dropped to zero. i repeated this test 4 times and wrote down the values.
i did more testing than this but i couldnt write everything down.

CN3
pin1 pin2 pin3 pin4
68,26,10,18
62,16,26,18
69,22,27,12
47,21,17,18

CN4
pin1 pin2 pin3 pin4
56,25,27,24
100,27,24,24
92,17,17,15
66,18,29,17

one thing i seemed to notice was that when i got the higher voltages, it coincided with a brighter flash. it flashes really lightly sometimes. i could only tell from looking at the back. and even then just barely. other times it flashes quite brightly. but always just for a moment.

anyway hope this tells you something. i wont try to guess what it all means. i got voltages on all pins thats all in know...

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

so...any ideas?

[budm]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

OK, I found picture of your board at shojimmy, it looks looks like it uses 4 strings of LED instead of 8 as I thought, there are 4 pairs of LED + and - as labeled at the connectors.
Can you take good clear pictures of the bottom side of the board so i can clearly how the connections are made around the LED connectors and the transformers (2) and those 8 didoes and caps close by? Too bad the service manual does not have any diagrams.
If I have to guest, I would say reading of 100V on CN4 PIN1 represent open LED string.
Do you know how to use your meter in DC AMP mode?

[budm]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

This is how I check to see if I have open LED strings, using AMP meter in series with the feeding wire of the LED string to see which one does not draw any current.

[triplefour]

Re: Sony Bravia 55 KDL-55EX640 no backlight / flashes once

ok heres the pictures of the board. sorry i cant get a good one of the whole board right now because my camera sucks, daylight is fading, and apparently i just dont understand lighting for circuit board photography well enough.

but i managed to get a good shot of the part you were talking about.

as for testing with my meter in amps mode i can safely say i have never done it. if you can explain clearly which probe to put where though, im sure i will understand it afterwards.