toshiba 58l4300uc no back light

[tonytiger]

hi everybody,
i have this toshiba 58l4300uc that when power on will just quick flash and green power lead stay on(green)
checked with flash light test and yes i have picture in the back..

i also checked all my voltage on power board and i do have my 5v and 12v where all indicated.

the only place i missing voltage is on one line(lB1-)0v
but i do have all voltage on lB1+ to lB5+ (power on 127v on all/power off and on again it goes to 200v and come back to 127v)

on lB1- ov but on all the others(lB2- to lB5- i have 18.4v

i did check the Q907b,Q907c,Q907d,Q907a black lead on panel and red lead pin on left and pin on right on all these giving me 12v

but Q907e giving me 0v on left or right side pin.

so i presume i do have a problem here....is there a way to check this line,i been testeing all small parts around it and in the back and they all seem to be ok....

i have no diagram for this tv. and also wondering is there a way to bypass my back light to test and how.

by the way i did took the front panel out and did a check of all leds individual with the 3v and was all ok. and also did a 135v or more on each 5 stipe led and the stipe led all came on ok.

by the way this is my first post so please if i do something wrong,please let me know,thank you.

[CapLeaker]

Re: toshiba 58l4300uc no back light

Since you tested out the LED strips,is the led driver Q907e shorted?
BTW, pretty good troubleshooting for a first poster.

[tonytiger]

Re: toshiba 58l4300uc no back light

thank you,since i been reading alote,i guest i been learning more than in school,and i do have to say you guys are very helpful also.

yes i did check the q907e onboard and i did took it out of the board and tested ok,and i even swamp it with q907d to make sure and q907d(with my old q907e) tested ok( 12v) but my new q907e(good q907d) was still 0v.

so i do know that this q907e is ok but must be something in this line,but with no diagram it is had to see....

[CapLeaker]

Re: toshiba 58l4300uc no back light

check transistors Q902(x). there are 5 of them. The bottom left one on your 4th pic. Pic is a bit blurry, but it looks like Q902E. Check U901 on pic4.

What transistor is Q907? There should be something printed on it.
What is U901?

[tonytiger]

Re: toshiba 58l4300uc no back light

ok,sorry for delay

i took some more photos in case,but as for the small transistors in the back q902e,tested ok and same for the 4 others,tested all the same reading.

as for the numbers printed on it,like you will see,i been trying alot of thing to try to read it but no luck.

and for the ic u901 (9908bgn
cd7318.2n
1302a2

is the number.tested all the small cap and res,ect....all around that so i guest it will be this ic,but still weired that only on line of led not receiving power from here....

[CapLeaker]

Re: toshiba 58l4300uc no back light

I can't read it. An oz9908agn maybe?

[dick_barton]

Re: toshiba 58l4300uc no back light

I can't read it. An oz9908agn maybe?

I think it's an OZ9908B from O2Micro

https://cdn.badcaps-static.com/pdfs/...6f8b3d4b96.pdf

[tonytiger]

Re: toshiba 58l4300uc no back light

yes i think your right with that number,it is very hard to read.
i will try some other test since you provided me the chip info tonight.

but at this point i guest the best thing will be buying a new power board.....

i let you know.

thank you very much guys for replying fast,very appreciated.

[Mayn]

Re: toshiba 58l4300uc no back light

Sorry to hijack this thread, but I can't start a new one yet.
I have this same tv, and I get picture. The problem is I can't adjust the brightness. It started flashing between normal and dim, and now it just stays dim. not black, just dim, and I can't adjust it to be brighter.
Any ideas?

[Mayn]

Re: toshiba 58l4300uc no back light

The image is perfect, no dark spots or anything. no obvious issues. I picked this up from someone else who already had the main board replaced and the problem was still there.

[Mayn]

Re: toshiba 58l4300uc no back light

Measured 114v on output from power supply to all led strips, and 3.4 on the BLON. Don't see anything labeled as DIM on this set.

Since the voltages seem fine, and the main board has been replaced with no difference, could a bad t-con board cause this? As i said, the image is perfect everywhere, just dim.

[Andrew F. Ali]

Re: toshiba 58l4300uc no back light

No backlight symptom is not a T-CON Bd problem...and since you do see a pic with the 'flashlight' test...this proves good T-CON Bd..... Look at the LEDs again and the interconnectors as well as the connector from PSU to LEDs....test each LED individually...Trace the source of that missing B1 voltage.

[Mayn]

Re: toshiba 58l4300uc no back light

The only thing that seems odd to me is the measurement I get on PWM on the P802 connector. I only get 0.4V. Any suggestions?

[Mayn]

Re: toshiba 58l4300uc no back light

I don't need to do the flashlight test as I have an image on the screen. just can't make it brighter

[Mayn]

Re: toshiba 58l4300uc no back light

Seems from looking around on these forums that PWM should be around 2.5V, so I'm guessing that's the problem. Anywhere I can 'steal' that from to make it work?

[tmhobadcap]

Re: toshiba 58l4300uc no back light

No backlight symptom is not a T-CON Bd problem...and since you do see a pic with the 'flashlight' test...this proves good T-CON Bd..... Look at the LEDs again and the interconnectors as well as the connector from PSU to LEDs....test each LED individually...Trace the source of that missing B1 voltage.

I have the same model tv and the same backlight problem. (But strange, the led is 6V instead of 3V.) I found the problem was poor connection at one of the interconnectors.