UN55B8000XF No Backlights/Picture on (only sound)

[icekillah]

I got a UN55B8000XF Samsung That is not Showing an image on screen nor does it have backlights.

When the tv is turned on i get the "Samsung" chime and can get audio via HDMI, also the red optical audio light does turn on, and the Standby light stays on and will take commands from the remote control. But no Backlights and No Image displayed(checked with flashlight). Disconnecting the TCON makes no difference. I have suspected the mainboard because when the mainboard is disconnected from the PSU the Backlights DO turn on. There is a repair kit for this PSU that has similar symptoms but not exactly the same. I have checked the output voltages from the PSU and can post all of them. Now for pin 1 the "Power On/Off" i only get .81 volts when the TV is on and of course 0 when its off. Is this normal? To me it seems to be low. Also get 12v on the Tcon fuses. It would be nice to know a testable Point of Failure, any ideas?

[budm]

Re: UN55B8000XF No Backlights/Picture on (only sound)

"when the mainboard is disconnected from the PSU the Backlights DO turn on" That means the power supply, LED drivers, LED strings are OK.
PWR-ON/OFF (PS-ON) >2.5V (typ.) = on, <1V = off
How about BL-ON and Dim control signal level?
So far it looks to be main board problem.
You should upload the pictures of the whole back side of the TV showing all the boards.

[icekillah]

Re: UN55B8000XF No Backlights/Picture on (only sound)

Sorry it took me so long to reply I wanted to be thourough on these voltage readings. Please see pics. 0v for BL_ON when mainboard is plugged in and 2.9v when the mainboard is disconnected. 0.5v for p-dim when the mainboard is plugged in and 4.1v when the mainboard is disconnected. I was able to get the service manual and still have some stuff to try. Some of the DC regulators/transistors have unusual makings that I cannot lookup (see pic, are the pin layouts usually the same on this transistor?). I'll take it since there is 5v on the mainboard there is suppose to be a 5v to 3.3v regulator that is not doing it's job? No IC's on this mainboard really get hot, bga a bit but that is probably normal. Any other ideas to try?

[Andrew F. Ali]

Re: UN55B8000XF No Backlights/Picture on (only sound)

OK. So you do have a Main Bd. issue. Are you going to replace the main Bd.? or are you going to try and fix??

[icekillah]

Re: UN55B8000XF No Backlights/Picture on (only sound)

I would like to repair the mainboard, some of the DC regulators are on the back which will make voltage checks difficult to take. Would it be possible to check them via diode mode, with the mainboard removed from the TV? I am confident in my smd soldering skills, and this mainboard is very difficult to find online. I would upload the service manual for this TV but I am sure that would be against forum rules.

[icekillah]

Re: UN55B8000XF No Backlights/Picture, (only sound)

ok I am going to pull the main board out to test the regulators on the back, it looks like I am not getting 3.3v. I am going to use the 5v stby, the 5v stby(ground) from the psu. Do I also need to ground the main board to the chassis/panel? Am I missing anything?

[Andrew F. Ali]

Re: UN55B8000XF No Backlights/Picture on (only sound)

I would like to repair the mainboard, some of the DC regulators are on the back which will make voltage checks difficult to take. Would it be possible to check them via diode mode, with the mainboard removed from the TV? I am confident in my smd soldering skills, and this mainboard is very difficult to find online. I would upload the service manual for this TV but I am sure that would be against forum rules.

I would be happy if you could upload the SM. It is not against forum rules. As for the 5V, I use a 9V battery with a 7805 regulator to power the Main Bd. and solder the battery Neg (-) to the GND on the Main Bd itself.

[icekillah]

Re: UN55B8000XF No Backlights/Picture on (only sound)

Sorry it took me so long to reply i had to go out of the country, but now am back and ready to work on this tv again. I tried to upload the pdf service manual but it was too large at 58MB, here is the download link:




That is a nice trick for a steady 5V supply i guess the next step is to work backwards from the mainboard to psu connector and try to find my mainboard's 3.3v power rail? or should i just start by checking the MOSFETS/DC Regulators for 3.3 on their outputs? Where would you start?

[nomoresonys]

Re: UN55B8000XF No Backlights/Picture on (only sound)

Theres a mainboard here BUT I'm not sure if its a match, it is on the shopjimmy substitute board list, will give both links. https://www.ebay.com/itm/Samsung-UN4...31n:rk:24:pf:0

[nomoresonys]

Re: UN55B8000XF No Backlights/Picture on (only sound)

https://www.shopjimmy.com/samsung-bn...b8000xfxza.htm

[icekillah]

Re: UN55B8000XF No Backlights/Picture on (only sound)

That is very interesting, according to shopjimmy the following boards are Substitute Parts:

BN94-02640A
BN94-02640B
BN94-02640M
BN94-02640C
BN94-02640D
BN94-02640E
BN94-02640F
BN94-02640K
BN94-02640L
BN94-02979J
BN94-02640N
BN94-02640P
BN94-02640Q
BN94-02640R
BN94-02640S
BN94-02640T
BN94-02640U
BN94-02640V
BN94-02640W
BN94-02661E
BN94-02661F
BN94-02661J
BN94-02787C
BN94-02787D
BN94-02787E
BN94-02979A
BN94-02979B
BN94-02979C
BN94-02979D
BN94-02979E
BN94-02979F
BN94-02979G
BN94-02979H
BN94-02979K
BN94-02979L
BN94-02979M


Has anybody tried a mainboard swap from a 46" into a 55" ?

I still prefer repairing my current mainboard, but its always good to have a plan B. I'll be able to get to it tomorrow. Now if I find a short to ground on this power rail what voltage and amperage should i set my Adjustable Power Supply to? I know low volts and high amps (would 2 volts be too much? amperage cranked to full?)

[icekillah]

Re: UN55B8000XF No Backlights/Picture on (only sound)

OMG nomoresonys i ordered your ebay link for a 46 mainboard that was on shopjimmy's list and..... It worked! Thank you I would have never trusted shopjimmy's list. Good for Samsung making interchangeable parts, a long time coming. I do have to use a different Samsung remote control but worth it. I am still going to try and figure out what happened with the original mainboard but for now the set is up and running with no issues at all. Thanks again

[nomoresonys]

Re: UN55B8000XF No Backlights/Picture on (only sound)

Glad it worked out.