RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

[triplefour]

a friend got this tv from another friend who wants to fix it. he's willing to pay 100$

at first tv would turn on after pressing power a bunch of times. but it would

just be a blank backlit screen. hdmi audio or video could not be verified in this state. my laptop didnt even notice HDMI was being plugged in.
this makes me think dead mainboard.

took apart and saw some kind of liquid drops all over the place. it doesnt look like water. water dries up eventually and this has been sitting for a while. in some places where the liquid had touched the metal of the chassis, white oxidization is occuring.
could be sulfer dioxide damage from the volcano. must investigate where this tv was living. we had an eruption recently and ive seen a few tvs with this kind of damage.

on pwr plug in, 5vsb present, standby led lights up.
on power btn press, red LED goes away, and psu powers up 12v and 5v rails.
pson is 5v
backlights do not turn on.
i see 50v on all strings.

there is a 3 pin connector on psu:
the BLON pin is 0 and the adj pin is 5v...


testing with led tester i see that each string powers up to 104v so 50v would
not be enough to light anything.

when i trick the psu with mb disconnected and pson to 5vsb and blon to 5v, i still see only 50v on each string. adj is 0.

putting 5v to adj pin changes nothing.


as a general course of troubleshooting for most tvs:
before i can be sure this is a mainboard problem, i have to see that the PSU
is able to power the backlights. do i have a bad PSU that can only do 50v, or
is there some other signal the psu is missing to turn the backlights all the
way up? (like the adj pin?) i dont see any other pins to mess with.

i dont see the bl voltage jump up to anything over 50v. it just goes right to 50 immediately.

any help will be greatly appreciated. thanks in advance!

[Andrew F. Ali]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

Are you using the chassis a GND for your voltage testing? What voltage are you getting across the big main capacitor at plug-in and when the pwr button is pressed? It does appear to be a Main Bd. problem from the testing you did already.

[petehall347]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

ant/cable socket is broken .. maybe not the problem but needs fixing by the looks of it .

[triplefour]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

the voltage on the main cap (which is rated 450v) is kinda strange...in standby its 30v, and when i power on the tv it goes up to around 270 but its not exactly stable. it jumps around and goes from as low as 255 and as high as 280. this is with the mainboard plugged in normally.

when i leave the mainboard out of the equation and use 1k resistors to jump pson to 5v and blon to 5v i see between 260 and 270v but it seems a bit more stable, not jumping around as much or as often.

and yes i did see that the tuner was broken but right now im not worried about the mainboard. i just want to figure out why backlights are not coming on even tho i am giving it (in my mind) the proper signals (PSON + BLON to 5v)

and yes i am using chassis ground for the negative lead on my voltmeter.

thanks for the input!

[budm]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

Always remember that when working and making Voltage measurement in the hot side you must use the negative legs of the main filter cap as the GND ref. for your meter, you can use chassis as the GND ref when you working in the cold side.
So when the TV is on you should have around 380 ~ 400 VDC measured right between the two legs of the main filter cap.

[budm]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

Can we see good clear straight shot of the whole top and bottom side of the power supply board?
From post #1 "just be a blank backlit screen" Are you saying you do have backlights or no backlights?

[triplefour]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

thanks budm for the reminder about ground reference. measuring with black probe on negative leg of primary filter cap, and red probe on positive leg i see 157v in standby and 390v after pressing power. they are stable. this looks normal now. i dont know how i forgot such a basic rule but i guess if u dont use it you lose it! making a big mental note from now on.

as for the blank backlit screen condition:
i had that condition at the very beginning when first looking at the tv (before taking it apart) but eventually it wouldnt turn on the backlights at all. not even a flash. before, it would be like...if i pushed power button a whole bunch of times, and then waited a few minutes, it would finally turn on with a blank backlit screen but not respond to any buttons or even turn off.
but alas, it stopped doing that even before i took it apart.

it looks like everything is fine on the PSU. i get all the 5v and 12v rails but the LED driver section is only putting out 50v instead of 104v i would expect according to the led tester.

i hope these pictures are good enough. i dont have the best camera and it is night time. maybe i can get better pictures during the day.

[triplefour]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

little help please? anyone?

[Andrew F. Ali]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

In the stdby mode that may be normal for you to get the 50V. I do believe you have a Main Bd. problem. Perhaps in the CPU/BGA system. You are not getting the BL-ON signal to the LED Driver.

[triplefour]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

I'm having trouble following that logic.
in my mind, when i put 5v to the PSON and BLON pin, then it is not in standby mode anymore because i am getting the 12v rails and i am getting some voltage (50v) at the led connectors.

in standby mode (whether mb is connected or not) there is no voltage to the LED's or 12v rails.

in my mind, if the PSU is working, there should always be a way to "trick" it into turning on the backlights without the motherboard being connected. therefore if i am unable to do this, the PSU has a problem.
am i wrong to think that?

[R_J]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

With the pson and blon bypassed, monitor the voltage at the led connector, see if this voltage goes to 100plus volts then drops back to 50 volts when the power supply is first pluged in, if it does it is likely there is a bad led in the panel.

[triplefour]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

i said it in the first post but allow me to clarify:
the voltage on the LED connectors never goes over 50v, and yes i did test this while plugging into power to make sure i didnt see a momentary jump up to a higher voltage and then back down to 50.
it just goes to 50. never over.

[budwich]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

question: are the "led voltages" that you are posting when you force on the power supply with the led strings unplugged or plugged?

[triplefour]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

those 50v will appear if i force the psu on, with or without the string connected, and also the same if the mainboard is connected and i press power button.

[R_J]

Re: RCA LED55C55R120Q powers on, no backlight, no response, wont power off.

It seems the led boost circuit is not working, Q601 is part of that circuit along with the fet (PQ403 )but it seems to be controlled by U601. What voltage do you have on pin 7 of U601?