RCA(Proscan) with two seconds of backlight

[Chuper]

Model: RLDED4633A-D
Main board: TP.MS3393.PC821

There is 10 led light strips with 6 led on each divided between two connectors/lines, so 30 on each connector/line in series, total of 60 led. 21 of these led were open. I changed all open ones. Now I got backlight for about 2 sec. It measures about 145V on two white connectors for led lines for a second without load and climbs up to 75V and slowly drops with them connected when they all briefly come on. I tried checking all strips separately with a power supply applying 20.4V to each and each read about 200mA on my ammeter.
Any suggestions on what the next step could be would be greatly appreciated.

[CapLeaker]

Re: RCA(Proscan) with two seconds of backlight

Not sure, but on your last picture is seems to me that some 20 or so LEDs are brighter than the others? The LED backlights are driven in current mode. So if the LED controller sees a mismatch in current on what the LEDs should draw and what they actually do draw, the LED controller senses that there is an error in the circuit or with the LEDs and it will shut itself down. Above that red cap CB325 is a little IC. What is the number on it? Should have something similar like OB3350CP. Pull up the spec sheet and see what happens around the pins. These have OVP, OCP, UVLO etc.
Something else you could try is to turn the backlights waaaay down to next to nothin and see if they stay on. The problem is that there still could be a bad LED hiding, that works for seconds and measures good, but once it gets hot it fails. That's why when that many LED's (like 20 of them) fail, replace them all...

[Chuper]

Re: RCA(Proscan) with two seconds of backlight

On some of the strips I replaced just one of them and on some as many as four but testing them individually they all lid up evenly and draw same current. That little IC is AS358M.
"Something else you could try is to turn the backlights waaaay down to next to nothin and see if they stay on." - maybe not exactly smart question but how do I do this?
Thank you CapLeaker for you help

[CapLeaker]

Re: RCA(Proscan) with two seconds of backlight

Well... that's the wrong IC... that would be a dual op amp. I am looking for the backlight PWM.
You have to turn the backlight down, in the menu settings / picture of the TV. Problem is you have no backlight, so you may have to shine a strong flashlight into the screen.

"It measures about 145V on two white connectors for led lines for a second without load and climbs up to 75V and slowly drops with them connected when they all briefly come on."
Could you explain that a little better? We want the exact voltages from each string with the backlights connected.

[budm]

Re: RCA(Proscan) with two seconds of backlight

We need the Voltage readings of the other two pins also, we know that 2 of the 4 pins are for the LED+ for feeding the Anode of each LED string, we now need the Voltage on the return pins to see what you have, you will have to leave the black probe on the chassis and read probe on the pin then turn on the TV while watching the meter to see what Voltage get, then turn off the TV and move the probe to another pin of the connector and repeat the same procedure, all this is done with EVERYTHING connected.
When there is no load the Voltage went up to 145V and then it goes into protection shutdown due to no LED's connected so the Voltage drops back down to 75V.
30 LEDs per string so the Vf of the LED string will be about 90V + Vdrops of about 20V on the PWM controlLed MOSFET, so the output Voltage at the LED connector with working LED connected should be around 110VDC or so.

[Chuper]

Re: RCA(Proscan) with two seconds of backlight

So I went to check voltages on LED- (cathode) against chassis. I don't believe in magic but it does work now. Here are voltage readings after TV powers on:
Connector 1: LED- 0.842Vdc; LED+ 99.7Vdc
Connector 2: LED- 0.885Vdc; LED+ 99.7Vdc
So the question is what did happen that it works now after a day of rest?

[budm]

Re: RCA(Proscan) with two seconds of backlight

We cannot tell because we did have all the Voltage readings of all the pins with LED connector in place when you were having problem.
But make sure to turn down the backlights level (not Brightness)to as low as you can

[Chuper]

Re: RCA(Proscan) with two seconds of backlight

budm are you referring to a setting in a TV menu?

[budm]

Re: RCA(Proscan) with two seconds of backlight

It should be in one of the user on screen setup page.

[budm]

Re: RCA(Proscan) with two seconds of backlight


It does not look like it has one, so you may have to try 'Picture Mode' to see if one of the setting will reduce the backlights level or not, otherwise you can expect the LEDs to fail again since they drive them at max rating.

[Chuper]

Re: RCA(Proscan) with two seconds of backlight

I did go through the menu and looks like there is no way to lower backlight level. Thank you budm for the manual. I guess this thread came to it's end.
P.S. amazingly helpful forum you got here guys!

[CapLeaker]

Re: RCA(Proscan) with two seconds of backlight

That is correct, there are some RCA models that one can't adjust the backlight level. I've ran into this once before a few years ago. I've just remembered because you said it. Why things are working now, I could be still a faulty LED, could be a dirty or bad connection somewhere, a cold solder joint, something along that line. I am glad you got it working for now.