RLDED3258A-C backlight repair and LED current adjustment

[@midnightman011]

Good video and explanations .. thanks for all the hard work with the camera Tom

[@supremelurker727]

Which strips are LED 2…LED 1 and LED 3? It’s not labeled on the metal piece.

[@dev1ator_yt]

Hi. How do you get these low mA readings ? if each LED is using/needs "300 mA" 21x led... i dont get this. I dont have clamp like this, but I connected new multimeter in series in front LEDs and got: 5.20-5.34 A, 112V. I dont know if this is correct current, first on mA got - OL, then on 10A - 5.20 A, I was surprised. If I connect A after LEDs - black wire will this show excess current or current of last LED ? I never done this kind of measuring. I plan simply add resistor in series with LEDs I could not find datasheet for my bd559 led driver and 4x R are 100k (104), wattage of resistor must be no less than 2 watt like 2W leds right ? or its better to put higher 3-5W resistors, more will endure probably. I guess in your video for 39LN5700 is 5W resistor, but did you also remove other resistors in series with MOSFET when you did test with that 4.3 resistor ? because one of 4.3 Ohms in front leds seems low.

[@tomtechtod9200]

1. There are always LEDs are in series, one string or several of them. When in series, currents don't add up, the current in one LED is the same as the one in series with it. 2. A multimeter will very likely give false readings when measuring switched currents. Don't forget, the LEDs are usually driven with a PWM signal of several hundreds Hz and more, , that's why I use a current probe and scope, the only reliable way. 3. Putting a resistor in series with the LEDs will not decrease the effective current in the LED until the PWM is maxed out. The "current sense" resistor value needs to be slightly increased so the LED driver "thinks" the current is higher than it actually is and will therefore decrease the PWM duty cycle. Most chips have probably other inputs to dim tge LEDs but most of the time that information is hard to find so I prefer increasing the current sense resistor value where possible or decrease the backlight setting in the TVs menu when available.

[@dev1ator_yt]

​@@tomtechtod9200 Thank you for explaining. Ill try to find that current sense resistor on PCB or try different solution with current modules, reducing backlight to 90% sadly didnt help me for long.

[@tomtechtod9200]

@@dev1ator_yt indeed, 90℅ is not much. I usually lower to 50 - 70℅. I just saw that in this TV they really drive with a DC current from the output of a boost converter. These TVs are even worse for finding information on the LED driver circuit, the principle remains the same though. Led current is sensed through some low Ohm current sense resistors and if you increase their value the LED current will drop.

[@DemiGod3]

Is there any ways to enable or turn on fully the OB3350CP chip, i have a broken board that i want to just use the led drive circut, but i cant get it to operate, its currently at 46volts, but the driver mosfet there is not signal on the gate for it to oscillate to 120volts to power the led array.

[@tomtechtod9200]

What are the voltages at the pins?

[@DemiGod3]

TomTechTod probably I didn’t make to mention that the logic side of the board isn’t working have couple blown chips on the lower logic level, I’m just trying to use the power supply side not much of the 12volts but more of the LED supply side to power another tv that have a bad backlight circuit. Just want to use the two boards to make one. One is turning on but no backlight and other has good power supply but I don’t know how to make the backlight circuit oscillate to full mode to drive the LED it’s at 46v but need (at least 120v) but the mosfet doesn’t have any gate drive so I know the donor board isn’t fully on as yet.

[@tomtechtod9200]

You will have to measure the controller IC voltage levels at all pins with an oscilloscope.

[@riyadabdulhafeed2899]

Thank you for this helpful video , regarding the led current why don't you decrease the backlight from service menu ? Would it decrease the backlight current also?

[@tomtechtod9200]

yes, that's possible too if that adjustment is available. (on some TVs it's not). I prefer doing it on the hardware side because on many TVs this menu adjustment has to be done for each input separately and many other menu options can possibly override the backlight adjustment, like ECO mode, Image mode etc. And I don't feel like reading the whole user manual. ;-)

[@mrsemifixit]

@@tomtechtod9200 What setting would you set it to for backlight to a safe level, 50% or lower?