LG 49LJ550M-UB Looking to do some mods

[KxngCrooked8]

Hello,

I have an old(er) LG 49LJ550M-UB that I'm looking to convert into a light source/audio player.

The only issue I'm currently experiencing is that I can't get the LED's to stay on permanently. I still have the intact LCD panel and have fiddled with all of the settings trying to get the LEDs to stay on with no luck. In lieu of that working, I'm hoping to trick the power board into keeping the LED's on. I think that I've found the correct signal that I need to send, >3V to the PDIM pin on the P201 connector on the power board. But I'm not sure where to grab that voltage from as most of the voltages on that connector seem to be 13.2V. Is there something simple I'm missing? Is there an easy jumper I can install or component that can be added to force the LEDs to stay on?

I've attached pictures of the power board for reference.

Thank you,

[maneatingmonkey]

How are you driving the PWR_ON signal? That should also be around 3.3V, no?

[KxngCrooked8]

I came across a service manual for my TV model that explains how to test the LEDs by using a 3V power supply (in the form of 2 x AA batteries) and connecting that 3V to PWR_ON and to PDIM with the ground going to PIN3. When I do that, I am able to get the LEDs to come on, but they flicker. Based on some measurements that I took before I disconnected anything in the TV, the PDIM was ~2.2V when the LEDs were being turned on by the logic board. What I'm assuming is happening is that 3V is a bit high and the power board isn't able to output that amount of power continuously. I'm hoping that I can drop the voltage on my AA power supply to ~2.2V and try again to see if that solves the flickering issue.

I'm open to ideas on if that sounds like the right track. Or to what you may think the flickering issue is caused by and how to solve it.

Thank for responding!

[maneatingmonkey]

Hey, so backlights on a TV are controlled by a PWM signal, whereby 50% duty cycle corresponds to 50% brightness, etc. You can't accurately measure this signal with a multimeter. A reading of 2.2V would correspond to a 3V signal at 75% duty cycle though. (Mathematically, it could also be a 220V signal at 1% duty cycle, but since PWR_ON is supposed to be 3V, we can assume a common signal voltage level).

So if you wanted to run your TV at max brightness, in theory all you need to do is supply a constant 3V DC to PDIM. I'm not sure why this isn't working for you. Can you measure the voltage across the batteries under load? And maybe also the current being drawn from them?

[KxngCrooked8]

I was able to measure voltage across the batteries. They are outputting 3.1V. I also verified that the 3.1V is making it to both the PWR_ON and the PWIM pins on the power board. This is true both when A/C power is connected and when it is not. I am not sure of how to measure the current. My electrical skills/experience are somewhat limited. I've only ever measure current with a clamp and I don't have one of them. I could possibly borrow one from work but will need to check. Since power is ultimately tied to both voltage and current, I assume the correct voltage with not enough current could also cause a lack of sufficient power to the LEDs?

Update: decided to try running it with only 1 AA battery, giving me an output of ~1.6V. When I do that, the LEDs still come on and flicker. I find this interesting because this means that the PWR_ON pin is also only getting 1.6V, yet everything is still turning on. I also measured the voltage going to the LEDs for both 1.5V and 3V input signals. I was surprised to find that they were the same: fluctuating between 105 and 110V. If I'm remembering correctly, when I tested the power to the LEDs with the cable connected to the logic board, I was getting ~130V.

Any thoughts? I'm honestly lost as to what is happening.

[maneatingmonkey]

DW about measuring the current. For a 3V digital signal the midpoint is 1.5V, so theoretically any voltage above that should be treated as ON an anything below that should be treated as OFF. It's a little more complicated than that in practice, so I wouldn't rely on using on one battery.

As for why the LEDs are flickering, I'm not sure. Does the service manual say anything about the MS pin? What voltage does it read when connected to the mainboard?

[KxngCrooked8]

Here is a table showing the expected voltages at each pin.

I plugged the cable back into the logic board, powered up the unit, and took a voltage measurement at the MS (DRV_ON) pin. It was reading ~3.1V.

As I'm looking at this again, I was jumping 3V to pins 1 and 11 (PWR_ON and MS (DRV_ON)). That means that I'm not pushing 3V to pin 12 (PDIM). Would a lack of PDIM signal cause flickering LEDs? Still open to any ideas.

[Diah]

@KxngCrooked8

what do you mean with "" I'm looking to convert into a light source/audio player."" ?
could you explain the mode you are trying in more detail ?
LG LED BL are critical as they used LED units build in zener diode

[KxngCrooked8]

Diah

What I'm looking to do is to modify/augment the wiring of the TV to allow for the backlights to come on and, if possible, still retain use of the audio input via the component ports on the logic board. What I've done is removed the case of the TV and the LCD panel. I am working on building a "window frame" to put the remains of the TV into. Then I will hang the "window" on a wall of my home in order to provide soft light into my home. Does that make more sense?

So far I've gotten the system/LEDs to power on using my little 3V jumper, but I can't get the LEDs to come on and stay on. They just flicker constantly. any input on to why this may be happening would be helpful.

The audio side of things I'm not as worried about as that seems to work with or without my mods.

[Diah]

okay, since the screen out.. check the LED BL if one turn off.. this could be the reason.
other wise as you said you want to use the audio... then keep the TV work as normal just with out screen...no need to do jumpers mode in case yours mainboard working

[KxngCrooked8]

Diah

I'm not sure what you mean by "check the LED BL if one turn off". Sorry. Could you explain a little more? The LEDs come on just fine if I power up the TV normally using the logic board. The issue is that the LEDs won't stay on for an unlimited amount of time. Something in the logic of the TV is shutting down the TV after some amount of idle time (seems to be 15-20 mins).

[Diah]

Diah

I'm not sure what you mean by "check the LED BL if one turn off". Sorry. Could you explain a little more? The LEDs come on just fine if I power up the TV normally using the logic board. The issue is that the LEDs won't stay on for an unlimited amount of time. Something in the logic of the TV is shutting down the TV after some amount of idle time (seems to be 15-20 mins).

forget what i wrote,, when you have working LED with MB connected... turning off because No signal. of course this function can turning it off from setting menu.

[KxngCrooked8]

I wish it were that easy. The reason that I am doing this project in the first place, is because the LCD screen is broken. So I can't see any of the menus to adjust things. I also tried to mess with the settings when the TV did work and I could never get it to stay on indefinitely. It would always timeout/go into standby after some amount of idle time.

I would also like to carry out these mods so that I can eventually use a variable voltage power supply to adjust the brightness of the LEDs.

[Diah]

well you have Video Out put on the mainboards which you can plug any other Tv or monitor to see the menu.
if you want to ignore the MB. then you need to Diy step down circuit to have 3.3V from yours PSU fed it to Power_ON and Drv_ON and Pdim...

[maneatingmonkey]

Would a lack of PDIM signal cause flickering LEDs? Still open to any ideas.

Yes a lack of PDIM signal could absolutely be causing the flicker.

But TBH I'm not sure why you want to bypass the signals from the mainboard if you want to use it anyway for audio. Unless a sleep timer has been specifically set, the screen shouldn't turn off so long at there is a video input connected.

So hook up a laptop to HDMI1 and load up a 10 hour YouTube video, making sure the PC audio is set to TV speakers. You can the change the input on the remote either by pressing SOURCE, DOWN, ENTER repeatedly (if the selection starts from the current source), or doing the same thing but each iteration pressing DOWN an additional time (if the selection starts from the top each time).

[KxngCrooked8]

maneatingmonkey I appreciate the input. I'll try sending the 3V to the PDIM pin as well and see what happens.

The reason that I want to bypass the signals is for the auto shutoff. You are correct that I could plug in a video input, but I'm hoping not to do that. I plan to mount this on a wall as a false window of sorts (my apartment only has windows on 1 side and having a more natural source of light from the other side will be nice) eventually nowhere near any video inputs. Additionally, this is just a fun little project for me to tinker around and try to learn a little bit.

I do plan on using the mainboard for the audio yes. I have a component to 3.5mm adapter and have the TV set to component input. With my testing, just the audio signal isn't enough to keep the LEDs on. I'm hoping to keep using the mainboard, while cutting the wires that are sending the signal I'm trying to emulate. Then I'll wire a small power supply into those signals to provide that signal constantly and hopefully keep the LEDs on. I'm hoping to use a variable voltage power supply for the PDIM signal as well to be able to vary the brightness.

Diah

thank you for the input. I'm currently using 2 x AA batteries to provide 3V as the signals as that is something I read in an LG service manual for testing backlights. If I need 3.3V, is there a relatively simple way to accomplish that using the voltages on the board already with some additional components? I'm going to look up the step down circuit but I'm a relative novice when it comes to designing electronic circuits.

[maneatingmonkey]

What I was getting at is that it might be possible to make a dummy plug to trick the TV to thinking there's an input. Or just repurpose an old Chromecast or Android TV box. Actually you could probably just plug in a USB with a JPEG and put the TV on slideshow mode.

But if you did want to hack the hardware, it would make more sense to find the power on signal on the motherboard and tie that to high if you are keeping the mobo in place. Otherwise remove the mobo and drive DRV_ON and PDIM directly.

[Diah]

[USER="303198"]thank you for the input. I'm currently using 2 x AA batteries to provide 3V as the signals as that is something I read in an LG service manual for testing backlights. If I need 3.3V, is there a relatively simple way to accomplish that using the voltages on the board already with some additional components? I'm going to look up the step down circuit but I'm a relative novice when it comes to designing electronic circuits.

Step-Down-Regler i use in hand always TS2596CM533, just because i can have out pot of many range V. but you can use only for 3,3V .. the out put put in Cap filter 22000uf 25V

[KxngCrooked8]

So, I was able to get my hands on a small variable voltage power supply. I hooked up 3.3V to PWR_ON, DRV_ON, and PDIM pins and the LEDs are still flickering. Plus, I'm getting the 105-110V at the LEDs which, according to the service manual I have, says that happens when the TV is in STBY mode. I'm not sure if that has something to do with the flickering. Any other ideas as to why the flickering may occur but only when powering it with my power supply? It still works fine for 15-20 mins if I plug the cable back into the main board.

If I can't solve the flickering, I'll probably go the dummied video in route using some HDMI signal generating device.

[Diah]

So, I was able to get my hands on a small variable voltage power supply. I hooked up 3.3V to PWR_ON, DRV_ON, and PDIM pins and the LEDs are still flickering. Plus, I'm getting the 105-110V at the LEDs which, according to the service manual I have, says that happens when the TV is in STBY mode. I'm not sure if that has something to do with the flickering. Any other ideas as to why the flickering may occur but only when powering it with my power supply? It still works fine for 15-20 mins if I plug the cable back into the main board.

If I can't solve the flickering, I'll probably go the dummied video in route using some HDMI signal generating device.

external power supply.. cant be used.. it had different Ferq as yours PSU.. this why we use step down circuit get fed from PSU and it out put will be same Freq.