Anyone with experience repairing a bad MAIN board?

[edugimeno]

Hi There! I already had another post with this but it got no attention from anyone so I'm starting a new one with a probably more descriptive subject

I have an LG 43LJ594V that would not even power up the stdby led. I checked the PS board and its 100% good. Provides 7.8V when STBY and 13V when ON, I checked it forcing DRV_IN input to high.

So all the other subPS's should be in the main board. The TV won't show any life in it'¡s stby led, won't turn on when pressing the phisical ON button nor the remote. My guess is that some subPS inside the MAIN board is bad.

I checked all capacitor and none shows any short. Also checked all D's and Q's and no shorts

I tested several points in the whole board and 7.8v is the only voltage I can find.

I have never tried to repair a main board, anyone can provide a hint on what to start with?

Thanks in advance!

[howardc64]

Re: Anyone with experience repairing a bad MAIN board?

Hi There! I already had another post with this but it got no attention from anyone so I'm starting a new one with a probably more descriptive subject

I have an LG 43LJ594V that would not even power up the stdby led. I checked the PS board and its 100% good. Provides 7.8V when STBY and 13V when ON, I checked it forcing DRV_IN input to high.

So all the other subPS's should be in the main board. The TV won't show any life in it'¡s stby led, won't turn on when pressing the phisical ON button nor the remote. My guess is that some subPS inside the MAIN board is bad.

I checked all capacitor and none shows any short. Also checked all D's and Q's and no shorts

I tested several points in the whole board and 7.8v is the only voltage I can find.

I have never tried to repair a main board, anyone can provide a hint on what to start with?

Thanks in advance!

I recently debugged a HiSense main that turns out to be a dead eMMC. It had all power rails but no activity (not even standby light) A good way to get a quick power rail status on the main is to probe all the inductor/coils. I think most if not all DC voltage output on the main has a coil/inductor on the output side. Just put meter in DC volt, ground on metal chassis and check either side of the inductor(s). Note that the generation side of the inductor may have AC components so will see that if in AC mode but in DC mode, will read same on both sides.

You'd want to see the following

~1v for CPU
~3.3v for lots of logic
~1.8v for eMMC
~1.5v for DDR memory
~5v for USB+WIFI+IR

Let us know how many of these voltages you see. I labeled all the inductors A-J on your board.

[edugimeno]

Re: Anyone with experience repairing a bad MAIN board?

Thanks for your answer Howard

Most of the dc-dc converters are OFF at this time. I only get voltages at the one you labeled J (near the tuner) and the voltage matches the main PS voltage, 7.8V (which doesnt make sense to me, why would a subPS be generating another voltage equal to the main voltage already provided?)

Thanks

[edugimeno]

Re: Anyone with experience repairing a bad MAIN board?

Also I get some voltage fluctuation in PS output, it's not steady 7.8v, but it swings from 7.8 thru 7.95v. I tried with main board disconnected and still fluctuates, so this is generated inside the PS board.

I removed both capacitors involved in this section of secondary and they tested 100% fine under a ESR meter (both exact capacity and low ESR).

Maybe this is ok for it to work but I wanted to mention it

[edugimeno]

Re: Anyone with experience repairing a bad MAIN board?

Now I've forced the PS to ON state (applying 3v to PS_ON and DRV_ON) and I get the expected 13v on PS output. Coil J now has 13V instead of 7.8v and now coil E has 0.8V....
All the other coils stay at 0v

[howardc64]

Re: Anyone with experience repairing a bad MAIN board?

Here is the service manual for this TV

https://tel-spb.ru/download/?brand=L...594V_LD75M.rar

Near the inductors are the DCDC converters (~6pins). Probably need to find the datasheet(s) for a few and see why no output. I think generally CPU turns on standby LED so need to trace why CPU rail and 5V rail (power to light LED) has no voltage.

[diif]

Re: Anyone with experience repairing a bad MAIN board?

I'd check the resistance to ground of each coil, with the power off.

[howardc64]

Re: Anyone with experience repairing a bad MAIN board?

I have a similar year working LG with same SOC (XD engine)

• Your main SOC on p43 on service manual (link in post #5)
• Mine main SOC 3rd pic from top ( see pic below, repair post link my failure was due to panel buffer board shorts)

On my working LG, voltage readings during boot are

Stage 0 : Initial Boot after plugging in AC without turning on TV power switch.

PSU

• PS_ON = 3.3V
• 13V = 13V

Main

• 1V, 1.5V, 3.3V, 5V all present on coils

Stage 1 : 20+s After Initial Boot plugging in AC without turning on TV power switch.

PSU

• PS_ON = 0V
• 13V = 7.8V

Main

• Only 3.3V present

When you first plug-in AC, you never see Stage 0 condition? I believe 3.3V PS_ON requires CPU to boot (which probably require most of main voltages) to turn it on.

On my main board, 3.3V coil and 6pin DCDC chip marked with red arrows

[edugimeno]

Re: Anyone with experience repairing a bad MAIN board?

Here is the service manual for this TV

https://tel-spb.ru/download/?brand=L...594V_LD75M.rar

Near the inductors are the DCDC converters (~6pins). Probably need to find the datasheet(s) for a few and see why no output. I think generally CPU turns on standby LED so need to trace why CPU rail and 5V rail (power to light LED) has no voltage.

Thanks, I had already tried that PDF but it doesn't get too deep in the repair, basically if main board is nor working-> replace main board

Yes I've seen the converter IC's and tried to find some datasheets but I can't find any information (example AHRH 937-> zero usable results...)

[edugimeno]

Re: Anyone with experience repairing a bad MAIN board?

I'd check the resistance to ground of each coil, with the power off.

I had already tried that. All coils show high Z to ground except H which shows 100 ohm in one direction and 125 reversing the polarity (probably goind thru a diode?)

I don't know if 100 ohm is a bad indicator...

[edugimeno]

Re: Anyone with experience repairing a bad MAIN board?

I have a similar year working LG with same SOC (XD engine)

...
When you first plug-in AC, you never see Stage 0 condition? I believe 3.3V PS_ON requires CPU to boot (which probably require most of main voltages) to turn it on.

On my main board, 3.3V coil and 6pin DCDC chip marked with red arrows

I see, in my case I think I have confirmed that the PSU is good, according to another service manual I found. Iy said that when PWR_ON and DRV_ON are left low, main voltage would be 7.8v meaning standby condition (this was checked) , but when you pull PWR_ON to high, PS voltage rises to 13V, and then when you pull DRV_ON high, BL LEDS will turn on. All this was checked correctly so I called this PSU good.

I haven't found any other voltages in my PSU other that the main PS voltage (either 7.8v, or 13v if I forced PWR_ON), but that's it, no "new" voltages generated inside. I don't even understand why the provided voltage is present at a coil, it was already generated outside! A DC-DC converter would be using that voltage to generate a new one!

I guess the led/ir board requires either 3.3v or 5v and none of this voltages are present anywere in the board so this is first thing, it can't even detect the turn on signal...

[howardc64]

Re: Anyone with experience repairing a bad MAIN board?

I see, in my case I think I have confirmed that the PSU is good, according to another service manual I found. Iy said that when PWR_ON and DRV_ON are left low, main voltage would be 7.8v meaning standby condition (this was checked) , but when you pull PWR_ON to high, PS voltage rises to 13V, and then when you pull DRV_ON high, BL LEDS will turn on. All this was checked correctly so I called this PSU good.

I haven't found any other voltages in my PSU other that the main PS voltage (either 7.8v, or 13v if I forced PWR_ON), but that's it, no "new" voltages generated inside.

Yes, I think no other voltages from PSU except backlights.

I don't even understand why the provided voltage is present at a coil, it was already generated outside! A DC-DC converter would be using that voltage to generate a new one!

I guess the led/ir board requires either 3.3v or 5v and none of this voltages are present anywere in the board so this is first thing, it can't even detect the turn on signal...

Unfortunately no schematics. Here is a source of many LG service manuals, you might look through here

https://www.electronica-pt.com/esque...erby,4/page,1/

I found this one with power tree. Its different SOC but maybe power tree is similar ( link p12 )

The HiSense main I debugged has similar power tree ( post link p48 of service manual )

Anyway, if you never see PS_ON and 13V without manually jumping it. Then probably trace the power path to IR and CPU. The 2 DCDC conversions nearest to the PSU connector (6pin and 8?pin chip near E and G ) might be good starting point. At least confirm 7.8V input gets there (use really fine probe tips and magnifying eyewear to avoid shorting between DCDC regulator pins) If you provide chip #s of all DCDC regulators, I can try to google for you.

[Davi.p]

Re: Anyone with experience repairing a bad MAIN board?

Sorry if i don't read entirely the discussion, i put my thoughts, my attention is towards 2 elements of this board, 1 is the microcontroller with a pencil mark on it, its firmware could be corrupted, it holds the standby software that is responsible of the state of the enabling signals that starts the smps dcdc stages, 2 is the dc dc that feeds the uC, probably a sot23 with 3,3v output near the uC..

[lotas]

Re: Anyone with experience repairing a bad MAIN board?

Yes I've seen the converter IC's and tried to find some datasheets but I can't find any information (example AHRH 937-> zero usable results...)

Here you go, this is MP8765

[howardc64]

Re: Anyone with experience repairing a bad MAIN board?

Here you go, this is MP8765

Looks like the 5V rail with high current (chip rated at 6A) Has a enable pin so maybe requires CPU to turn on?

OP needs to find the 1V CPU, 1.5V DDR, 3.3V/1.8V eMMC rails. Find datasheet and diagnose why not turning on.

[lotas]

Re: Anyone with experience repairing a bad MAIN board?

The problem here may be in the MICOM Renesas R5F100GEA (damage to the firmware), and it just gives all the starts to turn on.

[howardc64]

Re: Anyone with experience repairing a bad MAIN board?

The problem here may be in the MICOM Renesas R5F100GEA (damage to the firmware), and it just gives all the starts to turn on.

Ah make sense. With this uproc with onchip code, flash data, and RAM, main board can shutdown main CPU+memory+flash on standby. I was wondering why only 3.3v is present in standby mode on my same generation LG main board.

https://datasheetspdf.com/pdf-file/8.../R5F100GEAFB/1

Should check if 3.3V in on chip VDD. If not present, need to find why missing.

[lotas]

Re: Anyone with experience repairing a bad MAIN board?

Yes, micom 3.3v should be constantly powered.

[Davi.p]

Re: Anyone with experience repairing a bad MAIN board?

in the case of bad firmware i don't he can go far, does RT809 series have that support? i don't think

[edugimeno]

Re: Anyone with experience repairing a bad MAIN board?

Thanks everyone who showed up to help on this thread. I had already suspected of broken firmware or CPU but my understanding is that for the CPU to work at least one CPU-compatible voltage has to be present and all I can find along the whole board is 7.8V (qhich is already the voltage directly coming from the PS), so I would be expecting a 3.3v or 1.8v to at least have the CPU running. That's why I think there's s subpower issue in this board. All the coils but one have 0 or almost 0 volts, the other one has 7.8v which I see useless
So I can't figure out how a CPU would be executing a firmware when only 7.8v are present at the whole board
Does this make sense?

THANKS!