LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power board?

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

OK, there are two fuses by the white connector, one is black, one is white, they are labeled on the board with F1, F2. Use your ohm meter and report the resistance reading. do that on both board. I have a feeling the small black fuse is bad.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

budm, it seems like you are knowledgeable about this topic.

I am getting 9.3mV DC at that terminal ERR

so what does this reading mean?

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Since I do not have the spec on this board, the ERROR signal can be very low, such as 9.3mV = BAD, or it can be good. >3V can mean bad or good. It all depends on the logic voltage the circuits use.
So did you check both fuses on both inverter board yet. Do you also have the LCD panel model number as printed on the sticker, then you can look up the spec sheet which will tell you what kind of voltages you should have on the inverter board and on the T-CON board.
Just tell me what you find on the fuses first. It is so easy to test.
BY the way, the MASTER inverter board is not he same as the SLAVE Inverter board, MASTER has two fuses, SLAVE only has one fuse.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Here is the result


F2 (white fuse) both has .8 ohm
only left one has black fuse F1= 130K Ohm and changing continuously up.

I think this one connected to Capacitor and that is why resistance continuously going up. No fixed resistance for F2 black fuse.

So if the ERR reading is for power board or control board. I guess power board since the pin is on it?

I am getting 3.8mV DC with all the inverter board disconnected . ERR=3.8mV

Yes you are absolutely right. I didn;t know which one was master and which one slave . Now I know.

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

That black fuse is blown! That is what I thought, That is why you get about 0V Error voltage to go back to the maoin board to tell the cpu that the inverter board is not working, the power supply board is just a junction points for routing all the signals, ERROR is generated by the master inverter board.
You can jump that fuse with 1 Ohm 1/4 Watt resistor for testing.
I found out what screen you have but you should check anyway, AO OPTRONICS T420HW01 V.2.
Connector on the MASTER board:
PIN11, ERROR OUT SIGNAL PIN, LOW (<1V) = BAD, HIGH (>3V) = GOOD.
PIN 12, BL_ON (BACKLIGHTS ON/OFF CONTROL PIN) 3.3V = ON.
So check the black fuse, if that fuse is open, the PWM IC will not have the voltage for it to run, so the ERROR signal from this IC will be 0V = BAD.

See spec sheet of the LCD PANEL AND TEST POINTS.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

I am getting 3.30v at pin 12 VL_ON

I am getting 6.5mV at pin 11


what is mean that? power board faulty? You seems to know everything in this board. I will say expert.

[vinceroger69]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

did you change the fuse as in budm post 25? fuse f1 by side of the connector
You can jump that fuse with 1 Ohm 1/4 Watt resistor for testing

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

do I have to solder the fuse , as the fuse on the board is really tiny?

[vinceroger69]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

yes realy it can be done i am still a novice at repairing lcd s etc but have done a few of these fuses just take your time if you have not got a fuse use a 1 ohm quarter watt resistor just fot testing thats all i do once i know thats my only problem i order the correct fuse.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

I am getting Inverter ERROR PIN 11= 6.1mV and black fuse F1 very high resistance.

and VL_ON pin 12= 3.29V

Is this reading ok?

[vinceroger69]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

not sure budm is probs the best to ask i would re read post 25 still seems fuse is bad to me.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

yes budm looks very knowledgeable. He knows ins and out of this board. I will have to go get one fuse from radioshack. I will post the result after I put the resistor in.

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

That balck fuse is bad!
"So check the black fuse, if that fuse is open, the PWM IC will not have the voltage for it to run, so the ERROR signal from this IC will be 0V = BAD."

As I have explained, when that small fuse is blown, the pwm IC will not have the voltage to supply the power to make it run, the ERROR signal is generated by the PWM IC circuits, that is why the ERROR signal is 0 Volt as you measured.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

I could not find any 1 Ohm resistor at local store. I had to order it from ebay.

Can I just jump the resistor with small cable/wire for now and see what happen?

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Yes you can, just make sure that you did not create any solder bridge. By the way, what are the P/N the two inverter boards as printed on the labels?
To remove the old fuse, use good 40watts soldering iron and add more solder to both ends and go back and forth with the soldering tip while using the tweezer to remove the fuse.

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

By the way, worst case will be bad lamps.
https://www.badcaps.net/forum/showth...t=17639&page=3
https://www.badcaps.net/forum/showth...t=18596&page=2

It uses the same inverter boards as yours.
Two out of two have bad lamps.

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Latest news: https://www.badcaps.net/forum/showth...932#post266932

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

The tv was actually working last week. Had one issue only hafl screen darker for half an hour. So I think the ccfl is still good. I open the back and must have shorten something. The screen turned off suddenly and only sound was there.

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Let see if he finds any bad ballast caps in the lamp assembly, that will explain the half screen brightness, that symptom can also be only one of the two inverter is working.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

from my research the half screen darkness happens from bad joint on shift register. One UC chip. There is a technique to test it. I did and mine had bad shift register joint. The TV used to work perfectly bright after about half an hour. If you gently pull/twist the tv the tv get bright light. That is the way to test if the shift register connection bad or not.

I had this issue.

I am going to jump that black Fuse to see if I can get the back light work again