LG Flatron L194WT-SF - 2 seconds to black

[matthmal]

Hi Guys,

I have an LG Flatron L194WT-SF which around 5 years ago stopped working, I opened it and replaced a bulged cap on the power board and worked fine till now. Lately it started to do 2 seconds to black, the LG logo starts to come up and goes off instantly, I re-opened the monitor and found the remaining caps on the power board all bulged except the one which I replaced 5 years ago.
I replaced the remaining caps but unfortunately the monitor persisted with the problem.

Any suggestions please? P.S. I also tried the bulb test and the bulbs didn't turn on.

Thanks,
Matt

[Agent24]

Re: LG Flatron L194WT-SF - 2 seconds to black

What do you mean "The bulb test" ?

[matthmal]

Re: LG Flatron L194WT-SF - 2 seconds to black

With the bulb test I mean that I connected external CCFL bulbs (which I removed from other monitors) and the bulbs still didn't light up

[Agent24]

Re: LG Flatron L194WT-SF - 2 seconds to black

Check the power supply rails are OK and within spec and if anything changes between startup and after the image disappears.

Did you check with a torch or other bright light shone on the screen to see if the image stays on the screen after the backlights go out, or does the image disappear as well?


If you still have image, it sounds like an inverter problem.

Check for bad solder joints on the PCB. I see what could be bad joints on D101, D102, FB102 for example. Reflow any that look suspicious.

Test inverter transformer drive transistors U304 and U303 for shorts

Can you give the part number of inverter controller IC U301?

[matthmal]

Re: LG Flatron L194WT-SF - 2 seconds to black

Hi, I cannot see anything with a torch.

Any idea of how to test the transistors? searched the internet and it seems like you need a special board. i only have a multimeter.

the part number of the IC U301 is as follows:
OZ9938GN
A6DARU.3G
0710P

Thanks for your time,
Matt

[budm]

Re: LG Flatron L194WT-SF - 2 seconds to black

What DCV do you hvae on the 'ON (BL-ON, or INVER -ON)' pin and the DIM pin, and the 12V pin of the connector J1 of that power supply/inverter board?

[matthmal]

Re: LG Flatron L194WT-SF - 2 seconds to black

Hi,

Kindly note the below readings of the J1 connector:
12V -> 15.4V
ON -> 0V
DIM -> 0V

I also taken the voltage of the big cap, it is 383V, whereas it is rated 100uF 450V.

Any suggestions please?

Thanks,
Matt

[budm]

Re: LG Flatron L194WT-SF - 2 seconds to black

Hi,

Kindly note the below readings of the J1 connector:
12V -> 15.4V
ON -> 0V
DIM -> 0V

I also taken the voltage of the big cap, it is 383V, whereas it is rated 100uF 450V.

Any suggestions please?

Thanks,
Matt

If that is what you have when the monitor is turned on and connected to the running PC, then you have logic board problem, it is not sending out the ON (Backlights on signal, nor the Dimming control signal.

[TVRepairNoobie]

Re: LG Flatron L194WT-SF - 2 seconds to black

matthmal,

I have seen one bad CCFL will also result in the 2 second to black issue.
Do you have a CCFL tester to make sure your CCFL is good?

[matthmal]

Re: LG Flatron L194WT-SF - 2 seconds to black

If that is what you have when the monitor is turned on and connected to the running PC, then you have logic board problem, it is not sending out the ON (Backlights on signal, nor the Dimming control signal.

Those are with both boards connected to the power but not to the screen and pc, will retake the readings with everything connected. The negative probe should be on the GND right?

[matthmal]

Re: LG Flatron L194WT-SF - 2 seconds to black

matthmal,

I have seen one bad CCFL will also result in the 2 second to black issue.
Do you have a CCFL tester to make sure your CCFL is good?

I do not ave a CCFL tester but usually i connect extra CCFL bulbs that i have, i will retry the test.

[budm]

Re: LG Flatron L194WT-SF - 2 seconds to black

Those are with both boards connected to the power but not to the screen and pc, will retake the readings with everything connected. The negative probe should be on the GND right?

Yes, without the Video signals from PC most monitor will go into standby mode within seconds.
test lamps for testing the board:
https://www.badcaps.net/forum/showthread.php?t=19987

[matthmal]

Re: LG Flatron L194WT-SF - 2 seconds to black

Yes, without the Video signals from PC most monitor will go into standby mode within seconds.
test lamps for testing the board:
https://www.badcaps.net/forum/showthread.php?t=19987

Thanks, will retry the tests tonight and will let you know.

[matthmal]

Re: LG Flatron L194WT-SF - 2 seconds to black

Yes, without the Video signals from PC most monitor will go into standby mode within seconds.
test lamps for testing the board:
https://www.badcaps.net/forum/showthread.php?t=19987

Hi,

I have done the test as you suggested. The attached photo shows that one set of bulbs turn on and the other set tries to turn on with the result that both sets of bulbs turn off.

I have re-done the voltage tests as below:
12v -> 16.7V
12v -> 16.7V
GND
GND
5v -> 5V
5v -> 5V
GND
N.C -> 0V
ON -> 3.3V
DIM -> 3.3V
M/S -> 4.4V

Thanks,
Matt