AOC LM725 screen goes blank

[kharr]

i have an aoc lm 725, the problem is screen goes black but if you turn off the lcd then turn it on again you can see again the window screen for about maybe 3-4 seconds then goes blank again

here is the shot of my inverter, can anyone help me the common problem with this unit, thank you masters





on the 1st pic i have replaced those two encircled capacitors since i saw them buldging but then the problem still occurs can anyone help me?

[PlainBill]

Re: AOC LM725 screen goes blank

i have an aoc lm 725, the problem is screen goes black but if you turn off the lcd then turn it on again you can see again the window screen for about maybe 3-4 seconds then goes blank again

here is the shot of my inverter, can anyone help me the common problem with this unit, thank you masters

on the 1st pic i have replaced those two encircled capacitors since i saw them buldging but then the problem still occurs can anyone help me?

I have LOTS of help.

1. No d in bulging. Yeah, it's petty, but we have to start somewhere.

2. I've circled a few additional caps in green. As a general rule, replace ALL electrolytic caps (excepts the 100 uF, 450 Volt one) if any are bad. I recommend Panasonic FC or FM series, or equivalent low ESR high temp caps.

3. I've circled 4 transformers in the inverter section. Resolder every pin on those while you're waiting for the caps to arrive.

4. Try the monitor in a darkened room to see if you can see any signs of arcing, particularly around the transformers.

5. If it still doesn't work you have to determine if the problem is the inverter or the CCFLs. The best way to do this is to use the CCFLs from a CCFL case mod kit to test the inverter, and use the kit inverter to test the monitor CCFLs.

PlainBill

[kharr]

Re: AOC LM725 screen goes blank

woaaahhh, master plainbill sorry for the wrong spelling there man,

thanxxx for the very very fast response!!!=) you even drew the things i must do and detailed it very well, thank you, i will do all of your suggestions after i leave here in the office hopefully i can reach the electronic store on time so i can buy those elect. caps.

will update you soon and will picture every log i can do for this repair=) thanx=)

[PlainBill]

Re: AOC LM725 screen goes blank

woaaahhh, master plainbill sorry for the wrong spelling there man,

thanxxx for the very very fast response!!!=) you even drew the things i must do and detailed it very well, thank you, i will do all of your suggestions after i leave here in the office hopefully i can reach the electronic store on time so i can buy those elect. caps.

will update you soon and will picture every log i can do for this repair=) thanx=)

Make sure you get the proper replacements. You need caps rated for high temperature (105°C), low ESR. A list of suitable manufactures / series is here: https://www.badcaps.net/forum/showthread.php?t=2280

PlainBill

[kharr]

Re: AOC LM725 screen goes blank

Master I'm Back here are the pictures of the caps i got yesterday,



now here is the picture that i have replaced all of them and made sure resoldered the transformers, but the problem still occurs



now i did your final instruction, and was very happy cause i didin't know the ccfl kit can be used as a diagnosing thing for the lcd, haha, i tried now i am definitel sure that the inverter has the problem not the backlight=)

[kharr]

Re: AOC LM725 screen goes blank

for the arcing in low light there was none , problem still occurs boss

[PlainBill]

Re: AOC LM725 screen goes blank

for the arcing in low light there was none , problem still occurs boss

OK, things are about to get difficult. First, the little lecture on inverters. The usual design has a dedicated controller. That is probably the IC circled in blue in the attached picture. I also circled a less likely candidate in red. I'd like the part numbers of those. While we're doing colors, what is the component circled in brown?

The controller gets two signals from the logic card - on / off and brightness. The controller provides control signals to the transformer drivers and monitors the output voltage of the transformer and current through each CCFL. The usual sequence at start up is to drive the CCFLs at maximum power for a few seconds, then start monitoring voltage and current. If any of these are too high or too low, the controller shuts down the transformer drivers.

The first step is going to be to identify the controller and look up the application notes. Hopefully that will have a schematic. From there we may be able to figure out WHY it is shutting down. By the way, one typical reason is a shorted transformer.

An alternative is to use the inverters from a couple of backlight kits to replace the regular inverter. I'm not too wild about that approach. For one thing, you loose the ability to adjust brightness.

PlainBill

[kharr]

Re: AOC LM725 screen goes blank

hopefully we can solve it thanks for being so patient, and i am learning alot from this

here are the values you requested


problem by using the inverters from backlight kits you have to drive them from 12v source and i will be needing two of them if that happens, right?

[kharr]

Re: AOC LM725 screen goes blank

by the way i tested the transformers pins there were no short

[PlainBill]

Re: AOC LM725 screen goes blank

hopefully we can solve it thanks for being so patient, and i am learning alot from this

here are the values you requested

problem by using the inverters from backlight kits you have to drive them from 12v source and i will be needing two of them if that happens, right?

OK, the BA9741F is the controller. The 9435Ps drive the transformers.

12 volts should be no problem - typically the power supply provides 12 volts for the inverter section. If you look at the board carefully you should be able to locate the inverter power - suggestion - look at the two purple caps. Odds arer they are there to decouple the inverter power.

You misunderstand - a transformer may have a single shorted turn. That would be enough to reduce the voltage output.

PlainBill

[kharr]

Re: AOC LM725 screen goes blank

ok sir so BA9741F is controller and 9435P drives the transformer, what should i do next?

[kharr]

Re: AOC LM725 screen goes blank

should i replace these two? A9741F controller and 9435P ?

[PlainBill]

Re: AOC LM725 screen goes blank

should i replace these two? A9741F controller and 9435P ?

Not at this time. What exactly have you done to test the inverter? Have you hooked up known good CCFLs to it and verified they all work and are equally bright? SOMETHING is causing the controller to shut down. It could be a shorted turn in a transformer, a bad CCFL, or a bad component in the protection circuitry. It could be anything as simple as a cracked solder joint.

With a scope and a schematic, I could narrow it down within a few minutes. Without a schematic, it becomes more difficult. By remote control, it's nearly impossible.

PlainBill

[kharr]

Re: AOC LM725 screen goes blank

now i havent tried all the cfl ok will try each of them, and will try to get a schematic if i can, thanxxx...

and i'll be back=) hehe

[ratdude747]

Re: AOC LM725 screen goes blank

inverters can be a pain i wish you luck.

did you check for cold joints?

[Bobdee]

Re: AOC LM725 screen goes blank

Hello Kharr
Suggestion if I may
As this PCB does not seem to have any Inverter fuse it is possible for your B/lights to come on for the couple of seconds with a failed transistor and then shut down (one side fails and the other side works,)
Check the 4 transistor right of the 2 inverter transformers, they are above and below the 2 big blue ceramic caps (possibly C5707s or 6s)
Check with Diode test on DMM (in circuit) but first check there solder connections
B C E may be silkscreened on board, lets call that Pins 1 2 3 (to make it easier)
Top pair first
place the red on 1 and the black on 2 and take reading, do the same on 1 and 3
Then do the same to the other one of that pair, the readings should be the same
.
Bottom pair next
Do exactly the same as above pair, again this pair should read the the same
.
Compare the top reading pair to the bottom reading pair, if they are near enough the same then they are OK
If one pair reads a lot lower then there is a bad transistor in that pair

Or remove all 4 off board and check for shorts between pins = 1+2 1+3 2+3

And take diode readings on 1+2 1+3 (red on 1 pin) all 4 should read about the same as each other
Hope you can make sense of all that

[kharr]

Re: AOC LM725 screen goes blank

@ratdude
i did check for cold joints and almost had neck injury cause i just checked all continuity, hehe
they may be pain in the neck but with so much people here helping me even detailing me on what i should do, makes me even more driven to fix this thing so i could give my share on this experience here in this forum=) (i think tears are starting to flow now, haha)

thanxxx


@bobdee

woah alright! i'll try doing all of those instructions will log and post here all test that i have done, thank you so much i appreciate all of these, even detailing everything, don't worry i think i was able to make sesne with that=)

thank you thank you again =)
will be back later after the tests=)

[EGuevarae]

Re: AOC LM725 screen goes blank

Master I'm Back here are the pictures of the caps i got yesterday,



now here is the picture that i have replaced all of them and made sure resoldered the transformers, but the problem still occurs



now i did your final instruction, and was very happy cause i didin't know the ccfl kit can be used as a diagnosing thing for the lcd, haha, i tried now i am definitel sure that the inverter has the problem not the backlight=)

What caps did you use as replacements?

[kharr]

Re: AOC LM725 screen goes blank

@plainbill
done testing all bulbs with cfl mod kit all ok

here is the schematic you are requesting this is the service manual for lm729 but the inverter board for this model is exactly the same with lm725

download link



@bobdee

alright boss done removing the four transistor
tested pins = 1+2 1+3 2+3
the three transistors same no reading nbut the forth one has a short on pin 1 + 3 do i have to replace it?

the transistor label is C5706 and 5g below, will try to check our local seller if they have this one

[kharr]

Re: AOC LM725 screen goes blank

@eguevarae
nichicon sir