AOC LM725 screen goes blank

[Bobdee]

Re: AOC LM725 screen goes blank

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

Yes replace it, that no doubt was your problem (or part of it) these transistors are best replaced in pairs if you cannot get C5706s you can replace with C5707s, if you need to do that then replace all four
You can buy both on ebay type in 2SC5706 or 7

Sometimes these can be replaced and fail again due to bad solder joints on the inverter transformers, resolder inverter transformers these have ring crack as shown here

https://www.badcaps.net/forum/showpo...&postcount=231
best check all inverter joints with good magnifying glass

[kharr]

Re: AOC LM725 screen goes blank

alrighty boss, but probably will be able to buy the transistors on monday since our local electronic stores are closed on sundays

i will post tomorrow all the pictures i have taken during the testings

i'll check the board on my microscopes so no cold joint can escape, hehehe=)

thank you thank you, hope this will solve my problem (fingers crossed)

[freelander]

Re: AOC LM725 screen goes blank

I'm certainly not a repair guru here but I have tackled my fair share. This seems like a classic blown CCFL to me. You have two lamps each with two bulbs which is why you have four connectors. If one of the CCFLs is bad, the circuit will start up, hit the failed CCFL, then fail and shut down.

If you have a good working monitor or if the connector from your CPU lamps fit, try them one at a time in the circuit. Disconnect one connector, plug in one from a good working monitor, test, and move to the next. Make sure that you power down in between tests and changing connectors.

It's the four on the left in the picture above. This is a common connector for many bulbs.

[kharr]

Re: AOC LM725 screen goes blank

@freelander

ive done that dude, four of the bulbs are working great=) tested each with plainbill's ccfl mod kit trick=) thannxxx=)

[EGuevarae]

Re: AOC LM725 screen goes blank

@freelander

ive done that dude, four of the bulbs are working great=) tested each with plainbill's ccfl mod kit trick=) thannxxx=)

You recapped and tested the tubes already. If you check/fix cold solder points and replace the 5706/7s, I think that would do it.

[kharr]

Re: AOC LM725 screen goes blank

@eguevarae
which tube are you referring to?
a local shop here says that they have 5706 but they can't find it they will try to call me tomorrow and see if they can find where they kept it=)

[freelander]

Re: AOC LM725 screen goes blank

the tubes are the fluorescent lights.

[EGuevarae]

Re: AOC LM725 screen goes blank

@eguevarae
which tube are you referring to?
a local shop here says that they have 5706 but they can't find it they will try to call me tomorrow and see if they can find where they kept it=)

Tubes=CCFLs=Lamps
Sorry, I'll try to be more specific in the near future. You can search for "kits" on eBay. I've seen some that contain a picofuse, four 5707's and two fu-don't-remember-what-part-no (two fets that drive a pair of 5707 each) that are used to fix BenQ boards. That can be another way

[kharr]

Re: AOC LM725 screen goes blank

oic thank you guys for specifying sorry for being so stupid hehe=)

here are the pics where i tested the cfl





all cfl are okay=)

[kharr]

Re: AOC LM725 screen goes blank

updates:

here was the picture when i removed the 4 c5706 transistor which was taught by Bobdee




now i was able to get the c5706 from our local supplier, will install it tonight when i get home, hehe=)

[kharr]

Re: AOC LM725 screen goes blank

Update:

i have installed the c5706 transistor and resoldered again the transformers and now the lcd is working fine for 2 hours

total parts replaced:
2 x 16v 470uf electrolytic capacitors
2 x 16v 1000uf electrolytic capacitors
2 x 25v 150uf electrolytic capacitors
1 x 50v 4.7uf electrolytic capacitors
2 x c5706 transistor

all thanks to the following badcaps forum members lcd specialist
ratdude747
eguevarae
freelander

and special mention to:
Plainbill
Bobdee

for not giving up in helping and even gave me detailed instructions on what i should do

thank you very much to all of you=)

i will post the pictures later of the finish product, currently testing the unit now=)

and to all badcaps fellow members hope my log can help alot=)

[EGuevarae]

Re: AOC LM725 screen goes blank

and to all badcaps fellow members hope my log can help alot=)

You can bet that someone will benefit from this - the more detailed the thread, the more likely it can be used as a reference.
And congratulations on your now working unit

[kharr]

Re: AOC LM725 screen goes blank

update the lcd is still working for more than a week now=)

[chriscute42]

Re: AOC LM725 screen goes blank

hi kharr, where did you buy transistor c5706?.. im from manila philippines... thanks...

[gremlinmarti]

Re: AOC LM725 screen goes blank

This is not a hardware problem, all you need to do is reset the monitor to factory settings. I fixed mine that way. If your monitor goes off every 3 seconds (like mine did) you have to be fast or siwtch it on/off few times until you complete the steps to get it reset.

You need to get to the OSD menu of the monitor. So witch it on and press 'menu'. If you get the message 'OSD Locked'. Switch the monitor off and then press and hold 'menu' while you switch it on again.

Once you get the image, press 'menu' and then quickly 'press '<' (left) twice and 'menu' twice to get it reset.

It might go off, so switch it on again and if still goes off after 3/4 secs, try to reset it again.

I reset mine this way and now is working fine. Before it went blank after 3 secs (I actually had to try the reset few times as I could not read the options properly).

I tried resetting it as I did not understand why if it was a hardware problem when I could see the screen for 2/3 seconds and if pressed on/off it came back, it made no sense as a hardware problem. The sequence of keys is not 'magic', is just to go to the option 'Reset' and press 'menu' twice first to select and second to confirm (god that the option yes is selected by default). But memorizing it is good in case the monitor goes off...

[benja1475]

Re: AOC LM725 screen goes blank

hi... this is my first post, I search the internet about this problem and i landed in this forum... my problem is very similar with kharr, and followed all the instruction that mentioned here. in my case the other 2 transistors (TT2190) got the short. I search for datasheet of this two transistors and also the c5706 for possible replacement, i am wondering if i can replace the TT2190 with ordinary NPN with same characteristic of c5706 or replace it with c5706. This is what I'm thinking in case i cant find the TT2190.

[budm]

Re: AOC LM725 screen goes blank

@benja1475
Please post good clear pictures of your monitor's boards. using attachment function.
Are the TT2190 used for the inverter transformer drivers. I look at the spec of the TT2190 are nothing close to 2SC5706/5707 at all.

[retiredcaps]

Re: AOC LM725 screen goes blank

This is what I'm thinking in case i cant find the TT2190.

http://www.ebay.com/itm/250662312235

[budm]

Re: AOC LM725 screen goes blank

You should also check the tuning caps value and leakage resistance in the inverter output circuits, otherwise you may may blow up your new transistors.

[benja1475]

Re: AOC LM725 screen goes blank

guys thank you for the quick response on my query, I thought this thread is dead because its been 3 year since the last post dated... , about the picture of the board I will post it later, i need to borrow some high pixel cam to do the shot, but the board is very much the same with the picture from kharr in this thread.... about the datasheet from budm, Im confuse why the other 2 transistor is different type from c5706. on my observation they do the same task with c5706... Im sure they are used for the inverter transformer drivers. I do already tried replacing it with ordinary NPN transistor with same characteristic of c5706 because our local electronic store don't have any available TT2190 transistor... but still the same problem.