LCD 19" BENQ T905 problems

[dvorav]

Hi folks,
I possses a 19"BenQ T905 monitor. One day it went blank. (While using strong light, I'm able to see a picture). Guessing, it is backlight fault. It's very common fault for BenQ displays (as I know from Badcaps). As a read several threads, i have decided to openning it. I searched for blown 2SC5707. But found none. All caps seemed good as well. By the way the board is containing lot of SMDs and one IC 52 CJ5YK TL1451A. Finally i found one-way blown IRFU9024N. But now, it will last some time until I get the part. Meanwhile, can someone give me an advice if there is also some hidden fault... maybe the IC? I'll be glad for any advice. Thak you

[Krankshaft]

Re: LCD 19" BENQ T905 problems

How did you check the C5707s a visual inspection won't suffice here. You will have to measure for a short across them.

The reason I say this is because the IRFU FET you speak of usually blows because of a C5707 failure.

If the C5707s are indeed fine then check the 3 to 5 amp pico fuse and see if it's blown. If it is replace the IRFU and the fuse together and try powering it up.

[dvorav]

Re: LCD 19" BENQ T905 problems

Thanks for reply!

First of all, please, excuse my English. I'll try to do my best.

I checked the C5707s by multimeter, but none was shorted. That's strange, while thinking of reason why is the IRFU9024N blown (see attached photos).

Speaking of "blown", from closer look it seems rather fried (see violet color of a heat sink)

I had also attached photo of whole PCB. Items marked with * i have soldered off. The pico fuse was blown as well.

I'm wondering if I should replace also the other good IRFU.
And also questionable is condition of IC (see PCB)... whether is or isn't affected by the event of blowing the IRFU.

Unfortunately, there aren't any specialized shops in neighborhood, so I have to go to capital (Prague) to get new parts... it will be next week. Meanwhile, thanks for other suggestions.

[dvorav]

Re: LCD 19" BENQ T905 problems

Hi 2 All,

I got the new parts (IRFU9024Ns) recently. So I mannaged to change one 9024N and pico fuse 3.A. Guess what! Monitor is up and running. I'll let it run for a while to see if the fix is permanent. So far it seems that IC 52 CJ5YK TL1451A is good. And I think it will be for some time

Thanks for reading and replying. I hope this thread will help to someone.

See ya

[Wizard]

Re: LCD 19" BENQ T905 problems

And replaced the capacitors? These overstress the transistors.

Cheers, Wizard

[dvorav]

Re: LCD 19" BENQ T905 problems

No, I didn't replace capacitors. There is not any leaking, none of them is swollen. I assume that capacitors are fine. But the reason why the monitor failed could be connected with general absence of functional current limiting circuit (see davmax posts in another thread).

Do you mean some speciffic capacitor from your experience?

Monitor is running around several hours without any difficulties.

dvorav

[Wizard]

Re: LCD 19" BENQ T905 problems

They do dry out in hot area where lamp inverter supply is. Even it is not bloated at all.

Cheers, Wizard

[kc8adu]

Re: LCD 19" BENQ T905 problems

the elite caps in these benq units are usually in bad shape even if not bulging.
i replace them.

[dvorav]

Re: LCD 19" BENQ T905 problems

So you mean all 470uF 35V should be replaced, or ALL elite caps are worth replacing?

[carphone66]

Re: LCD 19" BENQ T905 problems

Check also the polyester capacitors at the switching transformers. C824 and C826 (220 nF, 160V) usually have dry solder joint in 90% of the cases due to ultrasonic vibration from the switching current.

Chris

[dvorav]

Re: LCD 19" BENQ T905 problems

Thanks for posts!!
I appreciate that. So far I have resoldered polyester capacitors. Monitor seem working.

Concerning Elite caps, there are obviously several types. Should I replace all of them or would it be enough to replace those 35V @ 470uF near/inside CCFL inverter side?

Dvorav

[Scenic]

Re: LCD 19" BENQ T905 problems

i'd replace all elite caps. pure junk.
had lots of LCDs with Elite crapcaps here. no visible signs of damage (bulging, leaking, etc.) but *all of them* were bad. and they smelled like rotten fish when i desoldered them (not to mention that their legs can be ripped out of the can very easily.. it's like the electrolyte in there ate up the terminals where the legs are/were connected to)

[Djadjo]

Re: LCD 19" BENQ T905 problems

Hi all

After 3 years..my benq also start to show problems...
when it turns on the picture only blinks...and the power on led shuts down..

inverter board model : 48.L1J02.A02

ok so i changed all (c5707) q809 ,q808,q815,q816..

also changed two irf9024n but only one of them have been blown...

ok so i start the monitor and voila the picture is back...but after minute or two...9024n (q812) gets so hot and burns....another (q805) 9024n is normal cold...and dont overheat when monitor is working...

only q812 as soon as i start the monitor gets rapidly hot...
ok so i change it with irf9530n (100V 12A) and that one too burns/dies...

i looked the high volt trafos...and everthing of components around or in that circle....i desoldered it and than look on multimeter...everething else is good...

the micro fuse is ok...never blown...

can i suspect the TL1451A or something else...



thnx to all

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Just to clarify, did you desolder the transformers? I can't understand what "it" is.

i looked the high volt trafos...and everthing of components around or in that circle....i desoldered it and than look on multimeter...everething else is good...

[Djadjo]

Re: LCD 19" BENQ T905 problems

sorry, I didnt said exactly what have i desolder...

ok i have desoldered next components : T801,802,803,804..they all have same instrument readings..the c824 c826 have exact values on lcr metar...
i have changed c821 elco (470uF 35V) the coils l801 and 802 apears normal too...I didnt touch annything on input part of the board..(the 220V to the mosfet q601...put him out and measure..and seems ok....




Djadjo

[Djadjo]

Re: LCD 19" BENQ T905 problems

repaired the monitor few seconds ago....

mos q812 (9024n) burned when i lit monitor...but it was my fault...after first testing i didnt re-measure c5707 again...so i burned two in total 9024n...and than realize that c5707 died tru first test...

after that it was easy...changed two c5707 and put new 9024n...and monitor works..without any overheating or any problem..



Thanks 2All

Djadjo

[capullaco]

Re: LCD 19" BENQ T905 problems

Hi, I got the same TFT with the same problem.
One day I was browsing youtube and suddenly the screen was fading to black.
Now I cant see anything, but the led turns on normally.

Could it be the inverter? Where is it? Which model/type is it (to order a new one)?
I dissassembled it as far to see the PCB, but I cant see the inverter bar (never seen one outside internet guides btw).


Im also a noob for electronic stuff, I got a tester but I dont know how to check the items on it.

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Hi, I got the same TFT with the same problem.
One day I was browsing youtube and suddenly the screen was fading to black.
Now I cant see anything, but the led turns on normally.

Post pics after reading this FAQ on pic posting.

https://www.badcaps.net/forum/showthread.php?t=1868

Do you get the screen for 2 seconds and then to black? Or do you get no flash or display at all?

[capullaco]

Re: LCD 19" BENQ T905 problems

Do you get the screen for 2 seconds and then to black? Or do you get no flash or display at all?

No,no display at all, not even the BenQ logo, but the led turns on right.

[retiredcaps]

Re: LCD 19" BENQ T905 problems

No,no display at all, not even the BenQ logo, but the led turns on right.

Benqs are notorious for blown picofuse, shorted transistors, and poorly solder transformers. I believe you will have one or all 3 of the above if you get no display or Benq logo, but have the power led on.

Post a pic and we can suggest test points.