[solved] Acer AL1721 LCD screen turns white when powered off

julien_saadeh

Good day all,

Recently, I bought a new 12V adapter for my LCD screen but I wish I didn't. The new adapter was faulty that everytime you touch it, it outputs 30V and then it gradually goes down to 12V. This, obviously, caused damage to the screen but thankfully not so severe..

The system board is working and the keypad is fine. But, the problem is that the screen's backlight never turns off. It's always on until the screen is unplugged. I found a service guide over the Internet and it seems to be helpful. But I don't know where to buy the parts that I need and what alternative routes I could choose to fix the problem.

Here is a photo of the parts: (They are 2 transistors, I suspect them to be NPN but I'm not sure)

The 1st transistor (to the left) receives 3.3V when the screen is "officially" powered on via the keypad.
Can they be replaced? If not, can I actually get bigger (standard) ones and hard-wire them?

Thank you!

Best Regards!

shant

err, can you get a better picture? (set the camera to the icon with the flower if available)
bujikian or katranji should have those

J4D

Can you tell me the part numbers on both transistors ? Good lighting conditions and a magnifier will generally help :)

julien_saadeh

Sorry for the bad photo, I can't do any better :(

Here's a basic circuit photo that I made:

The second transistor's Emitter is linked to one of the pins (which are used to link the system board). C is linked to somewhere else which I can't really tell.

julien_saadeh

The 1st transistor has 2 labels:

86 (horizontally)
05 vertically

Second transistor is labelled 76 horizontally

By the way, Jad, I'm shy around you.

J4D

julien_saadeh wroteBy the way, Jad, I'm shy around you.

Whyyyy !!?

julien_saadeh

J4D wrote
julien_saadeh wroteBy the way, Jad, I'm shy around you.
Whyyyy !!?

You always try to help, I'm thankful for that.

The problem seems to be somewhere else.
I am still trying to work on it using the service guide. I will keep you updated.

julien_saadeh

Here is the online service guide: http://www.manualowl.com/m/Acer%20Computers/AL1722/Manual/114956
You might need to open it with Firefox.
See page 58 where it shows the schematic of the inverter board. The transistors are shown and one of them (second one above) is PNP.

The biggest question is: What the hell could keep the backlight always on? Is it perhaps being "always started"?? That would be a heck of a damage to the lamp.

shant

well, this is beyond my basic understanding, jad knows better

J4D

According to what you said, without inspection. I think the problem either originates from the two transistors above : DTA144WKAT146TR-ND and DTC144WKAT146TR-ND or extends to the IC TL1451ACD which is a PWM controller that controls the brightness of the screen ( back light ).

You can try replacing these parts, but my suggestion to you is keep everything as is. unplug when unused. Your best chance would be replacing the complete lamp inverter.

julien_saadeh

According to my tests, the transistors are fine.. When the screen is officially turned off, no voltage is on pin 9 (Vcc) of the TL1451ACD IC. When powered on, I get 12V on that pin. The most important problem is that I get some flickering on the screen when I first turn it on (which drives me nuts).

Narrowing down the issue:
The problem resides in the circuit on the second part of the IC (from pin 9 to pin 16) because there are 2 backlight cables, upper and lower. So, when the screen is plugged in and powered off, only the upper part of the screen is lit and that section of the IC is responsible of the upper backlighting.

What I did now is skip those transistors by linking 12V to Pin 9 directly from the adapter's jack. That helped eliminate the flickering, hoping to increase the lifetime a bit more.

julien_saadeh

Guys, I think I found the glitch, It's in the IC Q203. It's a SOIC-8 . Both sides are actually shorted out except the 4th pin (G). I have the datasheet and I found a bunch of alternatives with same specs on the Internet. Do you know of a place (or a good electronician) where I could go to replace this one?

Can you, Jad? I will pay you what you need.

Thanks.

shant

julien_saadeh wroteGuys, I think I found the glitch, It's in the IC Q203. It's an 4-pin IC (on each side). Do you know of a place (or a good electronician) where I could go to replace this one?

Can you, Jad? I will pay you what you need.

Thanks.

hahaha, ive been waiting for jad to replace a few capacitors for 6 months :p

julien_saadeh

NEWS FLASH: I replaced the AO4411 IC with a MOSFET and it seems to do the job well. But there is still some glitch....

The backlight turns on but very dimmed and turns off (as if the screen is used on a lower rating adapter, I've done this once and it exhibits the same symptoms now but on the original 12V input).
There is nothing obvious about it anymore. I checked all capacitors for shortings, resistors too. Chances are with those tiny ones that are around the IC TL1451ACD, so it's not likely to be fixed.

Some resistors are different from the service guide. So I really can't be sure of the actual ratings. A resistor was showing 30kOhm while it's coded 20kOhm.. I replaced it with a 22kOhm. That's all about it.

EDIT:
Final solution: I put back the 30kOhm resistor and I bought a universal adapter. Now I'm using it at 15V. Everything is working alright. There is no heating or anything like that.