Benq t720 - 0 seconds to black

[guirnab]

Hello guys,

i have Benq T720 (for about 10 years) and it started to suffer from "2s to black"...well, it was more like 500ms to black. I had to turn off and on the display for about 30 times to stop blacking out. Then i found a "fix" - i lowered the brightness to minimum and it just worked®. But now, the display won't light up at all. It's still just the backlight issue - i can see the picture with flashlight.

I disassebled the LCD and found five 470uF and one 1000uF capacitors visually bad so i bought replacements and replaced them. But it didn't help at all. Could you please point me out where to look next? The board is quite complicated for me. Here is the photo:

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I highlighted replaced capacitors with green color. I tested the yellow one on capacitance meter and resoldered it, because it seems to be ok.

Thanks for any help! :-)

[budm]

Re: Benq t720 - 0 seconds to black

Resolder the joints on the transfomer's legs, transistors/MOSFET, all that area in the inverter section.
Check the transistiors/MOSFET for shorts or leakage.

[guirnab]

Re: Benq t720 - 0 seconds to black

Thanks for (quick) reply :-) I tried to resolder everything suspicious with no luck.

Acording to the 2s-to-black guide I :
- measured highlighted MOSFETs. Everything has more than 200 ohms.

- measured secondary windings on highlighted transformers. All four had 750 ohms.

Any other ideas?:-(

[budm]

Re: Benq t720 - 0 seconds to black

Are those transistors (4 of them) by the small rectangular grey caps labeled as C5706, C5707?

[selldoor]

Re: Benq t720 - 0 seconds to black

After resoldering if there is no flash from the lights check PF751 which I think is a fuse.

[guirnab]

Re: Benq t720 - 0 seconds to black

Are those transistors (4 of them) by the small rectangular grey caps labeled as C5706, C5707?

yes, C5707

[budm]

Re: Benq t720 - 0 seconds to black

I forget to tell you to check the fuse F751 for the inverter circuit, when this fuse is bad, it will show 3~4 Ohms, it should be <1 Ohms + your probes resistance.

[guirnab]

Re: Benq t720 - 0 seconds to black

Thanks guys, I couldn't find the fuse before Well, that may be the case. It definitely has more than 1 Ohm (out of range on multimeter). I guess it won't be easy to find replacement around here and I am afraid to just short it :-)

I'll try to find some tomorrow.....

[budm]

Re: Benq t720 - 0 seconds to black

I believe it just a 2A or 3A fuse (see what is printed on the body) so any 2A or 3A fuse with 25V rating or higher will work. That fuse is made by BEL FUSE.

[Lumberjack777]

Re: Benq t720 - 0 seconds to black

In my experience with those BenQ's it's always 1 of those transistors (or mosfets) that has shorted. And that causes the fuse to blow. I'd probably recheck those out of circuit first. Because if he just replaces the fuse; it will probably blow again.

[stj]

Re: Benq t720 - 0 seconds to black

if lowering the brightness made it work then it was detecting a lamp overload.
at 10years old, it's probably got bad tubes.

[budm]

Re: Benq t720 - 0 seconds to black

Well I did tell him to test those C5706/5707 and the MOSFET,s so hopefully he tested them correctly.

[guirnab]

Re: Benq t720 - 0 seconds to black

I am back. I was at local store, but they had only 2A picofuse (or 5A) and i need 3A. So I bought the 2A and 3,15A (big) tube fuse as backup.

First I tried the 2A picofuse. Backlight worked for 1s and then the fuse blew up (yea, I was kind of expecting that, but at least I know the backlight works).

Then I soldered in the big 3,15A fuse. I know it's not ideal, but it worked perfectly! (well...for the first try). After I assembled everything back together it stoped working again...but with different symptoms:

YouTube video

[selldoor]

Re: Benq t720 - 0 seconds to black

Is it connected to a PC?

[guirnab]

Re: Benq t720 - 0 seconds to black

Is it connected to a PC?

Not on the video, but it does exactly the same with VGA connected.

[budm]

Re: Benq t720 - 0 seconds to black

That looks like the 2-seconds to black, the LED power light stays on but the backlights go off. It can be the lamps or something is still not right in the inverter section.
If you have test lamps then it will help narrowing down the cause of the problem.

[selldoor]

Re: Benq t720 - 0 seconds to black

Budm - could it be a faulty startup cap?

[guirnab]

Re: Benq t720 - 0 seconds to black

That looks like the 2-seconds to black, the LED power light stays on but the backlights go off. It can be the lamps or something is still not right in the inverter section.
If you have test lamps then it will help narrowing down the cause of the problem.

It's not (pure) 2s-to-black. On the video top of the screen is brighter than it should be. I'am afraid to let it turned on for longer than seen on the video...

[guirnab]

Re: Benq t720 - 0 seconds to black

Budm - could it be a faulty startup cap?

Startup cap is the biggest one (150uF 450V)?

[selldoor]

Re: Benq t720 - 0 seconds to black

Startup cap is the biggest one (150uF 450V)?

No it is a very small one in the primary/hot side often 50v 100uf perhaps one of the 2 green ones bottom right in your picture.

It could be the bulbs as Bud says - tried turning the brightness down again?