Benq t720 - 0 seconds to black

[Agent24]

Re: Benq t720 - 0 seconds to black

Could very well be the lamps.

If you have another inverter you can test the lamps on that, too. Pink tint or flickering and they're bad for sure.

[Lumberjack777]

Re: Benq t720 - 0 seconds to black

As long as the lamps are about the same size, or a little longer, it won't hurt anything. Just make sure the thick wire, and the thin wire, are the same where you plug it in, as the original lamps.

I'd maybe recheck those 4 transistors (c5707) and the 2 mosfets because that's been the case in 99% of those Benq's I repaired. Once the transistor shorts, it takes out the fuse with it. But it wouldn't hurt to check the lamps I suppose.

[guirnab]

Re: Benq t720 - 0 seconds to black

I tried to turn it on again (with PC connected). Something is not right. If I remember correctly, the Benq logo sould dissapear right away. It seems to me that its there for too long now.

Btw, its not perfectly visible on the video, but the second "flash" is brighter than the first one...

And after the second "flash" I think it's not displaying any image (from PC). I tried to shine a flashlight on it while moving notepad window (that worked before )

Well, I have here another LCD monitor (LG Flatron L206WU). Do you think I could connect the lamps to it and its lamps to the Benq board to cross check it? I don't know if it is "compatible" - I don't want to break another monitor

[selldoor]

Re: Benq t720 - 0 seconds to black

Yes - he means these:
Inverter testing using old CFL:
https://www.badcaps.net/forum/showthr...er+testing+cfl

[budm]

Re: Benq t720 - 0 seconds to black

Or make two of these test lamps.

[selldoor]

Re: Benq t720 - 0 seconds to black

I have also been looking again at the video it is as you say also I have paused it and
the bottom left also looks a bit pink ( another sign of a faulty bulb)- well does to me though I dont know what the Benq start screen looks like.
I did have a monitor that did this once and took me two or three goes at reflowing the inverter joints to get it going.

If you dont have spare bulbs perhaps try changing the leads from bulb to bulb - that is to say change the leads around on the power board swap the tow at the top in your picture for the two at the bottom.

[budm]

Re: Benq t720 - 0 seconds to black

Well, I see on the video, you hit the power switch, the LED turns green and then the screen lights up with BENQ on the screen, then the screen turns dark and flicker back on for seconds and goes black again and but the green LED is still on solid during all these times. If it is startup cap, the power supply would have cycle off and the LED would have blink.
I also notice that the bottom left of the screen is not as bright as the right side. I still suspect the lamps or the inverter problem.

[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?

[guirnab]

Re: Benq t720 - 0 seconds to black

Budm - could it be a faulty startup cap?

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

[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...

[selldoor]

Re: Benq t720 - 0 seconds to black

Budm - could it be a faulty startup cap?

[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.

[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.

[selldoor]

Re: Benq t720 - 0 seconds to black

Is it connected to a PC?

[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

[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.

[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.

[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.

[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.

[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.....