LCD 19" BENQ T905 problems

[Grav]

Re: LCD 19" BENQ T905 problems

ok.

My bad: the caliber is 2000 ohms. No decimal display, the lowest number i can get is 0001 - if we except 0000.

For a set of transistor, i have 1 ohm on each B/C/E junctions.
For the other set, i have around 660 ohms for BC/CE/BE/CB junctions, 900 ohms for EB, and 100 ohms for EC. I assume they are ok

For the picofuse, 1 ohm, same as failed transistors. I think this is the rounding.

For 9024, i have junctions BCE going from 800 to 1200 ohms base values, always increasing (i assume they are corrects and coupled in some way with a capacitor).

I unsoldered the polyester caps, inductances, the transformers, and reflow them. grrr... one track half removed on a transformer leg, but it seems ok (i have 40w iron). I quickly resolder the c5707 to see if i have identical troubles - display for 1s then shutdown - so assuming the transformers are powering the neons and are not badly soldered.

Bought on ebay a set of 4x C5707 2x 9024 1x picofuse for 4 bucks. Next time for the real repairing

(btw, i have not replaced the caps except the bad shape - slightly growth - 1000uF / 10v)

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Just to confirm, i have to unsold c5707, 9024 and picofuse to test them; i have to simply check if they are not shorted in anyway (except the picofuse which must be short).

You can test everything "in circuit". If the components test bad "in circuit" then unsolder them and retest them "out of circuit".

[Grav]

Re: LCD 19" BENQ T905 problems

Thank you retiredcaps, this is exactly i was aiming to do.

Just to confirm, i have to unsold c5707, 9024 and picofuse to test them; i have to simply check if they are not shorted in anyway (except the picofuse which must be short).

Thank you for your advices !

[retiredcaps]

Re: LCD 19" BENQ T905 problems

I have a low cost multimeter i bought at least 15 years ago. 2 calibers (2000kohm, 200ohms with 2 digits), max 200 V DC so i can't even check the transformer output.

All you need to test the 1 picofuse, 4 c5707 transistors, and 2 fn9024 fets is the 200 ohm scale. Test the transistors and fets to see if they are shorted (i.e. less than 20 ohms). It should take about 5 minutes to test these 7 components.

On Benq boards, it is rare to have the inverter transformer be bad. More likely, it just needs to be resoldered with new solder.

I plan to replace the picofuse in all ways, because i guess i'd not be able to precisely measure its state.

A good fuse should measure less than 1.0 ohms. You don't need anything more precise than that.

[Grav]

Re: LCD 19" BENQ T905 problems

Hi retiredcaps,

I have a low cost multimeter i bought at least 15 years ago. 2 calibers (2000kohm, 200ohms with 2 digits), max 200 V DC so i can't even check the transformer output.

I plan to replace the picofuse in all ways, because i guess i'd not be able to precisely measure its state.

I 'd really appreciate an oscilloscope, but i clearly can't afford

[retiredcaps]

Re: LCD 19" BENQ T905 problems

I'm now in the mood of trying the big step:
- resolder the transformers
- change the picofuse
- change the FET
- change the c5707 transistor
(post #21 strict compliance)

Do you think i should replace some caps, too, or my failure seems to be transistor-related ?

Before changing anything, do you have a multimeter to do the tests mentioned in this thread?

[Grav]

Re: LCD 19" BENQ T905 problems

Hello.

I'm new too. I have the first failure described in this thread:
- Power on - Led stay green
- BenQ logo stays for 500ms
- auto power off

I can reproduce the problem.

I have read numerous threads about it. I have replaced 2 bad-shape 1000uF 10v caps.

I'm now in the mood of trying the big step:
- resolder the transformers
- change the picofuse
- change the FET
- change the c5707 transistor
(post #21 strict compliance)

Do you think i should replace some caps, too, or my failure seems to be transistor-related ?

How it could work a tiny moment, then power off ? It means the backlight is ok, so could it not mean the c5707 and FETs are ok , or exactly point they are not ? The part remains a mystery for my own comprehension

[retiredcaps]

Re: LCD 19" BENQ T905 problems

So im at the same point as the 1st time

Right now, I can't think of anything obvious.

Maybe someone else will have some suggestions.

[capullaco]

Re: LCD 19" BENQ T905 problems

So I replaced the jumper wire with an insulated one.

Rechecked the bridges I mentioned in posts before, and they seem fine. Sucked the tin and the points bridged were very,very near,almost touching, so I guess its ok.

Then added tin on some solders that had little (2 pins from my solders) and the rest seemed ok.

But I tried and... BenQ logo, but its a bit dark and the display was making a sound.
Turned off and on again, same BenQ logo with artifacts. Turned it off and unplugged it.

The fuse is gone, and one of the problematic transistors too. One combination of pin reading shows 7 ohms.
And both trans have low (or I think theyre low) resistance (135~ ohm) on some of their pins.

So im at the same point as the 1st time

[capullaco]

Re: LCD 19" BENQ T905 problems

Yeah,it was insulated. But I got a very short cut to made it as tiny asd possible,but at the end the plastic melted and was loose.

I think it doesnt make contact.But I will replace it.

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Just noticed that for your jumper wire, it is not insulated. I normally see insulated wire, not bare wire.

When you put the board back in, that bare wire could be touching something and causing a short?

[retiredcaps]

Re: LCD 19" BENQ T905 problems

What should I do now that I replaced the picofuse?

Put it away for a day. Take another look at it tomorrow when there is good sunlight and recheck the soldering.

If it looks good and transistors and fets are not shorted, try it again.

[capullaco]

Re: LCD 19" BENQ T905 problems

Ok,so only the picofuse is blown. The transistors that blown twice are also ok, and so the FU9024

The bridge I did to one of the pins from the lifted pads seems also ok, giving continuity.

So only the picofuse is blown, luckily I bought today a 125v one. (Already replaced it, thought havent assembled the board yet)

I attached some pics.

1- The transistors and the irfus.Theyre all ok.
2- Just now I noticed this soldering with the daylight.Its more golden. I just sucked it and replaced with new tin.
3- This golden tin is from one of the pins of the TR601

And the rest is the PCB in general (you can notice my fuse pins improvisement :p)

What should I do now that I replaced the picofuse? Maybe it blown cause it was 220v instead of 125v? That wouldnt make sense,would it? Btw, it has the same amp (3A).
Or maybe the 220v protected the transistors (they arent blown) caused by something?

[retiredcaps]

Re: LCD 19" BENQ T905 problems

But the last time I checked all the 3 legged components,

If your board looks anything like the OP, this is the backside

https://www.badcaps.net/forum/attach...0&d=1226067862

showing where the FU9024 are in relation to the c5707.

[capullaco]

Re: LCD 19" BENQ T905 problems

Ok, tomorrow im gonna check all the componentes again.

But the last time I checked all the 3 legged components, they were ok except for the 2 transistors (I made a mistake in some post above saying there were 4 blown transistors, the other 2 were double diodes and they were also ok)

nice night guys, uni class starting tomorrow

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Diferent voltaje but same amp, the guy told me that the amp is the important stuff and it should work with a 220V fuse.

Amp is the most important. It should be the same.

The FET is most likely FU9024. It is a 3 legged component and looks like a transistor. If you find, test to see if it is shorted (same procedure as testing a transistor).

[capullaco]

Re: LCD 19" BENQ T905 problems

Diferent voltaje but same amp, the guy told me that the amp is the important stuff and it should work with a 220V fuse.

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Yeah, I changed all the blown things, the picofuse (actually used a normal fuse) and 4 transistors.

Normal fuse? Is this fuse rated the same amp and voltage as the picofuse?

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Im gonna test all the things again,and try to make a better pic, but cant do miracles with the mobile phone (a relative has my cam and he will be out for a long time)

The FETs could be on the backside on the board.

Take the boards to a window on a sunny day. Turn off flash. If your phone has macro mode, use that. Look at your pics on the computer to make sure everything is legible. Some of us don't have PlainBill's 20/20 vision.

[capullaco]

Re: LCD 19" BENQ T905 problems

Yeah, I changed all the blown things, the picofuse (actually used a normal fuse) and 4 transistors.

BTW, the caps are orientated right, Im 100% sure. (white painted side with the negative side, the shorter pin)

Which ones are the fets? When I checked the transistors ,only 4 were blown, the rest were ok.

Im gonna test all the things again,and try to make a better pic, but cant do miracles with the mobile phone (a relative has my cam and he will be out for a long time)