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Old 08-03-2010, 03:46 PM   #21
Krankshaft
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Default Re: LCD 19" BENQ T905 problems

My standard operating procedure with these monitors is:
  • Replace all of the secondary caps.
  • Desolder all 4 inverter transformers legs and reflow them with new solder.
  • Desolder all 4 transistors and the polyester caps (between the transistors) and inductors (nearby the transistors) for the inverter and reflow them.
  • Check the rest of the board over with a magnifier for other bad joints.

If you haven't done all 4 steps in the list you will have a repeat failure.
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Last edited by Krankshaft; 08-03-2010 at 03:51 PM..
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Old 08-04-2010, 05:15 PM   #22
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Default Re: LCD 19" BENQ T905 problems

Sorry I cant post images now.
I dont have a camera atm (long story) and my mobile phone just broke :/

But the PCB is the same as the one posted by dvorav.

And like I said, im a botcher in soldering and electronics in general

Any tip for a soldering iron for this kind of jobs? small solderings
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Old 08-04-2010, 09:38 PM   #23
retiredcaps
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by capullaco View Post
And like I said, im a botcher in soldering and electronics in general

Any tip for a soldering iron for this kind of jobs? small solderings
Curious Inventor video on soldering is a really good one.

http://www.youtube.com/watch?v=I_NU2ruzyc4
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Old 08-22-2010, 01:09 PM   #24
capullaco
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Default Re: LCD 19" BENQ T905 problems

I connected the display, the pc recognizes it, the led powers on and work normally, etc.Except for the no image,it works correctly.
I tried to put a strong light in front of the display, and couldnt see anything.

Here are some pics,sry for the quality.






What should I do now?
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Old 08-22-2010, 05:18 PM   #25
retiredcaps
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by capullaco View Post
Here are some pics,sry for the quality.
What should I do now?
Re-read post #18 on how to post pictures. The URLs you supplied show nothing but a postage stamp.

Did you try testing anything in post #20 and #21?
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Old 08-22-2010, 06:28 PM   #26
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Default Re: LCD 19" BENQ T905 problems

Sry, now I uploaded the images into the forum.

Im gonna buy a proper soldering iron to do this job, so I havent tried the post #21 advices yet.

Like I said, im a newb, and also English isnt my main language,so can you help on those points please?

Code:
Replace all of the secondary caps.
You mean capacitators? Or all the electronic stuff in general? How do I get the stuff type/info to buy them in a electronic store?

Code:
Desolder all 4 inverter transformers legs and reflow them with new solder.
Where are the inverters on mi screenies?

Code:
Desolder all 4 transistors and the polyester caps (between the transistors) and inductors (nearby the transistors) for the inverter and reflow them.
Can you mark those on the screenies?

Im a total noob,but also I like this :p
Im starting to buy some electronic/soldering items to do those jobs properly, I have on my mind to try to repair this TFT and an old Game Gear

Thanks
Attached Images
File Type: jpg 1.jpg (296.8 KB, 173 views)
File Type: jpg 2.jpg (232.7 KB, 121 views)
File Type: jpg 3.jpg (226.9 KB, 99 views)
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Old 08-22-2010, 08:50 PM   #27
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by capullaco View Post
Im a total noob,but also I like this :p
I suggest you start by studying this book

http://www.lcdmonitorrepairebook.com...ir%20ebook.pdf

It will walk you through the basics and id all the components on the boards.

In addition, your pictures are horribly out of focus. Take your boards to a window on a sunny day, use macro mode, and turn flash off. Here is an example of clear focused pictures ...

http://www.badcaps.net/forum/showpos...08&postcount=3
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Old 08-26-2010, 01:25 PM   #28
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Default Re: LCD 19" BENQ T905 problems

Ok Im gonna change 2 capacitators that seem a bit out of shape (just a bit, I bet theyre ok, but just in case) and the fuse, that seems is a common problem.

But I saw 2 transistors that seemed to have burnt a bit the board.
One just a bit, and the other, you can see the burnt a lot. Also this transistor seems to have a very short middle pin, or its broken, I dont know.
I have marked it on the pic attached.

I have checked them with a multimeter, configured it in Ohms, and tested the middle ping with the others.

Before saying anymore, I have to say that its what I read on Internet on how to check transistors ,so I dunno if I did properly.

Well, I had values from both transistors, so Im not sure there broken after all...

As a last note, I must say that I checked them on board, ie. I didnt desolder them.

Thanks
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File Type: jpg burnt.JPG (291.5 KB, 101 views)

Last edited by capullaco; 08-26-2010 at 01:28 PM..
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Old 08-26-2010, 01:36 PM   #29
retiredcaps
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by capullaco View Post
But I saw 2 transistors that seemed to have burnt a bit the board.
Some browning is normal.

Quote:
Well, I had values from both transistors, so Im not sure there broken after all...

As a last note, I must say that I checked them on board, ie. I didnt desolder them.
Here is how I test transistors in circuit.

a) put black probe (com) on pin 1 and red probe on pin 2 - read ohm
b) put black probe (com) on pin 1 and red probe on pin 3 - read ohm
c) put black probe (com) on pin 2 and red probe on pin 3 - read ohm

If any ohm reading is less than 50 ohm you have a shorted transistor and it will need to be replaced.

edit:

Some boards will have a fuse or picofuse and they may be open because a transistor was shorted or some other component went bad.

You can test for open fuses by (with power off and unplugged)

a) put black probe (com) on one pin and red probe on other pin - read ohm

If the reading is 0.3 or 0.4 ohm you have a good fuse. Anything higher is a sign of an open fuse. Do NOT replace fuses and turn on the power back on without checking for what caused the fuse to go open otherwise you will just be replacing the fuse again.
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Old 08-26-2010, 03:09 PM   #30
capullaco
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Default Re: LCD 19" BENQ T905 problems

Thanks retiredcaps, I gonna check them.
But one question, which ohm switch should I use? I got : 200,2k,20k.... up to 200M.

You said 50 ohm, so I guess I should use "200"? Or did you mean 50k ohm?

Edit:

In the board, aside the picofuses there is a 'nromal' fuse, like the one attached on the pic.
How many ohms should it give?

Edit 2:
Ok, the picofuse is blown, with multimeter at 200k ohms, it went from ~160 up to 200.
Attached Images
File Type: jpg 3AB%20FUSE%20314,326%20SERIES.jpg (60.3 KB, 27 views)

Last edited by capullaco; 08-26-2010 at 03:14 PM..
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Old 08-26-2010, 03:15 PM   #31
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by capullaco View Post
But one question, which ohm switch should I use? I got : 200,2k,20k.... up to 200M.

You said 50 ohm, so I guess I should use "200"? Or did you mean 50k ohm?
200 ohms. A out of range reading should read 0L or "1" (on the left side of your multimeter).

Quote:
In the board, aside the picofuses there is a 'nromal' fuse, like the one attached on the pic.
How many ohms should it give?
All fuses should read 0.3 ohms.
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Old 08-26-2010, 03:25 PM   #32
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Default Re: LCD 19" BENQ T905 problems

Ok,the pico shows "1.", the normal one shows "0,2" so seems this one is ok.

The transistor that burnt the board also seems ok, almost everytime showed a range out of range (showing "1.") and a few times showed a high value ,like 197 for a second then went out of range.
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Old 08-26-2010, 03:28 PM   #33
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by capullaco View Post
Ok,the pico shows "1.", the normal one shows "0,2" so seems this one is ok.
Your picofuse, as we suspected, is open or BAD. You will need to replace it.

Quote:
The transistor that burnt the board also seems ok, almost everytime showed a range out of range (showing "1.") and a few times showed a high value ,like 197 for a second then went out of range.
The showing of 197 is your multimeter not reacting fast enough. For your own fun and learning, try testing the same transistor on the 2k, 20k, and 200k scale and see what you get.
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Old 08-26-2010, 03:42 PM   #34
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Default Re: LCD 19" BENQ T905 problems

Ok checked the other "fuses". At least they seem like fuses but:

1- Some of them have a numeration starting with "L", ex: L605 . Those seem to be ok (values 0.1 , 0.2)
2- The other ones I think they arent fuses, they start with "D", the numbers on them are far more visible and go out of range.
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Old 08-26-2010, 03:48 PM   #35
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by retiredcaps View Post
The showing of 197 is your multimeter not reacting fast enough. For your own fun and learning, try testing the same transistor on the 2k, 20k, and 200k scale and see what you get.
Where I said one second, it really was a tenth part , barely got time to see it

Tried it with 200k and showed high numbers, so guess that it has a lot more resistance

Edit:

btw,the blown fuse means that theres also some other thing blowed,because of that? I think that fuses are meant to protect those kind of things, but im asking just in cause. Or maybe a better question would be, what caused the fuse to blow.

Last edited by capullaco; 08-26-2010 at 03:51 PM..
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Old 08-26-2010, 03:50 PM   #36
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by capullaco View Post
1- Some of them have a numeration starting with "L", ex: L605 . Those seem to be ok (values 0.1 , 0.2)
2- The other ones I think they arent fuses, they start with "D", the numbers on them are far more visible and go out of range.
F = Fuse
I = Inductor (they should measure around 0.2 ohms)
D = Diodes

Test them with your diode setting. One way should measure 0.4 to 0.7V. Reserve and the other way should measure "1" or 0L.
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Old 08-26-2010, 03:53 PM   #37
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by capullaco View Post
btw,the blown fuse means that theres some other thing blowed also,because of that?
Fuses can go bad just by themselves. But they blow for a reason. It is likely that one of your transistors is shorted or the transformers are poorly soldered. Hence the recommendations on post #21.

This Benq board is widely known to have the problems mentioned in post #20 and #21. If you read or search for Benq here, you will see the same things to check and preventative recommendations over and over.
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Old 08-26-2010, 04:08 PM   #38
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by retiredcaps View Post
Test them with your diode setting. One way should measure 0.4 to 0.7V. Reserve and the other way should measure "1" or 0L.
Which one is the diode setting?
Can it be the the arrow one? -->| with a music note? Or one called "hFE" ?


Quote:
Originally Posted by retiredcaps View Post
This Benq board is widely known to have the problems mentioned in post #20 and #21. If you read or search for Benq here, you will see the same things to check and preventative recommendations over and over.
The problem is that I dont know what is what. The transformers are those squares with white/yellow tape over them?

Cant upload proper images, because I dont have my good camera,only this old one without macro mode.And my mobile phone is broken :<
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Old 08-26-2010, 04:49 PM   #39
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Default Re: LCD 19" BENQ T905 problems

So far seems that all the transistors are ok but one.

Also I just notice there are some coil like things named also with "L". All those "L" things (what does the "L" mean?) Show 0,1 - 0,2 ohms
.
Im still not sure how to mesure diodes.
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Old 08-26-2010, 08:43 PM   #40
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by capullaco View Post
So far seems that all the transistors are ok but one.

Also I just notice there are some coil like things named also with "L". All those "L" things (what does the "L" mean?) Show 0,1 - 0,2 ohms
.
Im still not sure how to mesure diodes.
Always replace transistors in pairs.

Sorry, I made a typo above L = inductor, not I. To measure diodes see this youtube video.

http://www.youtube.com/watch?v=hRKzZ...eature=related

In fact, you can search youtube for "multimeter tutorial".
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