LCD 19" BENQ T905 problems

retiredcaps · 09-08-2010, 11:25 AM

Re: LCD 19" BENQ T905 problems

Originally posted by capullaco

Its like at first, with the green led working ok.

Check your solder work again. Check the orientation of your caps, transistors to make sure they were installed properly.

Your previous pictures are very hard to see (fuzzy, out of focus, and at an angle). Take a look at this picture

https://www.badcaps.net/forum/attach...6&d=1282133540

and compare it to yours. Our the other pic, we can see all the PCB designations CLEARLY.

The reason I bring up the picture quality is I cannot tell from your pics where things are located.

So I'm guessing, but on most benq boards, there are 4 C5707 transistors and 2 Fu9024 FETs.

If you see no picture, but have a green power LED, there are 7 things to check. 1 picofuse, 4 c5707s, and 2 fu9024s. Check to make sure the picofuse is not open and the transistors and fets are not shorted.

capullaco · 09-08-2010, 10:18 AM

Re: LCD 19" BENQ T905 problems

So I tested it...no luck

Didnt even show up the BenQ logo. When I plugged it, I heard a spark, but dunno if it was from inside or just the normal spark sound that sometimes is heard when you plug something.

Its like at first, with the green led working ok.

capullaco · 09-08-2010, 09:32 AM

Re: LCD 19" BENQ T905 problems

Ok,gonna try with that pin being dry soldered !

PlainBill · 09-08-2010, 06:33 AM

Re: LCD 19" BENQ T905 problems

Originally posted by capullaco

Ok, I can solder a wire easily on the pin from the 3rd pic.
But the pin from the 1st and 2nd pic its alone! no traces to anything

Then you don't have to worry about it. They often use a standard bobbin; some pins are unused. You are concerned about electrical integrity, not the mechanical strength. Unless, of course, you are planning on using your monitor on an ATV.

PlainBill

capullaco · 09-08-2010, 04:24 AM

Re: LCD 19" BENQ T905 problems

Ok, I can solder a wire easily on the pin from the 3rd pic.
But the pin from the 1st and 2nd pic its alone! no traces to anything

retiredcaps · 09-07-2010, 11:22 PM

Re: LCD 19" BENQ T905 problems

Originally posted by capullaco

Maybe scratching the board a bit? What if I just leave a ball of tin? Will it be a dry joint? Maybe painting the place with a pencil? (dunno why this came to my mind)

How about soldering the pins to another bit of the board? On the same trace,of course.

I haven't done the scratching board so I can't say. However, I have soldered pins together when I lifted a solder pad and run a small wire from one pin to the joint. They were on the same trace.

As you can see in this post, some people run trace wires or other hacks ...

https://www.badcaps.net/forum/attach...5&d=1278637697

capullaco · 09-07-2010, 07:03 PM

Re: LCD 19" BENQ T905 problems

I used the desolder pump to desolder all the items (thought I didnt add more
solder,never saw that technique on the videos of desoldering I have seen) and only those 2 gave problems.

Maybe scratching the board a bit? What if I just leave a ball of tin? Will it be a dry joint? Maybe painting the place with a pencil? (dunno why this came to my mind)

How about soldering the pins to another bit of the board? On the same trace,of course.

I dont wanna give up yet!

retiredcaps · 09-07-2010, 01:29 PM

Re: LCD 19" BENQ T905 problems

Originally posted by capullaco

How can I fix this?

If you lifted the solder pads, there isn't much you can do.

For this exercise, you are removing all the old solder.

I like to add a little bit of solder first to the old solder/joint and then I use my desolder pump to suck everything up. Then I clean up with a little bit of alcohol and add fresh solder.

capullaco · 09-07-2010, 01:17 PM

Re: LCD 19" BENQ T905 problems

Ok, so I was resoldering all the things you marked.
When I removed 2 of the transformers, one of their pins were without that silver layer on the board, so the tins doesnt sticks well.

I attached some pics, 1 and 2 are the same and the 3 is the other pin.

How can I fix this?

Attached Files

capullaco · 09-03-2010, 12:49 PM

Re: LCD 19" BENQ T905 problems

Oh, thats not the blown fuse, its one on the top right on the pic.Next to the heatsink, only half of it is visible.
Its the picofuse, but I replaced it the 1st time with a cristal one.

Also the normal voltage in Europe is 220V, just saying, dunno if that affects.

retiredcaps · 09-03-2010, 12:46 PM

Re: LCD 19" BENQ T905 problems

Originally posted by capullaco

Ok,went to buy the components but they didnt have the fuse at 125V.
Would one of 220V do the job?

See your pic at

https://www.badcaps.net/forum/attach...6&d=1283482087

See where it says re CAUTION:

Others may have different opinions on the fuse, but the warning CAUTION is there for a reason. I know it is not the picofuse, but the same caution applies.

capullaco · 09-03-2010, 12:32 PM

Re: LCD 19" BENQ T905 problems

Ok,went to buy the components but they didnt have the fuse at 125V.
Would one of 220V do the job?

retiredcaps · 09-03-2010, 09:28 AM

Re: LCD 19" BENQ T905 problems

Originally posted by capullaco

One thing im still not sure:

Code:

Desolder all 4 transistors and the polyester caps (between the transistors

Is the polyester cap the 'K' cap from my first pic? Theyre 2 so it makes sense, but just to be sure.

Yes, the K cap is what Krankshaft is referring to.

About the dual diodes, yes they are.Theyre named "DXXX" and they have this printed on one side:

YG902C2
-->|--|<-- CI
40 121

I managed to unstick them from the heatsink.
But how do I test them? I tried this but I dont know if its correct:

- Black (COM) on 2nd pin RED on 3rd: 400
- RED on 1st pin BLACK on 2nd: 400

Those values are with the diode setting ( -->|) of the polimeter. Other combinations didnt show results.

Yes, that is correct. Excellent job. 400mV (millivolts) indicates that the dual diode is good. A good diode should read between 0.4V and 0.7V.

The things I need to resolder, I just need to unsolder them and sold again with new tin or is there any other action involved? I ask because the soldering on those things seems ok , at least on looks.
One more thing, should I resolder those things marked as '?' on the 6th pic?

Krankshaft writes to desolder (remove all the old solder) and resolder with new solder.

Another analogy (I spelled it right this time) might be painting a door.

You have a door that needs to be painted. For the most part the paint looks good, but it could use some touch up.

The lazy way is to just put another coat over it without cleaning it or stripping it down to the wood. This is akin to reflow new solder on top of the old solder.

The "proper" way to paint this door is to remove all the old paint, sand it down so that is smooth, apply protective coat, and then paint it. This is akin to removing the old solder and adding new fresh solder.

Yes, resolder those ? marked items.

PS. You have made huge progress in the last 24 hours. Keep up the good work.

PlainBill · 09-03-2010, 06:11 AM

Re: LCD 19" BENQ T905 problems

Originally posted by retiredcaps

Yes, because you chose to ignore 3 people telling you 4 times to resolder the transformers. This problem with the Benqs has been known for about 2 years now. When people say you *must* resolder the transformer it is because they have experienced what you have.

We can lead a horse to water ...

A poor analog might be the following (I can see PlainBill shuddering right now). Imagine a car engine as the transformer. When the bolts holding down the engine are loose, it shakes all the components of car.

You take the car to the mechanic (Eric the car guy). Eric says I can fix the loose radio, loose hoses, loose steering wheel, etc from all the damaged caused by the engine for $500.

Eric also HIGHLY recommends that he rebolts and remounts the engine. It will cost $1000. You balk. 2 days later, the engine causes the same damage. This time Eric charges you $1000 because the engine caused more damage than the first time.

Eric again recommends remounting and rebolting the engine, but this time it is $1500 because of more damage. You balk again.

And so on ...

Actually, that is an excellent analogy. You teach well, Grasshopper!!

PlainBill

capullaco · 09-03-2010, 05:14 AM

Re: LCD 19" BENQ T905 problems

Thanks again retiredcaps.

If I didnt resolder the transformers, its because I wasnt sure which ones were them, until jetadm123 pointed them out.

That being said, I also didnt research more, because after checking that the fuse and the transistors were blown, I thought that those were all the problems the PCB had.

One thing im still not sure:

Code:

Desolder all 4 transistors and the polyester caps (between the transistors

Is the polyester cap the 'K' cap from my first pic? Theyre 2 so it makes sense, but just to be sure.

About the dual diodes, yes they are.Theyre named "DXXX" and they have this printed on one side:

YG902C2
-->|--|<-- CI
40 121

I managed to unstick them from the heatsink.
But how do I test them? I tried this but I dont know if its correct:

- Black (COM) on 2nd pin RED on 3rd: 400
- RED on 1st pin BLACK on 2nd: 400

Those values are with the diode setting ( -->|) of the polimeter. Other combinations didnt show results.

The things I need to resolder, I just need to unsolder them and sold again with new tin or is there any other action involved? I ask because the soldering on those things seems ok , at least on looks.
One more thing, should I resolder those things marked as '?' on the 6th pic?

retiredcaps · 09-02-2010, 11:45 PM

Re: LCD 19" BENQ T905 problems

One other suggestion.

Once the new parts come in, check them all out of circuit to ensure they are good.

When you get all the repairs done, put away the monitor for 2 days. Come back 48 hours later with post #21 printed and go through each sentence top to bottom and bottom to top (i.e doublecheck all your work).

That will give your brain some time to process the repair.

Additionally, BEFORE you power up. Doublecheck all the components again to make sure they are soldered in properly, oriented correctly (not backwards) and retest them with your mulitmeter "in circuit".

Then and only then, apply power.

retiredcaps · 09-02-2010, 11:40 PM

Re: LCD 19" BENQ T905 problems

Originally posted by capullaco

1. Those are the transistors that were originally blown.Theyre blow again (the left arrow actually indicates the trans,not the cap :p)

Yes, because you chose to ignore 3 people telling you 4 times to resolder the transformers. This problem with the Benqs has been known for about 2 years now. When people say you *must* resolder the transformer it is because they have experienced what you have.

We can lead a horse to water ...

Here is the technical explanation why the transistors short out.

https://www.badcaps.net/forum/showpo...&postcount=269

A poor analog might be the following (I can see PlainBill shuddering right now). Imagine a car engine as the transformer. When the bolts holding down the engine are loose, it shakes all the components of car.

You take the car to the mechanic (Eric the car guy). Eric says I can fix the loose radio, loose hoses, loose steering wheel, etc from all the damaged caused by the engine for $500.

Eric also HIGHLY recommends that he rebolts and remounts the engine. It will cost $1000. You balk. 2 days later, the engine causes the same damage. This time Eric charges you $1000 because the engine caused more damage than the first time.

Eric again recommends remounting and rebolting the engine, but this time it is $1500 because of more damage. You balk again.

And so on ...

2.
Im not sure if they should be bridged... One is a pin of the 'K' cap of the 1st pic and the other one is a SMD named "JR10"

If you follow the circuit green pcb trace you should be able to tell if they are on the same circuit path or not. If they are, then it is not a problem if they are bridged.

I started here on these forums back in April 2010, but I can't remember a single case where a bridged connection has been documented as a problem.

I asked a similar question. See

https://www.badcaps.net/forum/showth...+solder+joints

4.I tried to compare those bridged with the image posted here, but im not sure, so I marked them

That is a good idea, but be aware that there are possibly many revisions of this board. Manufacturers can change the boards as they see fit.

5.Two new blown transistors.I dont know if they were blow before, but theyre now.

The picture's angle makes it hard to see, but those are probably dual diodes? They should have 2 diode symbols pointed towards the center.

What measurements did you get for these dual diodes?

6.Those are the things I have to resolder right? Are they marked correctly? What are those two things marked as '?' ? Should I resolder them also?

Yes, resolder those. L indicate an inductor. T is a transformer.

7.Finally, this is the bit I said it was burnt.but the transistor under it its fine.

Some browning is normal. As long as the transistor underneath tests okay, it is fine.

capullaco · 09-02-2010, 08:45 PM

Re: LCD 19" BENQ T905 problems

Ok, my mobile is working again.

Took photos of 'problematic' bits:

Code:

Photo number. Description

1. Those are the transistors that were originally blown.Theyre blow again (the left arrow actually indicates the trans,not the cap :p)

2.The bottomside of those transistors.Those 2 yellow points,are bridged (they have continuity and the polimeter beeps with the diode/continuity beep setting).
Im not sure if they should be bridged... One is a pin of the 'K' cap of the 1st pic and the other one is a SMD named "JR10"

3.Those are another bridged point.The cap pin is bridged with a drop of tin that it was already on the board.I think its ok

4.I tried to compare those bridged with the image posted here, but im not sure, so I marked them

5.Two new blown transistors.I dont know if they were blow before, but theyre now.They have a heatsink and theyre glued to it.
Also, theyre special and are larger with a hole for a screw. Its one piece, I mean its not a transistor glued to another one. Wonder if I will be able to buy that type of transistor...Also, I dont know they ID, but I guess the guy in the shop will know.

6.Those are the things I have to resolder right? Are they marked correctly? What are those two things marked as '?' ? Should I resolder them also?

7.Finally, this is the bit I said it was burnt.but the transistor under it its fine.

Attached Files