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Old 01-26-2011, 09:33 AM   #101
tabbiati
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Default Re: LCD 19" BENQ T905 problems

Hi to all,
changed transistor and with 7 dollar monitor work ok...
Thanks to all in this forum
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Old 03-03-2011, 06:26 PM   #102
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Default Re: LCD 19" BENQ T905 problems

Wanted to stop in and post and say that this helped me in repairing my monitor. Thanks to all who contributed, and contribute to other threads like this.

cheers
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Old 03-07-2011, 05:52 PM   #103
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Default Re: LCD 19" BENQ T905 problems

Seems Im the only one who couldnt repair his TFT
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Old 05-03-2011, 02:13 PM   #104
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Default Re: LCD 19" BENQ T905 problems

Hello everyone, I too have a benq t905 which is not working. I opened it and I found a blown fuse so I thought the problem was easily solvable. However replacing the fuse was a no go. Lamps don't light up, I can see the faint "benq" logo when I turn on the monitor. I'm going to check capacitors tomorrow. Any advice would be useful, thanks.
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Old 05-03-2011, 02:39 PM   #105
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Default Re: LCD 19" BENQ T905 problems

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Originally Posted by chriz74 View Post
Lamps don't light up, I can see the faint "benq" logo when I turn on the monitor.
Post clear focused pictures of your boards using the manage attachments function. Please do not post pictures inline.

Benq style power/inverter boards typically have more than one fuse. The main fuse by the AC plug and the picofuse.
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Old 05-03-2011, 03:03 PM   #106
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by retiredcaps View Post
Post clear focused pictures of your boards using the manage attachments function. Please do not post pictures inline.

Benq style power/inverter boards typically have more than one fuse. The main fuse by the AC plug and the picofuse.
Is this clear enough? There's no sun light available at the moment.
I tested the PF801 (is this the pico fuse?). Strange thing happens, the tester is stuck at 1 but some times it gives values then goes back to 1. Any clue?


Last edited by chriz74; 05-03-2011 at 03:13 PM..
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Old 05-03-2011, 06:31 PM   #107
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by chriz74 View Post
Is this clear enough? There's no sun light available at the moment.
I tested the PF801 (is this the pico fuse?). Strange thing happens, the tester is stuck at 1 but some times it gives values then goes back to 1. Any clue?
(Sigh) Standard problem. The solder joints on the transformer pins crack. This results in one of the transistors staying on; it shorts, and blows the fuse.

Replace the fuse without replacing the shorted transistors and it will blow again. Replace the transistors without resoldering the pins on the 4 inverter transformers and a transistor will short again.

This inverter design is a Royer oscillator, it is used by Benq who supplies monitors to both Dell and HP. It MIGHT not have been discussed in this thread, now it is.

However you are correct about one thing. This is an excellent opportunity to recap.

And to repeat what retiredcaps said, don't post pictures inline. Use Manage Attachments below the test entry area.

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Last edited by PlainBill; 05-03-2011 at 06:34 PM..
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Old 05-04-2011, 01:02 AM   #108
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Default Re: LCD 19" BENQ T905 problems

I don't see any crack in the solder joints, the fuse I replaced didn't blow again. I did not replace the pf801 . Btw what are the specs for this PF?
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Old 05-04-2011, 01:11 AM   #109
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Default Re: LCD 19" BENQ T905 problems

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Originally Posted by chriz74 View Post
Btw what are the specs for this PF?
The picofuse for most Benq boards is 3A 125V.

We KNOW this board very well. If you don't resolder as PlainBill states, you will have problems in the future. If you look at all the Benq board posts, it is the same advice over and over.
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Old 05-04-2011, 02:22 AM   #110
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Default Re: LCD 19" BENQ T905 problems

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Originally Posted by retiredcaps View Post
The picofuse for most Benq boards is 3A 125V.

We KNOW this board very well. If you don't resolder as PlainBill states, you will have problems in the future. If you look at all the Benq board posts, it is the same advice over and over.
125V, are you positive? Cause the main fuse is 2A 250v.
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Old 05-04-2011, 05:28 AM   #111
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Default Re: LCD 19" BENQ T905 problems

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125V, are you positive? Cause the main fuse is 2A 250v.
The main fuse is rated for 250 volts because the monitor may be plugged into circuits carrying up to 230 volts. The picofuse is rated at 125 volts because it is in a circuit carrying only 12 volts.

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Old 05-04-2011, 09:14 AM   #112
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Default Re: LCD 19" BENQ T905 problems

update. I found two "2sc5707" are faulty and also the "1200AP40 CPBY 05475".
I tried to test the lamps and i get no value, is this normal?

Last edited by chriz74; 05-04-2011 at 09:26 AM..
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Old 05-04-2011, 10:36 AM   #113
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Default Re: LCD 19" BENQ T905 problems

Quote:
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update. I found two "2sc5707" are faulty and also the "1200AP40 CPBY 05475".
I tried to test the lamps and i get no value, is this normal?
How did you try to test the lamps? A CCFL requires about 600-1000 volts to ignite. Unless you used a tester that produces that kind of voltage, you wouldn't get any results.

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Old 05-04-2011, 10:49 AM   #114
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Default Re: LCD 19" BENQ T905 problems

It was really a relative of mine so I don't know what kind of tester he used. I'll ask. What about the 1200AP40 BLOWN, how does that sound to you? Is this normal fault? As a result there was no power being spread to the lamp connectors.
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Old 05-04-2011, 11:14 AM   #115
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Default Re: LCD 19" BENQ T905 problems

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What about the 1200AP40 BLOWN, how does that sound to you?
How did you determine that this is blown???
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Old 05-04-2011, 04:34 PM   #116
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Default Re: LCD 19" BENQ T905 problems

Because it doesnt read 12v but 300v
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Old 05-04-2011, 08:05 PM   #117
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by chriz74 View Post
update. I found two "2sc5707" are faulty and also the "1200AP40 CPBY 05475".
I tried to test the lamps and i get no value, is this normal?
The lamps are likely not a problem.
Can't speak for the 1200AP40, as you didn't really tell us what you measured, and how you determined.

As many have tried to tell you, this monitor has a very common failure mode.
Bad solder joints on the inverter transformers cause transistors to short, and blow pico fuses. Check ALL the 5707s, and the FETs, replace any bad ones, replace the fuse, while you're in there, recap, and then fire it up.

Of the 22 or so monitors I've repaired in the last year or so (and I'm still a newbie learning) several were this model or similar, and had this common failure.
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Old 05-04-2011, 09:24 PM   #118
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Default Re: LCD 19" BENQ T905 problems

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Originally Posted by retiredcaps View Post
How did you determine that this is blown???
The only part corresponding to 1200AP40 is a SMPS controller. If you can still see the Benq logo as you mentioned in post 7 the 1200AP40 is good.

Really, this is a waste of time in my opinion. It's difficult enough assisting in troubleshooting a monitor if the person doing the real work is interacting with us. If there is an intermediate party (you), communication becomes impossible.

1. Check for 5V and 12V from the power supply. If they are present:

2. Remove as much solder as possible from the transformer pins and resolder with real solder, not this lead free junk Benq inflicted on everyone.

3. Replace both 2SC5707s in a pair if either are shorted

4. Check both FU9024N, replace any that are associated with a shorted 2sc5707

5. Replace the picofuse if it is open.

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Old 05-05-2011, 02:04 AM   #119
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Default Re: LCD 19" BENQ T905 problems

Thanks for the advices, I'll tell what turns out. BTW I can't see the BENQ logo anymore so I suppose the 1200AP40 is really gone. Oh I got the schematics from BENQ, I think it could be useful for future reference.

Attached Images
File Type: png 20110505084252.png (53.2 KB, 203 views)

Last edited by chriz74; 05-05-2011 at 02:08 AM..
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Old 05-05-2011, 11:45 AM   #120
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Default Re: LCD 19" BENQ T905 problems

Quote:
Originally Posted by chriz74 View Post
BTW I can't see the BENQ logo anymore so I suppose the 1200AP40 is really gone.
If the power LED is green or yellow, the power supply and logic card are likely to be working. That is, 1200AP40 is working.
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