LCD 19" BENQ T905 problems

[PlainBill]

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?

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.

PlainBill

[chriz74]

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?

[PlainBill]

Re: LCD 19" BENQ T905 problems

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.

PlainBill

[chriz74]

Re: LCD 19" BENQ T905 problems

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.

[retiredcaps]

Re: LCD 19" BENQ T905 problems

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.

[chriz74]

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?

[PlainBill]

Re: LCD 19" BENQ T905 problems

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.

PlainBill

[chriz74]

Re: LCD 19" BENQ T905 problems

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?

[retiredcaps]

Re: LCD 19" BENQ T905 problems

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.

[chriz74]

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.

[capullaco]

Re: LCD 19" BENQ T905 problems

Seems Im the only one who couldnt repair his TFT

[icewilly]

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

[tabbiati]

Re: LCD 19" BENQ T905 problems

Hi to all,
changed transistor and with 7 dollar monitor work ok...
Thanks to all in this forum

[tabbiati]

Re: LCD 19" BENQ T905 problems

Regards to all,
I'm new in this debate but I have the same problems already described for this monitor.
Thanks to your question I found the capacitor C824 signed with a bad weld
redone that has not solved the problem but after further tests I found that the two transistors
next to it are damaged (probably due to the welding missing).
I bought on EBay and just reach those components hope to be able to report that the monitor
is "alive again."

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Only the 5707 were burnt. After the soldering of transformers & various elements, the lcd is working very well.

Glad to hear it is working. The resoldering of the components mentioned earlier seems to be the key to keeping these monitors working.

[Grav]

Re: LCD 19" BENQ T905 problems

So, for the end of the story:

The soldering iron heat detached a new track for the board when i changed 1000uF caps. I had to wrap a wire.

Only the 5707 were burnt. After the soldering of transformers & various elements, the lcd is working very well.

Thank you both !

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Does anyone know if the 3A pico fuse at PF801 is fast acting or slow blow?

I believe fast blow based on these posts ...

https://www.badcaps.net/forum/showth...benq+fast+blow

https://www.badcaps.net/forum/showth...blow#post49446

[LRN]

Re: LCD 19" BENQ T905 problems

Does anyone know if the 3A pico fuse at PF801 is fast acting or slow blow?

My board has STD10PF06-1 parts in the place of the IRFU9024 parts at Q805 and Q812. The difference appears to be 10A continuous drain current as opposed to the 11A for the IRFU9024.

[Grav]

Re: LCD 19" BENQ T905 problems

exactly 4.55$, with shipping. First i tryed to order the components at Farnells, a French (worldwide ? idk) reseller. But at the end, it was up with too much for a 'tutorial' repair:

2€ per c5707 so 4€ total
3A fuses per 10 only for 5.5€ total
2€ par 9024 - wasn't planning to buy one, but it can help -
4 € for various caps (was interested in replacing all elite's)
and 12€ of shipping...

With the Chinese eBay provider 's set, i can afford a not working c5707 in the kit, and even burn one or two more. I could wait to reach a local store for caps. Oh, i guess i'll not be shipped before next week, but this is not a problem.

Thank you for your excellent advices, Retiredcaps.

[retiredcaps]

Re: LCD 19" BENQ T905 problems

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 first set of transistor, 1 ohm indicates that it is shorted. Unsolder that set and retest out of circuit to verify.

The other set with 660, 900, and 100 ohms looks okay.

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

The picofuse is good.

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).

The 9024 readings look good.

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)

4 bucks is reasonable for a kit.