Hp 1740

[Dew_32]

I got a scrap HP 1740 monitor and am trying to repair it. I have been reading this post and see allot of the same problems. My monitor lights up for about one sec. then goes dark I can see the display is there but there is no back light. So I order this repair kit and replaced 4 c5707, 2 fu9024n, Pico Fuse, 2 220uf 35v cap, 2 470uf 35v cap. I also re soldered the transformers as suggested in the form. But my monitor is still doing the same thing. Only displays for about one sec then goes black. Is there a way to find out which component is failing?

[PlainBill]

Re: Hp 1740

I got a scrap HP 1740 monitor and am trying to repair it. I have been reading this post and see allot of the same problems. My monitor lights up for about one sec. then goes dark I can see the display is there but there is no back light. So I order this repair kit and replaced 4 c5707, 2 fu9024n, Pico Fuse, 2 220uf 35v cap, 2 470uf 35v cap. I also re soldered the transformers as suggested in the form. But my monitor is still doing the same thing. Only displays for about one sec then goes black. Is there a way to find out which component is failing?

Yes, but it will require a little testing. This is a very interesting circuit; I don't believe I've ever seen it before. The transistors drive both primaries, the secondaries are connected in series.... OK, I've got a handle on it.

On the top side of the board there is a long jumper that runs between the pairs of transformers. J5 runs between T801 and T802. Another runs between T803 and T804, I can't see the number. Mount the power supply / inverter board in place with all the screws. Use either screw at the bottom of the board (as seen in the picture of the top side) as ground, and measure the voltage at each jumper in the second or so before the screen goes dark. This should be only a couple of volts DC.

PlainBill

[Dew_32]

Re: Hp 1740

Here are the readings I got
starting at the bottom
J10 .84
J20 .84
J5 .72
J17 1.68

[sofTest]

Re: Hp 1740

You might want to test the D801 and D803 diodes on the back of the board. I repaired a 1940 a while ago, where one of them was shorted. I also replaced all capacitors. You might have a look in this thread for ideas.

[Dew_32]

Re: Hp 1740

Where can I get those diodes if I find that they are shorted ?

[Dew_32]

Re: Hp 1740

Ok I pulled both diodes from D801 and D803 and tested them they were both good so I put them back on. I do see dark color on the board above the 4 resistors on the other side would that be my problem? R27-R30, R33-R36

[sofTest]

Re: Hp 1740

All Benq-boards I've seen had those dark spots.

You might have other bad capacitors beside the four you have replaced. Personally I've repaired five HP monitors with Benq-boards, and not one of them had so few as four bad electrolytic capacitors. As a minimum, I always replace all capacitors with value 220uF and up. I even saw someone who found one of the capacitors that sits between the c5707s to be bad. Also, what make and series of capacitors did this kit you bought come with?

[Dew_32]

Re: Hp 1740

So are you saying I should replace the four blue cap located at c830,c832,c834,c836.

But my main question would still be is there a way to test which component is failing and replace that one instead of blindly replacing parts untill I get lucky and make the monitor work again. or then again maybe not?

[sofTest]

Re: Hp 1740

To test if a capacitor is defective reliably, you need an ESR-tester. Unless you plan to repair electronics regularly, it doesn't pay to buy one. Just replace all. Also, if some of the capacitors of dubious quality (Elite, Lelon, Jamicon) is starting to fail, the rest probably are in line for failing also. So, unless you like opening up monitor to search for failed components regularly, replace them all.

I don't find my notes on the board with the same layout you got, so I can't verify the locations, but my monitor worked after replacing all capacitors with value 220uF and up.

Again, what make and series capacitors did you get with the kit?

[sofTest]

Re: Hp 1740

Edit time out, I'm making dinner:
No not the four blue ones, the ones I mentioned are the grey film capacitors. Anyway, the other electrolytics are more likely to be bad than the film capacitors.

[Dew_32]

Re: Hp 1740

here is the link from where I got the kit
http://www.lcdrepair.us/Kit-HP1940-inverter-power.html

The one I took out were all Elite

The ones I replaced them with veried the 2 220uf are Chengx, the 2 1000uf are nichican, I can't tell what the 440uf are

[sofTest]

Re: Hp 1740

Hmm, I wouldn't put any Chengx capacitors in a monitor of mine. They are likely to fail within short time. Nichican? Not Nichicon? If, Nichican, not them either. Nichicon maybe, depending on series. If you are in the US, I would have ordered a set of new known good capacitors from the owner of this site, or from DigiKey.

[Dew_32]

Re: Hp 1740

Ok all that being said this may still be a stupid question. If I just replaced all the capacitors that I told you plus the 4 c5707 and the 2 fu9024N shouldn't my monitor be working. which it is not. so I am thinking there is still something else going on there? Granted they might be poor components, but they would last for a while uless something else on the board is knocking them out right away. By the way none of the old cap looked bad had buldging on the top nor do any of the new ones. of course i have no way of testing them.

[sofTest]

Re: Hp 1740

OK, a little capacitor failure tutorial. In addition to when you can see the bulging top/leaking bottom, the capacitor can be shorted or open internally. The most common failure though, is an elevated ESR-value. ESR is a form of resistance in the capacitors. Elevated ESR is so common, that most of us here own an ESR-meter. Even a hobbyist like me. When we replace capacitors, we use low-ESR capacitors with same or better specifications than the original, from a known good manufacturer.

Those Chengx you got with the kit, are of unknown specifications, but they are very likely not low-ESR like the originals, witch I'm guessing were Elite ES-series. You might have replaced Elites with elevated ESR-values with capacitors equally high original ESR-values. The Chengx might work as they are manufactured for, but their specification might be to low to work in this specific application.

As for something else going on, since you got image, but no backlight, I'm guessing first capacitors (the ones you haven't replaced and the ones you have replaced), second a bad CCFL-lamp. To test the lamps you need another inverter. We usually use an inverter from a PC-case lamp to test the CCFLs.

[Dew_32]

Re: Hp 1740

Yes I already tested with another good inverter from a HP 1703 monitor I use on this computer and the screen works good, backlights stay on. so I guess I should get a good set of capacitor and try repalacing them again.

[EGuevarae]

Re: Hp 1740

Yes I already tested with another good inverter from a HP 1703 monitor I use on this computer and the screen works good, backlights stay on. so I guess I should get a good set of capacitor and try repalacing them again.

Did you resolder all the transformer solder points, and other suspecting solder points?

[Dew_32]

Re: Hp 1740

Yes I did all the transformer points I read that in other post and tried that already

[Madre]

Re: Hp 1740

And finally how you resolve the problem???