Dell SE198WFPf - Backlights go off after 2 seconds

[cynz]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

I got the same model(SE198WFPf) Dell monitor and it has the same problem. The date of mfg is May 2008.
But it looks weird.
a) When set the brightness and the contrast to 50%, the backlight turns off after 1 or 2 seconds, but the power LED stays on, and u can see the shadow of the image.
b) When set the brightness and the contrast to 100%, it can stay longer (about 30 minutes), then the backlight turns off.
And have checked the power board and logic board, no sign of burn or bad-solder. Also checked all the caps and readings r ok. Then checked 4 CCFLs individually, all works fine, but not that even to spread onto the screen, the end side(opposite to the tube connector) is weak.

Is that because the CCFL is getting old(but not dead yet), then the protection circuit shuts it off? Or the higher voltage power can activate the backlight much longer?

Have no clue, any help would really appreciate.

[smason]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

The stinginess of the Scots, while legendary,

We can thank the Scots for copper wire. They "invented" it.

2 Scotsmen fighting over a penny..

I can say that, I'm part Scottish, and have some Scotch in my blood as I type this...

[PlainBill]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

found the bad solder joint on one of the bulbs, re soldered but in the process i messed up the rubber that was covering it, so now it has carefully placed electrical tape around it lol, but to my surprise no arcing, no hissing or other sounds, and no lights going out. Silent as can be. I figured i'd get a spark show but for now all is good. Now if i can get it back together without messing it up in the process

I appriciate the help very much from you all. If i post again i'll be sure bring a little more to the table next time so i don't pull the annoying noob post again, sorry about that.

I would suggest you NOT reassemble it that way. Instead, either get a new end cap or use romansroad's suggestion and use clear silicone sealant. The adhesive on electrical tape tends to soften as it ages.

PlainBill

[WoR496]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

found the bad solder joint on one of the bulbs, re soldered but in the process i messed up the rubber that was covering it, so now it has carefully placed electrical tape around it lol, but to my surprise no arcing, no hissing or other sounds, and no lights going out. Silent as can be. I figured i'd get a spark show but for now all is good. Now if i can get it back together without messing it up in the process

I appriciate the help very much from you all. If i post again i'll be sure bring a little more to the table next time so i don't pull the annoying noob post again, sorry about that.

[romansroad]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

The stinginess of the Scots, while legendary, is a poor second to that of a farmer of German descent who grew up in the Depression. I see absolutely nothing wrong about reusing or modifying parts to achieve something not readily available. In one of the electronics magazines of the 60's or 70's (Radio Electronics, Popular Electronics?), I saw an article on using a triangular file to notch carbon composite resistors to adjust their resistance.
Compared to that, reusing a heating element is normal.

PlainBill

I bow (in my kilt) to your superior stinginess!

[romansroad]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

Ah Hah!

Bad ccfl wiring.....I'm so sorry you had to open the screen, but you're getting off cheaply with only a rewire and not buying a new tube (although they're not that much).

Be careful on the re-assembly to not kink the wire. I use clear silicone on the ends after appling heatshrink, and all has been well.

Nicely done! You're officially not a noob, and remember that a a bunch of voltage tests can't substitute for a close visual inspection (or in this case a nice high voltage shock!)

[alexanna]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

You have found the problem, Ccfls do not leak and continue to work, you probably have a wire you have to re solder .
With power off gently tug on the wires at the ends of the ccfl tubes if it has a poor solder joint you will find it
Al.

[WoR496]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

It would help to know what tests you have done so far so people aren't suggesting things that you have already tried.

sorry i've got myself tied in a knot and so much going on i forgot what all i did, but luckily i stumbled across something now that i have the ccfl's out of the display, one has a HV leak, touched the metal case and got a nice suprise. stuck the pos lead of my DMM on it and it started beeping and going nuts. I suppose this could cause the problems i'm having? I oppologize in advance for being a pain in the rear noob with this stuff, i really don't have a whole lot of experience with component level electronics troubleshooting. Wish i had a spare ccfl around. Yes i know, use a proper resistor but at this point im holding off on that since one i'm not 100% sure what i need, and two i don't want to wait a week to get one lol.

So i guess i'll try and hunt down a good ccfl or maybe this one can be helped? lol

oh and for some reason now i can get them to stay on for longer than i care to wait to see when they will shut off (over an hour), but has hissing sound from the leaking ccfl, also i noticed one white wire running across the top of the metal on the ccfl were it enters the other end of the bulb is burnt.

[jetadm123]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

more info.. voltage from transistors at 12 oclock and 1:30 locations looking from top of board. Don't know if this helps but it seemed odd that the one at 12oclock would do what it is doing when the ccfl's go out.

12oclock transistor - when ccfl's are working normally
1- ~3.2V
2- ~16v
3- ~3.2V

12oclock transistor -when ccfl's go out and stay out
1- Alternates between ~1.85V and ~2.35V
2- Stays at ~19V
3- Alternates between ~1.85V and ~2.35V

1:30 transistor - stays same weather ccfl's are on or off
1- 0v
2- 5v
3- 0v

Without knowing how these transistors are connected, it's very difficult to determine if they're functioning properly. The reading for the 1:30 transistor (if it is a transistor) looks off, but we don't know if that's being caused by another component. The "transistor" could very well be the duo diode pack that outputs the 5 volts. Look carefully at the silk screening near the component. DXXX is a diode, QXXX is a transistor. A diode is NOT the same as a transistor. If necessary, please provide a photo with the component(s) that you tested circled. Pictures say much more than words. Even providing a part number will help determine what the component is. Believe me, if you make people second guess as to what you're doing, then you'll receive little help.

[retiredcaps]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

I'm working threw the tests described in retiredcaps tutorial and so far nothing bad found.

It would help to know what tests you have done so far so people aren't suggesting things that you have already tried.

[PlainBill]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

I agree. A sloppy tolerance on the protection circuit will allow overheating, while the tighter ones just shut the secondary down before the xformer has a chance to cook the board. As you know, the resistances can be so slight that less expensive DMM's have a hard time differentiating them.

BTW, for your resistor trick, I've used the tungsten filament from a discarded hair dryer to custom make various resistances. They handle the wattage of this application easily, and can be cut to length to dial in the resitance. I'm Scottish....and cheap!

The stinginess of the Scots, while legendary, is a poor second to that of a farmer of German descent who grew up in the Depression. I see absolutely nothing wrong about reusing or modifying parts to achieve something not readily available. In one of the electronics magazines of the 60's or 70's (Radio Electronics, Popular Electronics?), I saw an article on using a triangular file to notch carbon composite resistors to adjust their resistance.
Compared to that, reusing a heating element is normal.

PlainBill

[romansroad]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

It depends. I've seen some that showed up clearly with a resistance check AND ringing, but the transformer looked perfect. And there was a picture posted recently with obvious physical damage to a transformer. I'd say it depends on how close the tolerances of the protection circuit is.

PlainBill

I agree. A sloppy tolerance on the protection circuit will allow overheating, while the tighter ones just shut the secondary down before the xformer has a chance to cook the board. As you know, the resistances can be so slight that less expensive DMM's have a hard time differentiating them.

BTW, for your resistor trick, I've used the tungsten filament from a discarded hair dryer to custom make various resistances. They handle the wattage of this application easily, and can be cut to length to dial in the resitance. I'm Scottish....and cheap!

[romansroad]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

I see several options. If you have a working ccfl, wire it in sequentially to each high voltage output until all backlights stay on. This assumes that you only have ONE backlight out.

PlainBill had a post to substitute a resistor for a backlight, and that would do the same thing, only it would be able to compensate for 3 of the 4 backlights. You could then wire in three resistors and see if the remaining backlight stayed on. If so, then remove the next resistor, and then the next until your original problem returned. That would identify that you have a backlight problem, and also show you which one was defective.

I think I read you resoldered the leads to the backlights....did you mean at the electode from the backlight itself? If so, sounds like you already have the screen opened. Try a known good ccfl or the resistor trick.

[WoR496]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

more info.. voltage from transistors at 12 oclock and 1:30 locations looking from top of board. Don't know if this helps but it seemed odd that the one at 12oclock would do what it is doing when the ccfl's go out.

12oclock transistor - when ccfl's are working normally
1- ~3.2V
2- ~16v
3- ~3.2V

12oclock transistor -when ccfl's go out and stay out
1- Alternates between ~1.85V and ~2.35V
2- Stays at ~19V
3- Alternates between ~1.85V and ~2.35V

1:30 transistor - stays same weather ccfl's are on or off
1- 0v
2- 5v
3- 0v

[WoR496]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

well i resoldered the inverters and most joints in between it and the ccfl connectors, the first time i powered it up after that it did nothing different, went off after a couple seconds, but the second time i powered it up it stays on for about 1 min. then starts to kind of flicker, then shuts off. And this is repeateable.. but it seems that if i have the power board or wires just right it will only come on for the 1-2 sec again.

[WoR496]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

I'm working threw the tests described in retiredcaps tutorial and so far nothing bad found. I did happen to locate a *hopefully* equivalent datasheet for this inverter controller chip. The one on the board is a OZ9937GN and this sheet is for an OZ9938GN so im not 100% sure of the difference but the package is identical. Datasheet attached, start on page 10, rest is gibberish.

Never did troubleshooting on an IC before but i'm going to give it a shot lol, datasheet seems to give a decent picture of what exactly it does and what pins do what. (but i guess thats the point of a datasheet right?) lol

[PlainBill]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

As to checking the ccfl's with a DMM...it wont work. The secondary check on the xformer is a good idea but in my experience, a toasted xformer will overheat the board a bit and yellow around it on the PCB. Anything that keeps you from having to open the screen is worth it!

It depends. I've seen some that showed up clearly with a resistance check AND ringing, but the transformer looked perfect. And there was a picture posted recently with obvious physical damage to a transformer. I'd say it depends on how close the tolerances of the protection circuit is.

PlainBill

[alexanna]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

As to checking the ccfl's with a DMM...it wont work. The secondary check on the xformer is a good idea but in my experience, a toasted xformer will overheat the board a bit and yellow around it on the PCB. Anything that keeps you from having to open the screen is worth it!

Would you reword or explain in a different way the post, I am totally confused of what you're trying to say.
Al.

[romansroad]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

As to checking the ccfl's with a DMM...it wont work. The secondary check on the xformer is a good idea but in my experience, a toasted xformer will overheat the board a bit and yellow around it on the PCB. Anything that keeps you from having to open the screen is worth it!

[WoR496]

Re: Dell SE198WFPf - Backlights go off after 2 seconds

Obviously, my link in post #2 is written using invisible ink!

If i had known that what the link pertained to i might have paid more attention to its contents i thought it was supposed to be rules for the fourms but i didn't see any ( just skimmed over) so i didn't know that it was a link to your guide.. sorry..