Dell ST2410b Backlight Shuts Off

[budm]

Re: Dell ST2410b Backlight Shuts Off

OK, I see now, so the output side of the resistor is basically feeding shorted circuit, that is why it has 4.7V across it, Next we need to locate the component that is loading down the circuit, it can be shorted CERAMIC cap, IC, etc. Need beterr clear and close up to see what is connected to that end of the resistor.

[budm]

Re: Dell ST2410b Backlight Shuts Off

OK, lift one end of the jumper JW13 off the board and see what voltage you get on the filter cap C803 as circled in the picture, it should be 5VDC. Pin 11 of the PWM IC looks like the VCC pin, and that RED Glass diode may be Zener which can also be shorted out.

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

You must desolder the zener ZD803 and see if it's shorted after that.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Ok Guys, I think you might be spot on the ball with the Zener.

• Removed JW13 and measured the capacitor voltage: 5V more or less.

• Just to rule it out or rule it in, I removed the zener and tested the voltage across the 10ohm resistor; it had (I think) 0.8V drop across it

Do I have to replace the zener? Do you think it failed as a result of another failure in the circuit? How do I know the specs to replace the zener with? It was reverse biased (it seems) so the reverse bias break down voltage is important, I gather.

What do you think?

PS: Sorry about the photo, my camera is broken had to borrow a camera to take the below, hope that's high res. enough. I thought I'd clean the board with isopropyl but it ended up with hazes and streaks, tried to clean it to the best I could...

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

This is the result with isopropyl?! It seems all corroded..
For the zener you can download the inverter ic IC801 datasheet and see the Absolute maximum ratings, at the voice Vcc that is approx. the rate of the zener.
"Do I have to replace the zener?" yes. "Do you think it failed as a result of another failure in the circuit? " I don't know but it's probably an overvoltage that shorted the zener, maybe for one of the big caps.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

The datasheet is hard to come by. I've found out its O2micro. It's an OZ9933GN. Any ideas where to find the datasheet?

[selldoor]

Re: Dell ST2410b Backlight Shuts Off

This may help
https://www.badcaps.net/forum/showpo...8&postcount=36

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

Put it of 7V and go for sure.. its for protection you can trash it away and test the monitor all hooked up and see if works.. then replace the zener

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Put it of 7V and go for sure.. its for protection you can trash it away and test the monitor all hooked up and see if works.. then replace the zener

Sorry, what did you mean by "Put it of 7V and go for sure.."?

I have removed that zener diode and hooked it all up. There's a full image on the LCD. BL_ON about 2.5V but still no light. So I disconnected the the BL_ON from the Video board and routed 3.2V from the Power line in to the BL_ON for the slim chance that it may require TTL logic levels. Still no light.

Should I check the waveform being fed in to the output transformers to make sure there is activity on the output stage? Do you know what the voltage levels are in to the primary of the transformers that drive the CCFLs? My ameture scope can only handle about 80V input, so I don't want to fry that.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

This may help
https://www.badcaps.net/forum/showpo...8&postcount=36

That'll be very helpful, thanks. At least there's some labels to those pins.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Ahaa! The Asus VE205 has the identical inverter and power board as the Dell. That'll be handy to know when chasing down parts, ey? Thanks again for the selldoor.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Ok, I was impatient and tested the voltage across the primary of the CCFL transformers with a multimeter, then hooked up my scope. 0V and no activity, respectively.

Tested pin 11 got 5V.

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

"Sorry, what did you mean by "Put it of 7V and go for sure.."?"
I mean put it at 7v (the zener) and you are sure.
You have the pinout of the inverter ic so check the waveform of the 4 outputs, pin 6,8,9,10. If you find that waveform then check those sop8 chip dual mosfets Q801 and Q802.
As you can see from the diagram selldoor posted each dual mosfets have pin 3 at the supply voltage (Vin) for the inverter circuit , that is the same as the voltage at the cathode of D705.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Could you correct me if I am wrong; The inverter IC is supposed to take care of oscillating the gates on the mosfets to switch them on and off causing change in currents in the primary coil of the CCFL transformer thereby creating high voltages on the secondary of the same transformer, lighting up the CCFL's?

So I tested those pins, and they are all in a "high" state. About 3 volts to be exact. All pins, 6, 8, 9, and 10.

I wanted to make sure that the input pin PWM was indeed receiving a PWM, and it is. It seems to get about 90% high and 10% low, though I could probably take a measurement. I just wasn't sure if what i got was expected...?

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

Could you correct me if I am wrong; The inverter IC is supposed to take care of oscillating the gates ....

You are correct

So I tested those pins, and they are all in a "high" state. About 3 volts to be exact. All pins, 6, 8, 9, and 10.

if those pins are not oscillating you can control that Ena (BL_on) path is not interrupted, if no i think you have to consider a new inverter ic. Bye.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Ok, I'll check if the ENA pin on the chip is high, if so, inverter IC needs replacement.

I'd like to thank everyone that contributed and gave their suggestions. I think I am going to love this forum and hope I can contribute to the members here and give assistance to others as well as I get more and more experienced.

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

I'm not experienced, wait good ending for the greetings :-)

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Davi.p, you, budm and others who have contributed have been most helpful. Your suggestions, experience or otherwise have been an experience for me therefore I really appreciate it. Thank you all.

As for the final result, I have ordered several OZ9933GN ICs (just incase the first one I put in goes bad because of another fault in the cirsuit that we have not identified yet). I am waiting for the ICs to arrive and will let you know when the replacement is done and what the results are.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Hey people,

Chips came. I got impatient with the SMD work, ran it a bit too hot... Board browned out and I heard crackling sounds while desoldering. When I replaced it with the new chip and powered it on: power LED started to blink. GRRRR.

I've ran out of patience but does anyone still think it's work trying to save this board?

By the way I have 4 extra OZ9933 chips. I'll sell it out for about $5 each if anybody is interested?

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

Hi, i can't evaluate if there's a new damage due to bad soldering job, maybe a spotted picture of it can help. How about the chip orientation?