Dell ST2410b Backlight Shuts Off

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Its TOOOOOOOO long a story. I will try and disect it

What make and series caps have you used. If you have read any threads we tend only to recommend Panasonic FR FM & FC or Nichicon ot rubycon equivalents.
I dont understand what you say about 16v/35v caps - you can replace 16v with 35v
but not 35v with 16v. (they are likely to go bang)

I was expecting to get a comment about the length. I swear it whole truth

Ok, on one pair of caps the original voltage rating was 35V and I replaced it with 16V because I bought them incorrectly. They didn't go "BaNg!" so I wasn't too sure, upon measuring it though, voltage across the those caps are about 20V, probably not enough for pyro display

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Check if your new cap with 16v changed for the 35v is conductive..

Yeah one of them is reading 0 Ohms, I think that's had it.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Can I see good clear pictures of YOUR board, both sides?
"However, I thought I'll check the T601 and to my surprise, secondary winding is 0 Ohms on any combination of the terminals. On the primary I had existent but low resistance (<10 Ohms)." DCR of the power transformer will be really Low, since you are getting the 5V, the transformer is working. What DC do you get on those two caps that are supposed to be 35VDC?

Sure I'll attach it here but it's the identical board tot that PohTayToez posted in the beginning of the thread.

I measured the caps as you suggested and voltage across them is about 20V so 16V is definitely not suitable to be placed in there.

- Component Side
- Track Side

Also the LD1117 regulator is running at about 32 deg Celsius, for dropping 5V to 3V, does it sound expected? Guess I should probably check the datasheet for heat dissipation characteristics right...

[budm]

Re: Dell ST2410b Backlight Shuts Off

"Also the LD1117 regulator is running at about 32 deg Celsius, for dropping 5V to 3V, does it sound expected? " Is this regulator on the power supply board or on the main logic board? I also trying to locate that burnt resistor you have found. We need to find out also if the logic board is still sending the BL_ON to the power supply board.

[selldoor]

Re: Dell ST2410b Backlight Shuts Off

You seem to have replaced bad caps with possibly worse caps. not a bit of wonder the regulator is running hot.

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

the capacitor seems shorted, so test in diode mode the big diode near the 35v caps.
test also the voltage at the cathode at that diode... if you manage to fix this last problem you will have to check the lamps for darkened ends.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

"Also the LD1117 regulator is running at about 32 deg Celsius, for dropping 5V to 3V, does it sound expected? " Is this regulator on the power supply board or on the main logic board? I also trying to locate that burnt resistor you have found. We need to find out also if the logic board is still sending the BL_ON to the power supply board.

I've pointed out the regulator and the resistor you were referring to.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

You seem to have replaced bad caps with possibly worse caps. not a bit of wonder the regulator is running hot.

actually I've re-installed the original caps now, but still the ccfl don't light up at all. I just received the new 50V replacement caps for the 35V ones so, I'll soon go ahead and replace all of them see how we go...

[budm]

Re: Dell ST2410b Backlight Shuts Off

OK, if you get the voltages back after installing the correct caps, then you need to find out if the main board is sending the BL_ON signal to turn on the inverter circuit or not.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Excellent will do. What's the best way to isolate the BL_ON signal wire coming from the main board?

[uengin]

Re: Dell ST2410b Backlight Shuts Off

the capacitor seems shorted, so test in diode mode the big diode near the 35v caps.
test also the voltage at the cathode at that diode... if you manage to fix this last problem you will have to check the lamps for darkened ends.

By checking the voltages at the cathode we're ensuring that the diodes are functioning as expected and turning on? actually that would be misleading if they were reversed biased. Sorry I'm just trying to learn what these diodes are and what is accomplished by checking their voltages at the cathode, thanks.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Excellent will do. What's the best way to isolate the BL_ON signal wire coming from the main board?

Would I be correct in taking an educated guess and say it's the orange wire?

[selldoor]

Re: Dell ST2410b Backlight Shuts Off

Not sure why you have got 50v caps? what make are they?
Anyway if you now think you have all working caps please advise the voltage on each pin on the power supply to main board connector - use a ground screw for black/negative probe. If you have a problem getting the probes into the socket
insert a needle to the pin and test to that.

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

uengin you said that you get no more lighting up of the lamps and you found a capacitor that conducts, putting togheter the things i, with a short experience, think that the inverter can be no more powered, sorry i don't have watched the OP pictures, in yours lo-res i see that the output of the ldo voltage reg. goes to the burnt resis. and after goes to the inverter part, maybe to supply the inv. IC, i remember that you've changed that resistor, but maybe there's a faulty inv. IC.

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

watching the 1st poster photo the broken resistor comes from the 5v output not the reg. output and goes directly to pin 11 of inv. IC. So have you tested your 10ohm resistor before putting in? Now you can measure voltage at the 2 legs of it mounted..

[uengin]

Re: Dell ST2410b Backlight Shuts Off

OK chaps, Couldn't get this stuff done on the weekend, but now here's what's been done and corresponding test result where applicable:
- Replaced all the capacitors with the new ones.
- Connected up 2 CFL as per budm's thread: CLF's don't light up at all
- Tested for the BL_ON line:

> The orange wire (suspected BL_ON line) is an unchanging PWM as shown below. I am pretty sure this is the BL_ON as this orange and the white lines are the only ones that lead to the inverter IC. It's measuring at 3V with what seems to be an 800us off time and 5.5ms on time duty cycle.



> The white wire is steady at about 3.2V and on connection of the video cable momentarily goes to 0V as the screens are blanked and restores to 3.2V when images are restored. Actually, now seeing the operation I reckon the white wire is most likely the BL_ON line. So that would probably mean that BL_ON is active high.


> The blacks are ignored as they're ground wires. Next up are the red wires which measured at just under 5V. These are unchanging states, if the power is on, these are on, if the power is off these are off.


> Now lastly the yellow wires; they measured at just under 3V. These are also unchanging states, if the power is on, these are on, if the power is off these are off.


Current symptoms are still, completely pitch black with image visible when viewed with a torch.

Thanks again, your help and ideas are appreciated.

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

I repeat, can you measure voltage at the 2 legs of the 10ohm resistor that you have changed?
Have you ever tested the voltage at the shottky big diode near the transformer?

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Davi.p, I didn't quite understand your previous post; because I want to learn as well, I guess I was expecting an explanation on why we're doing it also. So, I would really appreciate to learn what and why we're looking for as well.

Ok, firstly my bad, the resistor I had in there was 100ohms not 10ohms, I think the resistor I used was E96 series which there was a confusion on the meaning of the colour bands.

Replaced it with a 10ohm E48 series this time with 5% tolerance.took about 20-30 seconds for it to overheat and darken in appearance again, but I tested it's resistance again while off, still at 10ohms so I continued the tests you asked for but I gather there's a short circuit causing this my fear is the inverter IC has had it.

Anyhow voltage across the resistor is about 4.7V with 5V on the regulator side and about 255mV on the other side. Voltage on the cathode of the D701 is 5V and about 42mV on the anode; all obviously with respect to ground.

So what does this mean? Inverter IC is shorted to ground and pulling too much current through the 10ohm resistor?

Thanks guys, love your help and learning more about different circuit systems.

[budm]

Re: Dell ST2410b Backlight Shuts Off

Where is this 10 ohms resistor? On the power supply/inverter board? This one in the picture?

[uengin]

Re: Dell ST2410b Backlight Shuts Off

budm, the resistor is the one I specified in a previous post, quoted below.

I've pointed out the regulator and the resistor you were referring to.