Samsung 2233SN no CFL backlight

[Spoonicks]

Hello,

Have a Samsung 2233SN with no CFL backlight. Testing with a flashlight, I can see the image is displayed, but the backlight fails after some seconds.
Sometimes, after being off for a while, the backlight will remain on for a couple minutes, but most of the times it fails immediately.




I tried to follow the guide present in this forum, but so far I haven't de-soldered any components. I need some advice on how to proceed to diagnose this display.

Following the guide I tested the following:

The transformer for the CFL: Between primary and secondary windings I get OL. I measure 1.14KΩ across both secondary windings. The weird part is that I measure 0.3 Ω between one of the primary windings, and 3 MΩ in the other primary winding. I added a image, where I marked in blue the pins I used to measure the secondary windings, and the pings I used to measure the primary windings.



The MOSFET, I noticed a AP4506GEH on the bottow of the board, which I believe is used to drive the transformer for the CFL.
Across pins 1 and 2 I got: 29.7 KΩ (same, if I invert the multimeter leads)
Pins 3 and 4: 5.1 KΩ (same, if I invert the multimeter leads)
Between any of the pins, and the "tab" at the top, I get a reading in the Mega ohms range that keeps changing, increasing slightly.
It doesn't appear to be shorted.




The Diodes, I think I will have to remove the diodes to test them.
D21, D22 and D23 are reading 0,282 V. In the non conductive direction, the reading starts at 0 V and increases until OL.
D24 and D25 are reading 0,167 V. In the non conductive direction they start at 0 V and goes up to 2.67 V.




Testing the 12 PIN connection the 16V pins had close to 19V. I am not sure if that could be consider normal, or if that value is miss printed.




What should be my next step? remove the caps to test them? I can see 7 electrolytic caps in the board, plus the main big filtering cap. I believe only 2 of them are related to the CFL backlight.
Look at the reading I got, are those readings in the transformer windings normal? I would expect them to be the same, but it was measured in circuit.

[R_J]

Re: Samsung 2233SN no CFL backlight

It may be a bad ccfl lamp, the transformer only has one primary winding, pins 6.7 and 8,9 (pins 5&10 are just for mounting) this schematic should be close.

Sometimes you can try: disconnecting all ccfl lamps and try connecting one ccfl lamp (plug) at a time and note if the ccfl lights the screen for a second or two, each lamp should flash the screen about the same brightness, You can try each ccfl into the same plug example CN72 pins 1,2

With all lamps connected, you could also try and monitor the voltage on R73, R71, R76, R78 to see if there is a difference, a different voltage could indicate if there is a bad ccfl

[Spoonicks]

Re: Samsung 2233SN no CFL backlight

Is there a way to test for that? if I don't have any spare lamps?
If one of the lamps fails, then both of them go out?

Can you help me understand what pins are for the primary winding? I thought it would basically be 2 separate transformers in a single package. So, 8 pins total.

[Spoonicks]

Re: Samsung 2233SN no CFL backlight

With all lamps connected, you could also try and monitor the voltage on R73, R71, R76, R78 to see if there is a difference, a different voltage could indicate if there is a bad ccfl

Can I test this, even if the lamps don't light up?
EDIT:
Answering my own question, with the backlight failing to work, I got 0 Volts across those resistors.

[R_J]

Re: Samsung 2233SN no CFL backlight

You said the backlights lit up for some seconds

I can see the image is displayed, but the backlight fails after some seconds.

You need to monitor the voltage as you turn on the monitor to see if there is any voltage before the backlights shut down.

[Spoonicks]

Re: Samsung 2233SN no CFL backlight

Yes.
It actually stayed on for 71 seconds atvone time. I measure this:
R76 1.05 V
R78 2.8 V
R71 2.53
R73 1.5 V

I tried to go back and read the valies on R78 and R71, and they came down from the 2V range to 1V. The backlight then failed again. I am not sure what isbthe normal value I should get.
Looking at the schematic I think I measure wrong, I was using DC in my multimeter, and the current is AC.
I will try to repeat the readings

[R_J]

Re: Samsung 2233SN no CFL backlight

The voltages across those resistors are DC volts, R76 and R73 are connected together, same goes for R71 and R78, so I guess it is not really a good test point.
These are the return points of the ccfl's so the voltages will be different depending on the condition of the lamps.
Did you try just connecting one lamp at a time? that way each lamp should light for a second and shut down, but if one ccfl is bad (broken) it will not light at all.

An open ccfl could also trigger the over voltage protection to the ic,

[Spoonicks]

Re: Samsung 2233SN no CFL backlight

I thought it would be AC since there is no retification after the transformer.
Havent tested one lamp at a time yet.

[R_J]

Re: Samsung 2233SN no CFL backlight

I thought it would be AC since there is no retification after the transformer.
Havent tested one lamp at a time yet.

After the current flows through the ccfl, it is rectified by D75~D78 before returning to ground via R73/R73 etc There are 3 dc voltages that the ic (pins 4,5 &9) monitors, one feeds via R52, one at R54 and one at R55
D71~D74 monitors the A/C from the transformer, rectifies it and feeds the Dc value to voltage divider R52/R53, through R51 to pin 5 (vs)
You could also try swapping the ccfl leads around on CN72 from 1,2 to 3,4 and do the same for CN71, and see if that makes any difference. Also check the 4 small blue disc caps (C76 etc.) to make sure they are not cracked or have a black burn mark on them, I have seen them fail before.

[Spoonicks]

Re: Samsung 2233SN no CFL backlight

I tested the ccfls.
Connected one at a time, one of them didn't emit any sort of visible light, and the transformer made a high pitch noise.
My procedure was to connect one ccfl, power on the display, look for light, power off the display, repeat for all 4 lamps.




Assume CCFL is the problem, it didn't work on any of the connectors.
Probably not worth it to replace the CCFLs. It was fun to attempt to diagnose it though, not something I usually do.

The voltages across those resistors are DC volts, R76 and R73 are connected together, same goes for R71 and R78, so I guess it is not really a good test point.
These are the return points of the ccfl's so the voltages will be different depending on the condition of the lamps.
Did you try just connecting one lamp at a time? that way each lamp should light for a second and shut down, but if one ccfl is bad (broken) it will not light at all.

An open ccfl could also trigger the over voltage protection to the ic,

Need to study more about this.

Thank you far all the help, R_J

[R_J]

Re: Samsung 2233SN no CFL backlight

Looks like one bad ccfl, find a recycler or repair shop that might have the same size monitor with a damaged lcd and get a couple ccfls and replace the bad one.

With the bad ccfl, (likely cracked) the high voltage is likely arcing in the tube causing the noise in the transformer.