Samsung Syncmaster 731N - No backlight

[lti]

This monitor suddenly lost the backlight one day. When I turn it on, the backlight doesn't even flicker. The screen stays solid black, but I can see the picture with a flashlight. I have no idea what would be causing this. Everything tests good, so I replaced the OZ9938GN. That did not solve the problem, but the voltage readings on each pin of the chip are slightly different.

Here are the voltage readings I get:

1. DRV1: 0V
2. VDDA: 4.9610V when off, 4.9142V when on
3. Timer: 0V when off, 0.0086V when on
4. DIM: 0.3431V when off, 0.3421V when on
5. ISEN: 0V
6. VSEN: 0.0120V when off, 0.0115V when on
7. OVPT: 0.3486 when off, 0.3469 when on
8. ENA: 0.0278V when off, 2.6243V when on
9. LCT: 0.7345V when off, 0.7269V when on
10. SSTCMP: 0V
11. CT: Rapidly changing, between 0V and 0.0060, reaches 0.1-0.15V for a few seconds after being turned on
12. DRVA: 0V

There doesn't seem to be anything wrong, but the OZ9938GN isn't driving the MOSFETs.

[budm]

Re: Samsung Syncmaster 731N - No backlight

What voltage do you have at the ON/OFF pin connector between the main and the power supply board? DC reading on C810?

[lti]

Re: Samsung Syncmaster 731N - No backlight

I get 3.20V at the ON/OFF pin with the monitor turned on. I measured 14.98V across C810, but the multimeter I am using gives unusually high readings under some circumstances.

[lti]

Re: Samsung Syncmaster 731N - No backlight

I found something that might be a problem. R508 (marked 66C) doesn't appear to be connected to anything. It is connected to VDD, but the other end is connected to a trace that goes under the OZ9938. I can't find where this end is connected.

[jetadm123]

Re: Samsung Syncmaster 731N - No backlight

Do you know if the two mosfet chips driven by the OZ9938 are good? Check them for shorts.

[lti]

Re: Samsung Syncmaster 731N - No backlight

They aren't shorted.

Based on the board layout and the application circuit in the OZ9938 datasheet, it appears that R508 is connected to pin 13 of the OZ9938. I did notice that the voltage on this pin was close to VDD with the original chip. I tried connecting that resistor to pin 13. It appears that there is a very brief flash when I turn the monitor on. I don't know if this is the backlight trying to come on or just something that occurs when the T-Con and panel receive power and a video signal.

The voltage on pin 7 (OVPT) seems low. It might be shutting down because of this, but I doubt it. Every component except the OZ9938 and the ceramic caps have been tested. I only checked the resistance across the caps to make sure they weren't shorted, and I don't know their original values anyway.

[jetadm123]

Re: Samsung Syncmaster 731N - No backlight

I take it you've tested the secondary winding resistance of each inverter transformer?

Also, have you verified each ccfl bulb is good? You can build a ccfl tester using budm's design:

https://www.badcaps.net/forum/showth...ht=ccfl+tester

[lti]

Re: Samsung Syncmaster 731N - No backlight

There is a difference in resistance on each transformer, but I don't think it is a problem. I don't think all four CCFLs would fail at the same time, but I can't test them right now. The MOSFETs aren't being driven by the OZ9938. It looks like it isn't even trying to light the CCFLs.

[jetadm123]

Re: Samsung Syncmaster 731N - No backlight

It's an all or nothing type of setup, therefore, only one ccfl needs to fail in order to shut down the backlight circuit. That's why you need to verify if the ccfl's are good.

What were the resistances of the secondary windings?

The DRV pin output of the 0Z9938 should be a pulse, but the frequency may be too high for your meter to read. It's these pins that drive the mosfets.

[lti]

Re: Samsung Syncmaster 731N - No backlight

If a CCFL was bad, wouldn't I have the "2 seconds to black" issue instead of no backlight at all?

I don't remember the exact resistance readings. I will have to check again.

Samsung's circuit design is a mess. The circuit around D505, D506, and D510 is really strange. All three diodes are connected to R522, which is connected to VDD for the OZ9938. D505 goes to the OVPT pin of the OZ9938, and D506 and D510 each connect to the secondary of one transformer through the through hole resistors R525 and R506.

[lti]

Re: Samsung Syncmaster 731N - No backlight

T501:0.6156K ohms
T502:0.6205K ohms

I am getting 1.0mVAC and 0.3mVDC at the gates of the MOSFETs when the monitor is turned on and nothing with the power off.

The voltage at the SSTCMP pin hits 0.442V for a couple seconds when I turn the monitor on.

[lti]

Re: Samsung Syncmaster 731N - No backlight

I thought I had found something wrong because I was getting constant DC at pin 4 of the OZ9938 when it was configured to receive a PWM dimming signal. It ended up being a PWM signal, but the duty cycle was 100% because I had turned the brightness all the way up.

The chip is receiving all the signals it needs to start driving the MOSFETs, but it isn't doing anything. I can see a 112KHz AC signal on pin 13 (CT) and the voltage on pin 12 (SSTCMP) does increase slightly when pin 10 gets pulled high, but both pins show no activity a second later. It is trying to start, but it isn't driving the MOSFETs () and shuts down.

[budm]

Re: Samsung Syncmaster 731N - No backlight

I download the spec of the OZ9938 which is 16-pin device, your pinouts on post 1 don not match what I am looking at. Do you have the spec of the IC that you have? I do not see DRV2 on your post either.

[lti]

Re: Samsung Syncmaster 731N - No backlight

I didn't list the two ground pins or the two pins that aren't connected.

I wrote DRVA instead of DRV2 for some reason.

[lti]

Re: Samsung Syncmaster 731N - No backlight

I also noticed that the voltage at pin 4 of the OZ9938 is only 0.8VDC at full brightness. It seems like this should be closer to the maximum 1.5V allowed on this pin since it is an external PWM control signal.

I also tried increasing the voltage on pin 7 by putting a 1M ohm resistor in location R509. This increased the voltage to 2.3V, but it didn't allow the backlight to turn on.

[budm]

Re: Samsung Syncmaster 731N - No backlight

The ENA PIN you post as pin 8, pin 8 is NC. I think you need to give us the correct reading on all the pin again. pin 10 is ENA.

[lti]

Re: Samsung Syncmaster 731N - No backlight

I just used a numbered list.

1. DRV1: 0V
2. VDDA: 4.9610V when off, 4.9142V when on
3. Timer: 0V when off, 0.0086V when on
4. DIM: 0.3431V when off, 0.3421V when on, 0.8V at maximum brightness
5. ISEN: 0V
6. VSEN: 0.0120V when off, 0.0115V when on
7. OVPT: 0.3486 when off, 0.3469 when on
8. Not connected
9. Not connected
10. ENA: 0.0278V when off, 2.6243V when on
11. LCT: 0.7345V when off, 0.7269V when on
12. SSTCMP: 0V when off, 0.422V for a second when turned on, then back to 0V
13. CT: 4.90V, 112KHz AC signal for a second when turned on, then back to 4.90VDC
14. GNDA
15. DRV2: 0V
16. PGND

All measurements are DC volts except the frequency on pin 13.

[lti]

Re: Samsung Syncmaster 731N - No backlight

Correction: The voltage at pin 4 is 0.9475V. That seems high enough.

Did I piss everyone off, or is nobody posting because they can't figure out what is wrong?

[lti]

Re: Samsung Syncmaster 731N - No backlight

If all else fails, has anyone used this? I have seen people on this forum using the larger blue inverter, but I don't think it will fit if I use the original power supply and just cut the inverter off the board.

I would also have to figure out how to mount the universal inverter.

[budm]

Re: Samsung Syncmaster 731N - No backlight

If you monitor DRV1, DRV2, do you see any sign of drive signal at the output at all when you first turn on the monitor?