Samsung LN40C630K1F LCD TV No Backlights and Flashlight Test Fail

[signal.quest]

Hi all,

I have been working on a Samsung LN40C630K1F LCD TV. On power up the backlights only briefly light up when first powered on then they go out (2 seconds to black). The flashlight test also fails. The sound works fine. Please see attached pics.

I have researched the forums a lot. I have watched the videos of trying to locate the bad CCFL or transformer and bypassing. I can't find any evidence of a bad transformer/CCFL. I verified I get 376V from the big caps to the fuse. I have used a small/cheap oscilloscope and it looks like the MOSFETs on the inverter are cycling when the TV first turns on before the supply is shutdown from protection. I have measured the test points on the TCON board that I can easily see marked and they seem to be measuring as expected.

I have not opened the panel to check the CCFL bulbs, yet. I would like to verify the LCD works before I attempt to repair the backlight. I was hoping to replace the CCFL backlight with LED. I believe this would be possible if I can disable the inverter circuit and tap power from the power board (BN44-00340B). Has any does this successfully with a similar model?

My next step was going to be to buy a replacement TCON board to see if that allows the flashlight test to work. Does anyone have better idea of what else to check? I appreciate your help! Thank you.

[CapLeaker]

How did you rule out the transformers? You got transformers to check on the PSU and on the backlight driver board. It could be a bad HV bypass cap, cold solder joint, secondary HV windings on the transformers out of spec, etc.

You can use an old twistie CCFL bulb and wire that to the CCFL driver broad to check for a bad lamp.
What is you BLon signal doing?

[EazyBone]

Put everything back to normal. Unplug main board connector AT PSU. Plug in the power board, lights should come on and stay on. Report back

Edit this works on newer Samsungs I am unfamiliar with older ones. Doesn't hurt to try

[signal.quest]

Thank you, both for posting and trying to help me. I greatly appreciate it. 😀

All of the electrolytic caps pass testing with ESR meter. I also tested all small caps including SMD caps for shorts.

Is there a way to test the HV iverter transformers on the PSU? I don't have a ring tester. I don't see any visible damage or dry solder joints with a jewelers loop. I don't smell anything burnt.

I have tested each transformer output on the balance board with a multimeter and the resistance is basically identical on all. I also checked for damage and dry solder joints with a jewelers loop. I also watched the youtube videos and attempted to read AC voltage spikes at the test points when powering on, but I could not detect and difference on any of the transformers.

I have tested using and old twistie CCFL bulb wired to the balance board on the output of each trasformer. The same behavior happens of 2 seconds to black. I assume I have more than one bad CFL bulb or a problem on the balance board, or a the inverter side of the power supply.

I have not checked the BLon signal - how would I test that? What would it indicate?

Unplugging the mainboard from the power supply and powering on does not change the behavior of the backlights. It is still 2 seconds to black.
From all that I have read this rules out the mainboard as a cause of the backlight issue.

I am most concerned in first seeing the image on the LCD with the flashlight test. I think I can do the LED conversion if I can at least get the image.

[EazyBone]

Generally with newer tvs you can use a backlight tester on a connector piece and see if they bulbs light up. Maybe try this? I am unfamiliar with older TVs but I'd assume this would work. And well it should show a short if a bulb is bad. They cheap on Amazon 15-20$

[signal.quest]

I do have a backlight CCFL tester. All the bulbs light up dimly when I test them individually. I am not sure if this test rules out the bulbs completely. I think if the bulbs are starting to go bad they could pull too much current and cause the inverter circuit to shutdown in protection.

[EazyBone]

Ahh unfamiliar with that. Generally my led tester will test a line of LEDs and say 93v and test the other it may say 90v so 3v discrepancy will show bad led likely. Does it give you anything like that? Probably not much help .. in sure someone from Europe be waking up soon and will answer.

With a tv this old why even invest time into it? You can get a 40 inch 4ktv off marketplace for $40-80 or a 1080p one sometimes free. Clearly there is a reason but curious as to why.

[signal.quest]

The CCFL tester only lights the CFL or does not light if it is bad. I am a hobbyist and enjoy repairing/modifying thigs - I enjoy the torture of it. But only if I am successful in the end. Haha.

[CapLeaker]

Test the back light transformers secondaries (HV winding) with your DMM in restistance mode. Post the result. You got 2 transformers on the PSU and the back light board.

[signal.quest]

Here are the measurements on Ohms on the balance board from top of the TV to bottom of the TV:

T1: 152.3
T2: 152.9
T3: 154.4
T4: 152.1
T5: 152.7
T6: 150.9
T7: 151.0
T8: 152.5
T9: 154.0
T10: 153.4
T11: 153.5
T12: 155.3

HV Transformers:

T1: 12.6
T2: 12.9

[CapLeaker]

You measured the HV secondary side where the lamps are, right?
How this works is that these transformers need to be within 2% of each other on their secondary winding. If one or more is / are out, thais board will shut the BL off as a safety measure. Yours are over 2% out. You maybe could override this on the driver chip, but I don’t recommend doing this unless you know exactly what you’re doing and the consequences it may have.

[signal.quest]

I did measure on the lamp side, yes. Any idea what would cause them to not be within 2%? Can I fix this somehow? I would override the protection if it is easy to do, but I don't know how - do you?

I still am more concerned about being able to see the image on the screen with the flashlight. I have a TCON board on the way from aliexpress, but not due to arrive for 2 more weeks. I then would do an LED conversion and remove/bypass the CCFL components. Thanks.

[CapLeaker]

The only proper way to fix it is either a new balancer board, or transformers. It’s possible to override the protection, but that is not something I suggest doing, unless you’re a professional. I don’t want to see your house burning down, because the protection is disabled. It is just something for testing purposes not a permanent “test”.

[signal.quest]

Thanks. Do you have any advice on the image flashlight issue besides replacing the TCON?

[CapLeaker]

Check the LVDS with an Oscilloscope (100Mhz or so) and flick the OSD on and off. You should see a change in the LVDS differential pairs. If you do, the main board is fine. If you don’t, you need a new main board.

[signal.quest]

Thanks. What pins on the LVDS should I check? Do I check with respect to ground? I don't have a remote - can I power cycle or hit the buttons on the TV? I hope I don't need a new main board because they seem hard to find now. That might end this project.

[signal.quest]

I received the new TCON board and have installed it. I still don't see an image on the screen using the flashlight.