Samsung LN40D550K1FXZA Backlight Problem

[classicmac]

I'm having trouble with a Samsung LN40D550K1FXZA where the backlight does not function properly. When I turn the television on the backlight flickers on momentarily and I can see the Samsung logo for half a second. The backlight then turns off.

The main board does seem to fully boot. I can hear sound when the television is on. I can also see picture doing the flashlight test.

I tried disconnected the main board to test the backlight as suggested in the troubleshooting manual attached to post https://www.badcaps.net/forum/showthread.php?p=792496. I get the same behavior when I connect power where the backlight just flickers on then turns off again. I'm attaching a picture of the power supply board. I don't see any visibly failed components or bulging/leaking capacitors.

Does anyone have any suggestions on where I should go from here? I would appreciate any help or advice.

Thanks!

Michael

[dick_barton]

Re: Samsung LN40D550K1FXZA Backlight Problem

So the backlights do not come on as per the fast-track guide, so possible inverter fault or / lamp fault.
Can you measure the voltages at CI801

Can you post pictures of all the circuit boards at the back of the set and how they connect to each other?

[neilc6]

Re: Samsung LN40D550K1FXZA Backlight Problem

Check this thread

https://www.badcaps.net/forum/showthread.php?t=58093

[nomoresonys]

Re: Samsung LN40D550K1FXZA Backlight Problem

If your power supply voltages are all good, probably a bad ccf/lamp or bad transformer on inverter board or something else wrong with inverter board, you could grab an inverter board and try it, usually pretty reasonable prices on them BUT won't fix issue if it's a bad ccfl/lamp, there's some good videos on testing inverter boards on youtube, also check any fuses that might be on inverter board, fuses will start with F on label next to them.

[budm]

Re: Samsung LN40D550K1FXZA Backlight Problem

Beside checking the PFC Voltage, check capacitance of CI818 also.

[classicmac]

Re: Samsung LN40D550K1FXZA Backlight Problem

Check this thread

https://www.badcaps.net/forum/showthread.php?t=58093

Thanks for the link to the other thread neilc6. Are the capacitors I highlighted in the attached photo the "main filter caps?" I'm getting 163V across them. The other thread seems to say that I should be seeing 380-400V.

Neilc6 I see you had a similar problem and replacing ICP801 fixed it. Is there anything I can do to verify ICP801 has failed before I order a replacement? Thank you everyone for your responses and suggestions.

[neilc6]

Re: Samsung LN40D550K1FXZA Backlight Problem

Yes those are the main filter caps. Voltage is measured with black on the negative lead and not chassis ground. In my case, the PFC voltage spiked up when first plugged in going into an overvoltage situation. If your backlights do come on momentarily, it would indicate the PFC voltage comes on. You will need to monitor it when the TV is both turned on and plugged in. Be careful, it's high voltage and alligator clips would work best.

I would also check the usual suspects CP820, CI818 and DP810 assuming the designators are the same for your power supply board.

[budm]

Re: Samsung LN40D550K1FXZA Backlight Problem

Thanks for the link to the other thread neilc6. Are the capacitors I highlighted in the attached photo the "main filter caps?" I'm getting 163V across them. The other thread seems to say that I should be seeing 380-400V.

Neilc6 I see you had a similar problem and replacing ICP801 fixed it. Is there anything I can do to verify ICP801 has failed before I order a replacement? Thank you everyone for your responses and suggestions.

So the 160V reading on the cap is measure when the TV is turned on? May be you leave the probes connected to the cap then turn the TV on to see if the meter shows the Voltage jumps up to 400V range or not, you will have to put meter scale at higher than 200V scale.
There is also PFC Diode DP810 that needs to be tested to find out if it has leakage or shorted out.

[nomoresonys]

Re: Samsung LN40D550K1FXZA Backlight Problem

Thanks for the link to the other thread neilc6. Are the capacitors I highlighted in the attached photo the "main filter caps?" I'm getting 163V across them. The other thread seems to say that I should be seeing 380-400V.

Neilc6 I see you had a similar problem and replacing ICP801 fixed it. Is there anything I can do to verify ICP801 has failed before I order a replacement? Thank you everyone for your responses and suggestions.

Yep, need to verify the PFC circuit, check those in post 7, CI818 is a problem in many power supplies.

[classicmac]

Re: Samsung LN40D550K1FXZA Backlight Problem

I did more testing and the voltage across the capacitors does jump to ~400 for a split second before dropping down to ~160. Is there anything else I should verify?

[dick_barton]

Re: Samsung LN40D550K1FXZA Backlight Problem

You need to check or replace the capacitor as mentioned by the others above since it tends to be one of the faults recognised for this power supply.

[classicmac]

Re: Samsung LN40D550K1FXZA Backlight Problem

I removed CI818 and tested it and was getting ~.47-.5. It should be a .47uf capacitor right? I'm attaching a picture for reference.

[nomoresonys]

Re: Samsung LN40D550K1FXZA Backlight Problem

I'd test the inverter board.

[dick_barton]

Re: Samsung LN40D550K1FXZA Backlight Problem

Yes, 2W474K value is 0.47uf 450VDC Polyester Film type.

[arenat]

Re: Samsung LN40D550K1FXZA Backlight Problem

I did more testing and the voltage across the capacitors does jump to ~400 for a split second before dropping down to ~160. Is there anything else I should verify?

Most Samung power supply of that era have an OVP , if go over 415V disable it.

Measure that voltage again .

[classicmac]

Re: Samsung LN40D550K1FXZA Backlight Problem

I'd test the inverter board.

I'm a little new to this. Is there something specific I should test on the inverter board?

[nomoresonys]

Re: Samsung LN40D550K1FXZA Backlight Problem

This one has a number that appears to match but the board looks different. $7.50 https://www.ebay.com/itm/Coby-Sony-L...YAAOSw-ldZZ5mG

[nomoresonys]

Re: Samsung LN40D550K1FXZA Backlight Problem

According to shopjimmy the KDL-40X400 is one of the boards that will work, if the seller has his info. correct.
http://www.shopjimmy.com/auo-19-40t0...t-inverter.htm Shopjimmy must have made an error here, because these boards look way different than the one in post #24.

[nomoresonys]

Re: Samsung LN40D550K1FXZA Backlight Problem

Can you upload a pic. of your inverter board?

[budm]

Re: Samsung LN40D550K1FXZA Backlight Problem

You have power supply/Inverter transformer driver board, the primary inverter transformers winding are driven by the your power supply/ Inverter driver board. I would check the resistance of the primary winding, but more concern right now is the PFC Voltage of over 400VDC.
Your Inverter transformer board should look something like this:
http://www.shopjimmy.com/samsung-lj9...t-inverter.htm
or
http://www.shopjimmy.com/auo-19-40t0...t-inverter.htm
Or: http://www.shopjimmy.com/cmo-27-d056...t-inverter.htm
Show us the pictures of your inverter transformer board.