Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

[gustagen]

Hello all! I've had great luck in repairing an LCD monitor and a Vizio LCD TV with the generous help and patience of the good folks here on the forum.

Yesterday I rescued a 46" Samsung LN46D550K1FXZA destined for the landfill. The backlights are not coming on. The display seems to work fine, as I can see the bouncy "Check Input Connection" message by using a flashlight.

The caps on the power board aren't bulging and I see no signs of leaks. No strange smells on the power board or the inverter board. I have continuity across fuses FS8012S and FS802S on the power board, but I did not remove them from the PCB to test.

I've attached photos of the power and inverter boards. Please let me know if you'd like more photos of a certain area.

Any suggestions on where to start? I have a cheap multimeter and VERY basic understanding of electronics. Thanks!!!

[sugar_ray]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

If you can see it using the flashlight? I would look into the tcon board

[budm]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

Do you have >360VDC (measured right at the two legs) at the main filter caps, the the two 450VDC rating caps laying on the side?
So the backlights DID NOT even flash on for seconds?
What DCV do you have on CN802 BLU_ON pin and the P_DIM pin?
What DCV on CNI801, LD pin?

[gustagen]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

I remounted the boards and powered up the TV. Here are my readings:

165 V from both large caps at center of power board

13 V at CN802 BLU-ON pin

13 V at CN802 P-DIM pin

12 V at CNI801 LD pin

I should have explained earlier what happens when I power up the TV:

- Plug in power cord, LED is solid red.
- Press power switch on panel.
- LED flashes red 5 times, then solid red.

The backlights never flash.

[tom66]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

Does the large cap voltage ever reach ~390V? or is it always 160V?

[gustagen]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

I dug up some leads with alligator clips so I could attach my meter and work the controls simultaneously.

Plug TV in. Both caps read between 165 V and 170 V.

Power TV on. Both caps read between 165 V and 170 V.

My meter is cheap and not auto-ranging. I see no significant changes in the meter while leaving the TV powered on for several minutes.

[budm]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

"13 V at CN802 BLU-ON pin

13 V at CN802 P-DIM pin

12 V at CNI801 LD pin"

What did you use for GND?

[gustagen]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

I used the mounting screws for the power board as a ground for all measurements, including the pin on the inverter board.

[budm]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

13V for BL-ON and Dimming control do not look right at all, way too high.
The power supply board is also indicating the PFC Voltage booster circuit is not working or the board is not getting the PS-ON (PWR-ON) from the main board, so you need to verify if the PS-ON is present or not.

[Alastair E]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

I used the mounting screws for the power board as a ground for all measurements, including the pin on the inverter board.

So--You used the screws as Gnd when checking the big caps for the 390V....?

Should measure ACROSS the cap, ie, one lead to one cap wire, other lead to the other (Same) cap--Not to chassis screws for checking the 390V....

--This is the HOT side of the PSU so No readings are done to chassis.....

[gustagen]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

I was incorrectly reading the chart for CN802 and attempting to take readings from the front of the board, where only half of the pins are accessible.

I have now flipped the power board, maintained all connections, and grounded the screw holes to the chassis with some alligator leads. I am using chassis as ground for the meter. I have kicked myself in the rear to save you guys the trouble. Here are the correct readings:

CN802 BLU_ON reads 5V with power on
CN802 P_DIM reads 1.67V at power up, fluctuates and hits 1.34 V for a second, then settles at 0.9V
CN802 PS_ON reads 3.3V with power on

I should also clarify that I took readings of the large caps at the caps' legs. I did not reference the chassis for ground.

I also just noticed a very faint high-pitched whine coming from the power board. When the TV is powered on, the whine begins and quickly rises to a high pitch. It maintains this pitch until the TV is powered off, and it quickly falls in pitch and almost dies out. It goes away quickly as soon as the power plug is pulled. It takes about 1/4 second for the sound to rise/fall from low/high pitch.

I power cycled the TV while searching for the source of the whine. Using a length of thin vinyl tubing I think I pinpointed the noise to the area of CM809.

I am lucky enough to be just a few minutes from a well-stocked electronics supply store. Is it worth throwing a few capacitors at the power board?

Sorry for the rookie mistakes and backtracking, and thanks for the help so far!

[budm]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

"CN802 BLU_ON reads 5V with power on
CN802 P_DIM reads 1.67V at power up, fluctuates and hits 1.34 V for a second, then settles at 0.9V
CN802 PS_ON reads 3.3V with power on

I should also clarify that I took readings of the large caps at the caps' legs. I did not reference the chassis for ground."
OK, that means you need to get the PFC Voltage booster circuit working first.

[gustagen]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

As I was looking over the power board, I noticed a strange mark in the area of CP820. It is not obvious in my previous photos because of a shadow that happens to fall in the same area.

Attached is a photo of CP820, as if you were laying under the TV looking up at the board. Looks like I found a bad cap after all.....

Unfortunately the nearby electronics shop is closed for the day, so I'm off to the scrap bin to scrounge for a cap!

[tom66]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

Yes, that could cause this fault, be aware it is possible the diode is also damaged (it is a snubber for the PFC diode)

[budm]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

The cap is probably showing as shorted circuit right now, so you also need to check the PFC Diode DP802, that will explain why you do not have boosted Voltage right now. Make sure the replacement cap is rated at 2KV or so.
What is printed on the body of the cap right now?

[gustagen]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

I set my cheap meter to the diode setting to check DP802. It read "000". I swapped the leads and it read "000" again.

CP820 has this printed on one side:

R
221K
1KV

[budm]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

That is what I expected, shorted.
It is 220pF 1KV, I would go with 1.5KV or 2KV instead since 1KV rating still break down.
Once you remove the cap, you might as well verify that the Diode is not shorted out either.

[gustagen]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

The electronics shop is closed on weekends, so I'll get a new cap and diode on Monday.

I will report back then. Thanks everyone!

[gustagen]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

SUCCESS!!!

I managed to find a 220pF 2KV cap on a modem PCB in my scrap heap.

I removed the blown cap and the diode seemed to check out fine.

I installed the new cap, held my breath, and powered the TV up. That Samsung splash screen never looked so good!

Thanks to everyone who chimed in!!! This makes 3 LCD devices you guys have helped me put back into service!

See you next time!

[tom66]

Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

Nice fix! Thanks for sharing....