No backlight, with sound Samsung lcd LN46B640R3F

[sandersen]

Looking for help on LCD tv. Tv was working fine then all of a sudden all goes dark. Sound still works can still see image on screen with flashlight test, just no backlight.

So far I have checked fuses at power supply both were good,all capacitors looked good as far as I could tell (no bulges). Tried unplugging the main board from the power supply to force the backlight on. Backlight did not turn on. Jumped the SB pin and PS pin at the power supply and checked voltages at all the pins. All looked good 5.1V pins read 5.21V, 13V pins read 12.94V. The pins I'm not sure about are INV-ON read 3.8v, I-PWM read 0V, E-PWM read 5.23V and 5V DET read 0V.

Any help would be appreciated.

Samsung LN46B640R3F
Power supply BN44-00265B

[budm]

Re: No backlight, with sound Samsung lcd LN46B640R3F

Can we see the whole back side of the TV so we can see how all the boards are connected together?

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

Budm
These are the pictures of the back. Left side first.

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

Here is the right side. Can't post all at once?

[budm]

Re: No backlight, with sound Samsung lcd LN46B640R3F

OK, is the fuse F1801 OK? This fuse feeds the b+ to the Inverter circuit.
Also check the DC Voltage right at the two legs of the main filter caps (the two 4501VDC caps laying on their side), it should have around 360~400VDC when TV is ON.
http://www.shopjimmy.com/samsung-bn4...er-supply.htm#

Sch in PDF.

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

These are the two fuses I checked. Both had 5 ohm resistance so they are good. The caps that I measured are cp802 and cp803 both, with a jumper on ps and sb, measured only 62VDC.

First fuse.

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

Here is the second fuse on the top left. The two caps I measured are the on the right.

[budm]

Re: No backlight, with sound Samsung lcd LN46B640R3F

"These are the two fuses I checked. Both had 5 ohm resistance so they are good"
Fuse should have the same resistance reading as when you touch the two probes of your meter together. 5 Ohms reading on the fuse is not right.

"The caps that I measured are cp802 and cp803 both, with a jumper on ps and sb, measured only 62VDC." Are you measuring RIGHT AT THE TWO LEGS OF THE CAPS as instructed? Just remember, you are making voltage measurement in the primary side of the circuits, you cannot use chassis as the ground ref point for the lack lead of the meter.
Based on what you indicated for the voltage reading, you must have used the chassis as the ground ref, unless your meter is bad.

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

When I put my leads together I do read .5 ohms. Also checked in diode mode there is continuity.

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

You are correct I did use chassis ground. Should have measured voltage across the caps. Don't know what I was thinking???

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

Will check voltage correctly tomorrow. Thanks for the help.

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

Measured correctly this time. I measured 384 VDC on both capacitors.

[budm]

Re: No backlight, with sound Samsung lcd LN46B640R3F

OK, that is good, it means the PFC Voltage booster is good and you do have invr_ON signal so now we have to look at the inverter circuit section next.

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

So the inv-on voltage when the sb and ps pins are jumped should be at 3.8 VDC . I assumed it should be 5.1 VDC?

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

Went ahead and measured the voltage at the ena pin on lx6503 (second to last pin on the left side of the picture). Voltage was 2.56VDC ref to chassis ground and the same voltage was on cap labled c1837.

[budm]

Re: No backlight, with sound Samsung lcd LN46B640R3F

> 2V = ON, <1V = off, so you are OK, it will not be 5V due to voltage divider in the circuit.

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

Sorry picture is rotated. Second pin from the top, on the right is where I measured.

[budm]

Re: No backlight, with sound Samsung lcd LN46B640R3F

What do you get on the VCC pin1? Also on drive 1 and drive 2 (check with DC and AC mode on the meter).
Also check the capacitance of C1804 (0.68uf 500V).

[stj]

Re: No backlight, with sound Samsung lcd LN46B640R3F

can you remove the big black plastic cover on the lamp panel and take a foto pls.

[sandersen]

Re: No backlight, with sound Samsung lcd LN46B640R3F

@stj here is the inverter board