Samsung LN40B630N1FUZA - Backlight Inop

[monte1967]

I have a Samsung LN40B630N1FUZA that was manufactured in February 2010 and purchased in April 2010. The TV was not used heavily the past four years and was recently unplugged (several hours after being powered down) before we went on a vacation. When we returned the TV was plugged back in and attempted to be used the next day. The TV powers on, I hear the startup sound, but get no picture and no OSD when the MENU button is pressed on the remote. If I play a DVD or use another input, I can hear sound and see a faint, clear picture if I shine a very bright flashlight close to the screen. I have not been able to get any decent estimates to repair the TV, so when I found this forum I decided to try some of the items I had found for this specific model.

I removed the back cover and the Power/Inverter board (BN44-00264A) for inspection. I found no burnt traces, no bulged caps, the two fuses (FS801 and FS802) checked okay and I obtained the following readings (taken with a Fluke 23III DVOM on Ohms Auto-Range) when going across the Bridge Rectifier (as seen in another thread) -

1 - 2 = 1.51 M Ohms
1 - 3 = 1.52 M Ohms
1 - 4 = Moved from 15 M Ohms to 2 M Ohms over a period of about a minute
2 - 3 = 0.896 M Ohms
2 - 4 = 1.494 M Ohms
3 - 4 = 1.541 M Ohms

If anyone has any ideas how to isolate a component failure on this board, or determine if the Logic Board or T-Con Board is the issue, I would greatly appreciate the help. I have access to a DVOM, soldering iron, etc., but no scope. I can read the wiring schematic for the board and know what most of the symbols mean, but do not have enough knowledge to develop a troubleshooting method. Thanks in advance for anything you can provide!

[tom66]

Re: Samsung LN40B630N1FUZA - Backlight Inop

Thank you for such clear photos!

Can you try checking the voltage across one of the 450V capacitors on the power supply with the TV switched on? You should get about 390V.

[budm]

Re: Samsung LN40B630N1FUZA - Backlight Inop

Also tell us what DC Voltage you have on CNM802 INV_ON/OFF pin. No backlights flash on for seconds either?

[monte1967]

Re: Samsung LN40B630N1FUZA - Backlight Inop

CP815 and CP816 both have 392 VDC when the TV is powered on, which slowly dissapates after power off. When measuring DC volts between pin 23 (Inverter On/Off) and pin 17 (Ground), I get a quick spike at power on and then a steady 4.69 VDC after. I do not see the backlight flash at all, but I have the TV face down on the bench right now. Guessing I should be able to clearly see the flash from the rear of the panel if it were present.

[Glitcher]

Re: Samsung LN40B630N1FUZA - Backlight Inop

Just curious what voltage do you get across the caps CP815 and CP816 while the tv is on standby (TV is plugged in but not turned on yet)?

[monte1967]

Re: Samsung LN40B630N1FUZA - Backlight Inop

CP815 and CP816 both have 172.5 VDC on them several minutes after entering standby mode. It does not appear to drop any further after that. I will recheck later.

[Glitcher]

Re: Samsung LN40B630N1FUZA - Backlight Inop

Try checking these resistors to see if any of them are out of spec.

[selldoor]

Re: Samsung LN40B630N1FUZA - Backlight Inop

In post 1 did you mean you tested FM802S

[budm]

Re: Samsung LN40B630N1FUZA - Backlight Inop

Here are good pictures from Shopjimmy, and the SCH from TOM.
Check the cap CI892, the voltage (negative of the probe on the negative leg of the main filter cap for primary side circuit ground ref. point on those points.

[monte1967]

Re: Samsung LN40B630N1FUZA - Backlight Inop

In post 1 did you mean you tested FM802S

I apologize for the error. Yes, I meant I checked FM802S and it was okay. Thanks for catching that one.

[monte1967]

Re: Samsung LN40B630N1FUZA - Backlight Inop

I will make the rest of the checks recommended later this evening. Bud - I really appreciate the help. Can you clarify the test you are wanting me to perform? I reviewed the redline diagram you attached, but am not 100% clear.

[budm]

Re: Samsung LN40B630N1FUZA - Backlight Inop

I will give you the details this evening.

[monte1967]

Re: Samsung LN40B630N1FUZA - Backlight Inop

Try checking these resistors to see if any of them are out of spec.

Resistor Spec Measurement
RI870 68 68.6
RI871 56 56.6
RI872 30K 25.4K
RI873 470 234.6 (parallels RI874)
RI874 470 234.7
RI875 68 68.1
RI876 56 56.2
RI877 30K 29.9K
RI878 470 234.2 (parallels with RI879)
RI879 470 234.1

I'm kind of concerned with RI872 as I am guessing it is a 1% or less tolerance resistor and does not match the corresponding resistor (RI877) in what appears to be a "twin" circuit, where I would expect all readings to be nearly the same. Thoughts?

[johnboy1313]

Re: Samsung LN40B630N1FUZA - Backlight Inop

In a dark room, does the screen flash at all at startup?

[Glitcher]

Re: Samsung LN40B630N1FUZA - Backlight Inop

Those readings look good, I was expecting one to be open circuit or very high resistance.

[monte1967]

Re: Samsung LN40B630N1FUZA - Backlight Inop

In a dark room, does the screen flash at all at startup?

I have it in a fairly dark room and never see anything flash in the way of a backlight when it is powered on.

[monte1967]

Re: Samsung LN40B630N1FUZA - Backlight Inop

Looks like a replacement board is not available for this thing after my searches without significant lead times. It would be nice to find the offending component and replace it, but at what time do I stop testing and order the board in hopes of getting one? Is everyone pretty sure this is in the Power/Inverter board, or are there still possibilities for the T-Con or Logic boards? If there was some sort of procedure I could use to eliminate them, it might make sense to do that and then order the Power/Inverter board before they become harder to find. I don't want to give up on this though. Thoughts?

[Glitcher]

Re: Samsung LN40B630N1FUZA - Backlight Inop

The problem is definitely in the power/inverter board. I had the same model and board with the same problem, and it turned out to be in the general area I circled and found an open resistor. Try testing the transistors there also.

[monte1967]

Re: Samsung LN40B630N1FUZA - Backlight Inop

UPDATE - I finally found a fairly cost effective Power/Inverter board on the internet and ordered it late last week. It arrived today and I installed it. TV is working perfectly now (and has for several hours). I am now going to try and take a closer look at this board and see if I can find anything wrong with it. Does anyone know if there is anyone who buys these boards to refurbish them? It would be nice if I could get some money back from it and help someone else out as it looks like these boards are getting hard to come by now. Thanks to everyone for their help!

[jamodus]

Re: Samsung LN40B630N1FUZA - Backlight Inop

don't know if any of you will see this but did you ever find the precise problem on that board? i was got one for free and thought i'd do the low-level troubleshoot also. so far everything checks out ok though.