A neighbor's Samsung experienced a voltage surge during a close lightning strike. When you plug the unit in, the LEDs light up though the push button joystick on/off switch does nothing. I get the 5V, 13V and 3V at the output of the power supply leading to the main board. There is no obvious physical damage to the power supply board, both fuses check out OK as well as all diodes. At the moment, I'm leaning toward main board failure. Anyone else have an opinion on this? Thanks.

I would agree with main board. I have seen lightning walk in thru cable and take out the main board more often than a line strike. It doesn't take much to kill a main board. I have seen one go dead from a static "tic" from a customer plugging in a HDMI cable on a cold dry day.

Before you condemn the signal board, you need to make sure the "run" part of the PS is working.

Take the meter and find the "PS on " line going to the power supply. It may not be labelled that--but will be similar. (pwr, pwr. on, etc).

While turning on the set, with the front button or remote--see if that line changes, (either from 0-3v. or more--or possibly the other way--3 to 0. it should be a noticeable, definite change.

It is does NOT--then you likely DO have a bad signal board.

If it DOES_-then something is wrong with the run supply on the PS.

You can VERY easily have full standby--but still have a run supply problem.

Before you condemn the signal board, you need to make sure the "run" part of the PS is working.

Take the meter and find the "PS on " line going to the power supply. It may not be labelled that--but will be similar. (pwr, pwr. on, etc).

While turning on the set, with the front button or remote--see if that line changes, (either from 0-3v. or more--or possibly the other way--3 to 0. it should be a noticeable, definite change.

It is does NOT--then you likely DO have a bad signal board.

If it DOES_-then something is wrong with the run supply on the PS.

You can VERY easily have full standby--but still have a run supply problem.

That's the strange thing... I have no power on or off. If I plug the unit in, the LED backlights come on though the power on/off switch does nothing. And of course if I unplug the unit, the LEDs go off.

When you say the LED backlight comes on are you referring to the panel backlight or the decorative lighting that some models have?

Do you hear a relay clicking?

In earlier Samsung LCDs a corrrupt EEPROM will stop the set from starting up and can cause the standby LED to stay off or on. The EEPROM could be corrupted by a power supply fault. You'd fix the power supply only to find it still wouldn't start up.

The EEPROM can be temporarily bypassed (usually by shorting the data and clk lines for a second or two while applying power) to get the set to start in service mode where the EEPROM can be reset. From memory service mode should be info -> menu -> mute -> power on the remote.

I don't know if this also applies to the model you have though.

Many modern flat panel sets (and some CRT-based sets, for that matter), do NOT use a relay for turn on--they just "gate on" the run supply, usually with a pulse from the signal board, rarely over a data line.

If...your backlights are running, and you have all of the run volts from the supply..but NO turn-on pulse from the signal board..(if it is strying high or low--and not changing wiht the power button)--THEN you likely have a signal board issue..


BUT..

If this set has a "Tcon board), (near the top, usually of the panel, though I have seen them at the bottom I think), the UNPLUG this--and re-check to see if the set will power off. it could have a data corruption, killing the on-off command.

Also--if the IR sensor is on a seperate board from the keyboard--unplug it. Then do the same--one at a time--for the keyboard.

FInally..unplug the BL inverter/driver board. There is a CHANCE --it may be corrupting data.

If not luck...likely bad signal board.

When you say the LED backlight comes on are you referring to the panel backlight or the decorative lighting that some models have?

Do you hear a relay clicking?

I was referring to the LED panel backlighting. No click. I've attached an image of the set below.

BUT..

If this set has a "Tcon board), (near the top, usually of the panel, though I have seen them at the bottom I think), the UNPLUG this--and re-check to see if the set will power off. it could have a data corruption, killing the on-off command.

Also--if the IR sensor is on a seperate board from the keyboard--unplug it. Then do the same--one at a time--for the keyboard.

FInally..unplug the BL inverter/driver board. There is a CHANCE --it may be corrupting data.

If not luck...likely bad signal board.

Firstly, thanks to everyone for advice and help.

I unplugged the t-con board and still I have no control with the power on/off switch. There isn't any keyboard as the power switch is a joystick like device that has multiple functions. The IR sensor is part of this switch. The driver for the LEDs is part of the power supply.

I am supposed to be getting 3 volts leading to the power switch from the main board though I'm only getting a tenth of a volt.

This is newer than the models I am most familiar with. EEPROM corruption could still be part of the problem you're having, but the failure mode is likely different.

It's unusual that the backlight is on as it is turned on by the signal board - usually by pulling the BL ON (or similar) line to +5V.

Something on the signal board pulling both BL ON and PS ON high while pulling the 3V rail down will probably end up leading back to the one big IC that does everything. I'd check any voltage regulators on the signal board, but beyond that it's probably only viable to replace the signal board.

In a way it's convenient that so many of these thin sets get cracked - there's usually used boards on eBay.

The set has a manufacturing date of 2012.

I had incredible luck yesterday and won all three bids on eBay for each board for $.99 each plus shipping (grand total of $22). I figured what the heck, all three cost me less than I would pay for the main board alone. I'll let everyone know the results later in the week.

This is newer than the models I am most familiar with. EEPROM corruption could still be part of the problem you're having, but the failure mode is likely different.

It's unusual that the backlight is on as it is turned on by the signal board - usually by pulling the BL ON (or similar) line to +5V.

Something on the signal board pulling both BL ON and PS ON high while pulling the 3V rail down will probably end up leading back to the one big IC that does everything. I'd check any voltage regulators on the signal board, but beyond that it's probably only viable to replace the signal board.

In a way it's convenient that so many of these thin sets get cracked - there's usually used boards on eBay.

NOT always....

IIRC....some recent sam models had a "PS on Low"..meaning--that they had a 3v or so pulse on the PS line when they are OFF_-and lifting that line caused the PS to go to full on . So, a dead signal board WOULD allow for this failure mode.

Hmm... I haven't ran across any of those yet. I have seen a couple of PS on low where all the rails would come up with no/dead signal board, but none as yet where the backlight would also come on.

As I indicated earlier, I got all three boards for a ridiculous price so I purchased all three instead of just the main board. I initially replaced both the main and the power supply boards thinking if there was something slightly out of whack with the original power supply board that I didn't catch, it might take the main board out again. Upon plugging it in I immediately noticed the backlights stayed off until the unit powered on and whoppe, a screen full of snow appeared (it was on TV input) and message boxes. Woohoo! I put a DISH signal through it and got picture and sound.

I was running an HD signal through it yet I wasn't getting full screen. In the menu it was set to 16:9 so I was a bit confused. Then I noticed a bit of graininess, kind of like sparkling in certain parts of the screen. T-Con board. :) So I replaced that and tada!! Everything is now OK. I upgraded the firmware to the latest and it's now running for a few hours to see if it remains stable. So that lightning strike definitely took out two of the three boards and most likely damaged the power supply board as well. Geezzz.

Nice to hear of your success, especially at bargain prices.