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Eng. Andres
Eng. Andres, Engineer
Category: Electronics
Satisfied Customers: 68
Experience:  Electronic Eng. Electronics Specialist. Circuit designer
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I have a Samsung LE46F71B that wont switch on. I have a blue

Customer Question

I have a Samsung LE46F71B that wont switch on. I have a blue LED on the middle power button but pushing that or the remote control will not swicth TV on

Submitted: 4 years ago.
Category: Electronics
Expert:  TV-Tech1 replied 4 years ago.

Hello, I'm TV Tech1 and I'll be working with you to identify and help you resolve your problem.


Can you darken your room and look at the TV screen when you press the power button? See if the screen stays pitch black or if you happen to see the screen light up to a subtle gray light level. If it does, does it stay on, or does it seem to light for a second or two, then go back to pitch black.


Customer: replied 4 years ago.

Tried what you have suggested. There is no change in the screen when I push the button. screen stays pitch black

Expert:  TV-Tech1 replied 4 years ago.

Ok, then the Backlight on command from the micro, OR...the power supply isn't turning on its backlight power section. We don't know which one yet.


If you have a voltmeter, we can check the TV's Backlight on command. That would tell us if the power or main board is the problem.


Another option is to disconnect the plug /cable between the two boards, install a jumper on the Power on and Backlight on wires on the power board and see if the TV screen lights up.


Are you willing to remove the TV's back cover, and possibly remove a giant metal box (schematics show it on the 40 inch TV) to get to the power supply and main board, and either -


1] Use a voltmeter to check the power supply voltages or

2] Use a couple pieces of solid wire like you find on a resistor or capacitor leg, and jumper the standby 5 volt line to the power on and backlight on pin holes?


Let me show you a picture of that --




It's not complicated to do, but you do need either a voltmeter or maybe a electronic hobbyist store in your area.

I will tell you this. I live in the USA, and we have Radio Shack stores in practically every little town here. They originate from the UK. In the UK they were named TANDY back in the 60's and 70's. I've asked several customers if they exist, but with no clear cut answer to that question.


Does Tandy or Radio Shack....or even "The Shack" exist? The last one is what Radio Shack claimed they were renaming their stores. It hasn't happened, and it's been about 2-3 years since they mentioned the renaming.


They have those alligator jumper wires I use all the time, plus resistors and capacitors you can use as solid wires.



Customer: replied 4 years ago.

I am fairly comfortable trying that. I have a voltmeter - give me 10minutes to try it. Im assuming once the power is disconnected it is safe to remove the back of the TV

Expert:  TV-Tech1 replied 4 years ago.

Yes, I do that as a rule, and get the TV prepared for testing. Then I plug it back in.


There is ONE cable that connects the power supply directly to the main board and that cable will have a STBY (standby power line) and a PS-on and a BL-on line all on that one cable.

Those names are usually found on the power supply board.


I'm talking in "generics" to give you a basic idea what and where to look for these.


With your voltmeter, you check the STBY need either 3.3 or 5 volts DC on that when the TV is plugged in. The TV doesn't have to be turned on yet....the PSon and the BLon lines should switch from 0 volts to 5 volts when the power button is pushed.


Now....this is kind of important. Usually before I TEST....I'll simply unplug the main board side of that cable, and see if the backlights light up. I look at the holes and slits cut into the LCD panel, because they usually start glowing if the backlight is lit.

If it doesn't light when unplugged....I'll check it with the voltmeter, verifying I have STBY, then press the power button and see if PSON and BLON both switch from 0 to either 3.3 or 5 Volts.

Then I follow up with jumpering the STBY to BOTH the PSON and BLON wires to see if the backlight lights up.


I always doublecheck and verify. I never assume the power board is good and the main board is bad. That's why I run the whole testing procedure that way.


I'll give you the details on our next response, but making pictures of this takes a little time.

I'm hoping with the basics, you'll find that cable that connects the power board to the main board, see the PSON and BLON and STBY markings by that plug, and then give me some details back so I can send details to you afterwards.


Customer: replied 4 years ago.

I have the back off and the metal case for the power board. have found the cable you discribe but it doesnot show a PSon or BLon see pic below it does show a P-On/off how does the connector number. ie is it 1-16 along one line and then back to blue cable end for 17 etc


with the connector disconnected there is no LED on the ON button and I cant see any backlight - though not sure what im looking for


Expert:  TV-Tech1 replied 4 years ago.

P-On/off - That is the MAIN power on off control line. BLon is the ON/OFF line on that white cable, separate from the cable you show.


I got a picture of the TV set, showing some basic information --



Then I found a fairly good close up of your actual plugs on the power board. This one really helps you.




Now...when you unplug that cable from the MAIN board, your TV won't turn on at all.

You have to jumper the STBY hole to the PSON hole, and run another wire from the STBY over to the ON/OFF hole on that 2nd cable. BOTH have to be disconnected from the MAIN board, not the power supply, because we are forcing the power supply to turn on.


You do that AFTER leaving the power and main connected. Your voltmeter should have the black probe touching the GND (ground) pin, or the OUTER SLEEVE of the RCA jacks on your main board. Then you take your voltmeter, set it to DC volts, and use your Red probe to check the STBY first....should always have 3.3 or 5 volts on it when the TV power cord is plugged in


When the Power button is pushed....

P-On/off should switch from 0 to 3.3 or 5 Volts DC and stay on

That On/off on the smaller cable should switch from 0 to 3.3 or 5 volts DC and stay on too.


Hope that clarifies the voltage testing, and also the "bypass the main board and force the power board on" testing.


That's how you test the TV.


Customer: replied 4 years ago.

okay - i thought you had gone so I have put the TV back together. will try this tomorrow. Can I get back to you for more questions ?

Expert:  TV-Tech1 replied 4 years ago.

Sure, you can actually save this page to your web browser by using your bookmark or favorites button and save this page to your list, then click on it anytime you want to contact me directly.


Check your voltages with the boards connected together. Make sure you always have voltage on the STBY line.

Power it up, see if the Power on and the on/off lines switch voltages and stay at that voltage.


Then unplug the cables on the main board and jumper the STBY to both the power on and the on/off lines to see if the TV screen lights up.


Get back to me after checking both with the details on what you found.



Customer: replied 4 years ago.

okay - thanks for all your help so far

Expert:  TV-Tech1 replied 4 years ago.
You're welcome. Hope my information has been helpful and not too confusing. I try my best to introduce you a little at a time so it's not overwhelming.