Samsung LN52B750U1FXZA - Standby Light - No Other Power

[CapLeaker]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

I have to say, that your pix are sweet. I wish all would be like that good.

[edy_b]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Hi everyone,

I'm new to the forum but I have the exact same issue with a Samsung LN46B610A5FXZC. The power supply board is: BN44-00265B, and the logic board is BN41-01149B.

My main issue is the TV has the standby light on but fails to power on. I get no response from the "touch-sensitive" buttons along the side front of the TV (around the standby light). I do not have a remote but I tried a number of universals and they did nothing either (the standby light didn't even blink).

I was wondering where to start... I can disconnect the logic board and see if the backlights turn on when I plug it in the wall. To check voltages getting supplied to the logic board do I check the pin-outs for the black ribbon cable going from the supply board to the logic board? I assume they are labelled on the board? Do I just use the ground pin there and check it relative to the voltages of the other pins?

I have a little experience repairing TV's but have mostly replaced obvious blown caps or changed components based on "repair kits", but have not done major deep-dive diagnostics.

The only thing I found is what looks like the proper schematic for the power board at the link below but I have no access to any service manuals or schematics of the logic board or power board otherwise.

https://ia601200.us.archive.org/13/i...N44-00265B.pdf

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

"I can disconnect the logic board and see if the backlights turn on when I plug it in the wall."
So do the backlights come on and stay on per the test?

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Ok I'm back with a new problem with the same TV. I am just writing back on this same thread since it already has pictures and info about all the boards.

The powerboard is: BN44-00287A

Would love a schematic if someone has one for this board.

So new problem:

The problem first was that I was only getting a picture for about 2 seconds. When first turned on everything is on and at full brightness. 1 second later the image dims about 50% and a second after that the screen and backlights turn off. I don't hear any clicking besides for the initial click when I turn the set on. Recently I haven't even been getting a picture just a flash on the screen.

Once on, the stand-by light remains off and the remote still works. I can turn the TV on and off with the remote. The on/off switch on the set itself also works.

Some pin voltages I have:

From Mainboard:
Power On/Off: 0.892V
H_Sync: 1.724V
STB 5.3V: 5.28V
GND: 0V
13VS: 12.8V
GND: 0V
GND: 0V
5.3V: 5.25V
5.3V: 5.25V
GND: 0V
13Vm: 12.77V
13Vm: 12.77V
INV On/Off: 5.6V
E-PMW: 382.8mV
Det_5V: 0V
GND: 0V

From Power Board to Inverter Board (or the long board connected to the CCFL tubes):
I used the ground on that connector and the tested the small pads on the board itself.
VCC_1: 12.77V
FB1: 0V
FB2: 0V
GND: 0V
GND: 0V
LD: 12.28V
LD: 12.28V

Large Caps are giving me 388V on both.

Just doing some probing around I did find that there are 2 MOSFETs connected to a black heat sink. They are right next to transformers that connect to the wires (one set being blue and white and the other set being red and white) that head out to the backlight bulbs. The bottom MOSFET (MI8015) and 2 small diode looking components (labeled LB806 and couldnt find the label for the 2nd one) was giving me 388V but the top MOSFET (MI8025) and 2 components labeled (LB807 and LN808) are giving me 0V.

Any help would be appreciated. It seems like the problem is the MOSFET but didn't want to start taking components off if you guys think something else could be effecting the 0V there.

Thanks!

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

"E-PMW: 382.8mV" Is that for sure what you are seeing? If that is the case, it is way too low, it is the dimming control signal. You should the probe on that pin and GND, then turn the TV on and look at the Voltage to see how high it goes before it drops down to 0.382V, you should see the backlights get brighter as the Voltage is high and as the Voltage is lower the the backlights will also get dimmer. I expect it to be 2~3V range for typical backlights brightness.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

When I first turn the set on I get 1.8V but which quickly drops to .380V once the relay clicks.

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

You can try disconnecting just the E-PWM wire going back to the main board to see it the backlights will stay on bright, and also measure the DC Voltage on the power supply board side to see what you get without that wire going back to the main board.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Disconnecting the E-PWM wire turned the backlight on. I was getting 5.25v from the E-PWM pin on the power supply board itself. Does this mean an issue with my mainboard.......again?

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Your main board is the problem.
You can try reset the TV using the remote using MUTE-1-8-2 or something like that, I have not done one for quite a while, you need to do research on that.
After resetting then put the E-PWM back in place to see if the backlights will stay on bright.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

It works! Thank you very much! Do you know why the factory reset would get the E-PWM pin to start to work properly again?

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Great, I ran into this problem before too.
I think something got corrupted.
So did you use the MUTE-1-8-2-power? I do not remember exactly how.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Yup. Quick google search came up with (while the TV is off) Mute 1 8 2 Power. That brought up a menu where I could select reset.

[TomLeogrande]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

I am having the exact same issue as fuzzyrunner. I bought a new mainboard and put it in... did not fix the problem.

Can someone give me the exact thing you did to solve this?

If I am reading all this correct I need to do the following.

1. Disconnect the E-PWM wire
2. Use the Mute 1 8 2 Power reset menu.

Any other steps?

Do I need to reattach Pin 25 if it appears to work?

Will this work long term?

Can someone please tell me which wire is the E-PWM?

I believe it is Pin 25 on this connection. I believe the Triangle signifies Pin 1 Does it go 1-15 down the same side then go to the next row and go 16-30. Making the two far end pins 15 & 30?



thanks in advance!
Tom

[TomLeogrande]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

For more information and for potential clarity for other newbies like myself trying to repair their TV. Here is what I have done... good bad or indifferent.

Original Problem.
Perfectly working TV one day, the next the power button stops working.
TV has power, which is noted by the constant red light near the bottom right of the TV and the flashing bright white light in the middle bottom of the TV when the Power button is pushed.

Watched a bunch of online videos and broke into this thing thinking I could do it. I used to build computers for a living in the 90s so I am new but not a complete moron when it comes to circuit boards etc..

When I disconnect the main board the backlight comes on and stays on.

When I reconnect the main board the original problem comes right back.

I ordered a new main board as several online videos pointed out that this is most likely the culprit when the symptoms above come up.

Replaced the main board with the new main board and the problem remains.

--------------------------------------------------------------------------------------
Today
--------------------------------------------------------------------------------------
1. I see this forum and this information
2. Looks like my solution might be here.. and could be along these lines..

1. Disconnect the E-PWM wire
2. Use the Mute 1 8 2 Power reset menu.

As mentioned above here are my current questions

Is the pin circled in my attached photos actually Pin #25?

Any other steps?

Do I need to reattach Pin 25 if it appears to work?

Will this work long term?

Can someone please tell me which wire is the E-PWM?

I believe it is Pin 25 on this connection. I believe the Triangle signifies Pin 1 Does it go 1-15 down the same side then go to the next row and go 16-30. Making the two far end pins 15 & 30?

[RDC55]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

That is pin 20 that you have circled - pin 25 is on the outer row. Start from pin 1,3,5,7....or go back from 29, 27, 25.

The point of disconnecting E-PWM is to see if the backlights get brighter, if they do as with fuzzyrunner then you confirmed the P/S is working and you may have an issue with the main board. Of course this is assuming when you said you had the exact same problem as fuzzyrunner you were referring to this issue, and not his initial post??

Once the P/S was confirmed working as the backlights got brighter with E-PWM disconnected, budm suggested the factory reset using the mute 1 8 2 power. At this point pin 25 wire would be reinstalled as it was removed earlier for test purposes and that process was completed.

[TomLeogrande]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Thank you RDC55!!

I have replaced the main board with one I purchased. That did not solve my problem... Which was the same as this thread.. So I am guessing that it is my Power Board.

I am more of a PC guy - but I would think that means it is my power board. Probably best to swap that board out as well.

Pin 3 is showing 10 on my multimeter.. Pin 3 is my standby power - I think that is a good sign I have an issue with the Power Board. Would you agree with that?


To be honest - probably cheaper to buy a new TV.. But would like to keep this one working since it was a gift from someone and why add to the world's junk pile problem - if I can keep it running for a few hundred bucks.

[RDC55]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

So OP originally posted:

"I have a Samsung LN52B750U1FXZA LCD TV that isn't turning on. When plugged in all I am getting is a steady red standby light but nothing else. I do get a quick flash from the center LED that lights up the "Samsung" lettering on the front of the TV when I first plug the TV in and when I unplug it as well."

Is this what you are dealing with?

"Pin 3 is showing 10 on my multimeter.. Pin 3 is my standby power - I think that is a good sign I have an issue with the Power Board. Would you agree with that?"

Well this would be way off, assuming you meant 10 VDC or was it maybe 10 mVDC? Are you sure you got pin 3 and are you using the metal chassis as ground for the negative lead of the DMM?

[TomLeogrande]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

OP is exactly as stated above - "I have a Samsung LN52B750U1FXZA LCD TV that isn't turning on. When plugged in all I am getting is a steady red standby light but nothing else. I do get a quick flash from the center LED that lights up the "Samsung" lettering on the front of the TV when I first plug the TV in and when I unplug it as well."


Yes, I am using the metal chasis for ground and Yes Pin 3 - Assuming the connector is at the top left as you look at it, it is the second pin up from the bottom on the left or outside row of pins.

I get 10 vdc on multimeter reading when the mainboard is connected. No other pin is showing me any power.

I have a new power board on the way, and already have a new mainboard installed - I am hoping that will solve my problem.

Appreciate your help. I can report back after I install that board.

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

10V reading is odd reading.
1) Battery of your meter is OK?
2) Try measuring the DCV of a battery to see what it shows.

[Davi.p]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Hi, i guess this thread is a bit confusing, first because it is not clear if the OP solved the problem, after because the OP's tv changed behaviour in the last posts... my idea is that power board has got bad capacitors in the main power supply section, the right way to begin this repair and every other one is to load the power board with resistors and see the tensions, also connecting 5,3v to switch pins is not so good because control transistor dosn't like that tension, must be use a diode.