Samsung LN52B750U1FXZA - Standby Light - No Other Power

[fuzzyrunner]

Hello,

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.

My mainboard: BN94-02585L (barcoded sticker) or BN41-01149B (printed on the board itself)

Power supply board: BN44-00287A

Observations:
When I unplug the main board from the power board the backlight turns on.

When the TV is plugged in, the transformer on the power supply board has a "hum" that lasts about 5 seconds then goes away for about 3 seconds. This repeats until I unplug the TV. While the power supply is connected to the main board, the POW ON_OFF pin cycles from ~19mV (while I hear the hum) then jumps to about 2.1v (when the hum stops) but only stays there for a split second.

When power supply is not connected to main board the hum is constant.

Pin readings when power supply is NOT plugged into main board (Only even pin#'s with a wire connection is Pin#2 and Pin#30):
1 POW ON/OFF: .800V
2 H_SYNC: 4.1mV
3 STB 5.3V: 5.33V
4~6 GND: 0
7~8 13 VS: 13.07V
9~12 GND: 0
13~16 5.3V: 5.33V
17~18 GND: 0
19 13Vm: 13V
20 GND: 0
21~22 13Vm: 13V
23 INV ON/OFF: 5.25V
24 NC: N/A
25 E-PWM: 5.33V
26 GND: N/A
27 DET_5V: 5.33V
28~30 GND: 0

Pin readings when power supply IS plugged into main board (Only even pin#'s with a wire connection is Pin#2 and Pin#30):
ALL (#V > 0) MEANS THAT IT CYCLES FROM THE VOLT# DOWN TO PRACTICALLY 0. This cycle coincides with the transformer humming on and off.
1 POW ON/OFF: 1.5v > 0V
2 H_SYNC: 0v
3 STB 5.3V: 5.33v
4~6 GND: 0
7~8 13 VS: 13v > 0
9~12 GND: 0
13~16 5.3V: 5v > 0
17~18 GND: 0
19 13Vm: 13.1V > 0
20 GND: 0
21~22 13Vm: 13V > 0
23 INV ON/OFF: 0V
24 NC: N/A
25 E-PWM: 4058 > 0
26 GND: N/A
27 DET_5V: 2.7V > 0
28~30 GND: 0

My first thought was a main board problem since I am getting proper voltages from the power board when the power supply is not connected to the main board and the back lights are lighting up. I bought a main board online but that did not solve my problem. I am assuming that the main board I bought is working properly and that is actually the power supply that is giving me issues.

Is there anything other troubleshooting things I can try to pinpoint this a little better. Also, if it is the power supply, do any of you have an idea as to what might be giving me this issue? I don't have any noticeable burns, marks on the power board.

[CapLeaker]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Your problem is that your main board that you have on there is not turning on the backlights.

23 INV ON/OFF: 0V

25 E-PWM: 4058 > 0
I got no idea what you are measuring there at 4058... mV maybe?

Since PSon and BLon are generated on the main board, you got a main board problem. You proofed already by disconnecting the main board, that the backlights come on. So someone sold you a faulty board.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Woops...That should be a 4.58V > 0 not 4058

Ugh..any other ways to test this? I can't believe the old and new-to-me board have the same fault.

I am also not sure how I can prove to the company that they sent me a faulty board.

[CapLeaker]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

As I already said before:

The PSon and BLon/Inv on are generated on the main board. The PSon thells the PSU to turn all the voltage rails on and the BLon/INV On tells the PSU to turn the backlights on. If you don't have those signals, you can test the singal path manually with your multi meter to see for bad connection, broken trace, bent pin etc. in order to see that IF the BLon signal actually reaches the PSU. If you have no BLon signal on the main board, well, the main board is faulty.

You could try leaving the main board connected and take the BLon pin out (so you don't back feed any voltage to the main board) and jump BLon with a 1k resistor to Stby voltage or PSon. See what happens...

[johnboy1313]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Try unplugging the tcon with everything else plugged in and see if it does anything.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Try unplugging the tcon with everything else plugged in and see if it does anything.

Yea I tried that but nothing changed.

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

"Observations:
When I unplug the main board from the power board the backlight turns on."
You have main board problem.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

As I already said before:

The PSon and BLon/Inv on are generated on the main board. The PSon thells the PSU to turn all the voltage rails on and the BLon/INV On tells the PSU to turn the backlights on. If you don't have those signals, you can test the singal path manually with your multi meter to see for bad connection, broken trace, bent pin etc. in order to see that IF the BLon signal actually reaches the PSU. If you have no BLon signal on the main board, well, the main board is faulty.

You could try leaving the main board connected and take the BLon pin out (so you don't back feed any voltage to the main board) and jump BLon with a 1k resistor to Stby voltage or PSon. See what happens...

Gave that a shot. The backlight turns on for about 3 seconds then off for about 5 seconds and just repeats that until I unplug it. It does this same sequence with or without the resistor in place. Also, I am getting 5.3v across the resistor when jumping STBY and INV on/Off and and when the resistor is jumping POW ON/OFF and INV On/Off I am getting a cycle with the voltages going from 19mV when the light is off then it jumps to ~2.2v when the light turns on but is only there for a fraction of a second before dropping to ~.900v, then back to 19mV when the light turns back off.

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

You DID remove the BL-ON wire that goes back to the main board first before forcing on, right?
"Pin readings when power supply IS plugged into main board (Only even pin#'s with a wire connection is Pin#2 and Pin#30):
ALL (#V > 0) MEANS THAT IT CYCLES FROM THE VOLT# DOWN TO PRACTICALLY 0. This cycle coincides with the transformer humming on and off.
1 POW ON/OFF: 1.5v > 0V"
Your PS-ON is also cycling ON and OFF as indicated in your post #1.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

"Observations:
When I unplug the main board from the power board the backlight turns on."
You have main board problem.

When I was first reading through this forum I think I believe I read a thread that you posted the same thing on. It made sense (with my limited electrical knowledge) to me after I tested the voltages coming from the power supply board that it would be a main board problem. I purchased a new-used board but it doesn't work either and gives me the exact same readings as the old board.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

You DID remove the BL-ON wire that goes back to the main board first before forcing on, right?
"Pin readings when power supply IS plugged into main board (Only even pin#'s with a wire connection is Pin#2 and Pin#30):
ALL (#V > 0) MEANS THAT IT CYCLES FROM THE VOLT# DOWN TO PRACTICALLY 0. This cycle coincides with the transformer humming on and off.
1 POW ON/OFF: 1.5v > 0V"
Your PS-ON is also cycling ON and OFF as indicated in your post #1.

Correct, I took that pin out of the wire harness before jumping.

ETA: If that is what you mean. All my readings from my OP were from when I would start to tv with everything plugged in like normal. No pin jumping or anything just plugged everything up and plugged the TV into power.

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

But your PS-ON is cycling, so it is still main board problem.
So the NEW main board is doing EXACTLY the same as the old board?
BTW, you can also force on the PS-ON pin too, but you will have to disconnect that wire first also.
Also up load good clear pictures of the whole backside of the TV showing all the boards.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

But your PS-ON is cycling, so it is still main board problem.
So the NEW main board is doing EXACTLY the same as the old board?
BTW, you can also force on the PS-ON pin too, but you will have to disconnect that wire first also.
Also up load good clear pictures of the whole backside of the TV showing all the boards.

Yes it is. Same cycling of the backlight, same cycling with the PS-ON, etc. It definitely could be coincidence.

I will try force on the PS-ON also.

Attached the pictures

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Very good pictures! What did you use for the lights source?

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Very good pictures! What did you use for the lights source?

Haha just the lobby lighting for my business. Just had the building upgraded to LED lighting which is noticeably brighter than the previous T8 fluorescent bulbs.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Very good pictures! What did you use for the lights source?

With PS_ON/OFF unplugged from the main board and having a 1k resistor jumping from 5.3v STBY to PS_ON/OFF pin and 1k resistor going from 5.3v STBY to INV ON/OFF (with INV ON/OFF unplugged from main board) the backlight turns on but that is it. I am getting about 5.2v through both resistors but nothing happens. If I take the resistors off and leave the PS and INV unplugged there is no difference. Backlight stays lit but no other signals from the TV.

Restested Pins at at this state:

1 POW ON/OFF: 5.2V > 0
2 H_SYNC: 1.6v > 1.8v
3 STB 5.3V: 5.27V
4~6 GND: 0
7~8 13 VS: 13.07V
9~12 GND: 0
13~16 5.3V: 5.33V
17~18 GND: 0
19 13Vm: 13.13V
20 GND: 0
21~22 13Vm: 13.13V
23 INV ON/OFF: 14mV
24 NC: N/A
25 E-PWM: 5.12V
26 GND: N/A
27 DET_5V: 5.25V
28~30 GND: 0

Looks like with the POW ON/OFF and INV ON/OFF Pins pulled that all my voltages are correct and stable (besides H_SYNC and POW ON/OFF). The resistors don't change anything when connected or disconnected.

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

With PS_ON/OFF unplugged from the main board and having a 1k resistor jumping from 5.3v STBY to PS_ON/OFF pin and 1k resistor going from 5.3v STBY to INV ON/OFF (with INV ON/OFF unplugged from main board) the backlight turns on but that is it. I am getting about 5.2v through both resistors but nothing happens. If I take the resistors off and leave the PS and INV unplugged there is no difference. Backlight stays lit but no other signals from the TV.

Restested Pins at at this state:

1 POW ON/OFF: 5.2V > 0
2 H_SYNC: 1.6v > 1.8v
3 STB 5.3V: 5.27V
4~6 GND: 0
7~8 13 VS: 13.07V
9~12 GND: 0
13~16 5.3V: 5.33V
17~18 GND: 0
19 13Vm: 13.13V
20 GND: 0
21~22 13Vm: 13.13V
23 INV ON/OFF: 14mV
24 NC: N/A
25 E-PWM: 5.12V
26 GND: N/A
27 DET_5V: 5.25V
28~30 GND: 0

Looks like with the POW ON/OFF and INV ON/OFF Pins pulled that all my voltages are correct and stable (besides H_SYNC and POW ON/OFF). The resistors don't change anything when connected or disconnected.

Well with all the connectors back in place but the PS-ON and BL-ON wires are disconnected for using force on resistors and you are able to get the backlights to stay on but you do not see any images on the screen, then it is telling me that beside having command signals problem you also have other problem on the main board too. You should be able to see the image on the screen if all other functions of the TV are OK.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

Well with all the connectors back in place but the PS-ON and BL-ON wires are disconnected for using force on resistors and you are able to get the backlights to stay on but you do not see any images on the screen, then it is telling me that beside having command signals problem you also have other problem on the main board too. You should be able to see the image on the screen if all other functions of the TV are OK.

I am a little confused.
When the entire main board is unplugged the backlight turns on and stays on. No incoming voltage to INV or PS.

When both the PS_ON and INV are unplugged but everything else is plugged in the backlight stays on. No incoming voltage to INV or PS.

If I supply the INV with 5v the backlight remains on but if I plug the power cable to the INV it turns off. If I keep the INV unplugged but plug the PS_ON back in then the backlight flashes on and off but if I feed the PS_ON 5v from STBY the back light stays on.

Anytime I feed 5v to INV shouldn't that shut off the backlight?
Why does the backlight turn on and off only when the PS_ON is plugged in and the INV is not plugged in? Why does the backlight not turn on and off when I supply 5v to PS_ON? haha I am confused. am I doing a really bad job of jumping the pins?

[budm]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

"When the entire main board is unplugged the backlight turns on and stays on. No incoming voltage to INV or PS.
When both the PS_ON and INV are unplugged but everything else is plugged in the backlight stays on. No incoming voltage to INV or PS."
That is because the power supply/inverter board has pull-up resistors built-in to do the power supply/inverter self-test for the Technicians in the field so they do have to the external resistors. You probably can remove those resistors you are using now and the backlights will stay on as long as the PS-ON wire and the BL-ON wire are not connected but everything else are connected.

'Why does the backlight turn on and off only when the PS_ON is plugged in and the INV is not plugged in?' The PS-ON from the main board is turning the power supply ON and OFF, the PS-ON should be on solid (>2.5VDC) and stays there until the TV is switched OFF, then it goes LOW to keep the power supply OFF, and since the power supply is being turning on and off, the Voltage to the inverter circuit will also being turning on and OFF. For the backlights to be on solid, the BL-ON to turn on the backlights has to be on solid, the backlights also need steady Voltage from the power supply to keep the inverter circuit running, if the power supply is going on and off even though the BL-ON is steady the backlights will flash. Just think of it this way, you have the lamp switch (BL-ON) to turn on your lamp, you leave the lamp switch ON but the AC (power supply source) coming into your house is going on and off, then your lamp will be flashing on and off even though the light switch is in the ON position.
Bottom line, you have bad main board, no need to look further.
I think you need to learn to understand how the TV functions and what it needs to function.

[fuzzyrunner]

Re: Samsung LN52B750U1FXZA - Standby Light - No Other Power

"When the entire main board is unplugged the backlight turns on and stays on. No incoming voltage to INV or PS.
When both the PS_ON and INV are unplugged but everything else is plugged in the backlight stays on. No incoming voltage to INV or PS."
That is because the power supply/inverter board has pull-up resistors built-in to do the power supply/inverter self-test for the Technicians in the field so they do have to the external resistors. You probably can remove those resistors you are using now and the backlights will stay on as long as the PS-ON wire and the BL-ON wire are not connected but everything else are connected.

'Why does the backlight turn on and off only when the PS_ON is plugged in and the INV is not plugged in?' The PS-ON from the main board is turning the power supply ON and OFF, the PS-ON should be on solid (>2.5VDC) and stays there until the TV is switched OFF, then it goes LOW to keep the power supply OFF, and since the power supply is being turning on and off, the Voltage to the inverter circuit will also being turning on and OFF. For the backlights to be on solid, the BL-ON to turn on the backlights has to be on solid, the backlights also need steady Voltage from the power supply to keep the inverter circuit running, if the power supply is going on and off even though the BL-ON is steady the backlights will flash. Just think of it this way, you have the lamp switch (BL-ON) to turn on your lamp, you leave the lamp switch ON but the AC (power supply source) coming into your house is going on and off, then your lamp will be flashing on and off even though the light switch is in the ON position.
Bottom line, you have bad main board, no need to look further.
I think you need to learn to understand how the TV functions and what it needs to function.

Thanks for the information!! I am always learning and I am definitely grasping this better. Thanks again!