SAMSUNG LN40B750U1FXZA, is not turning ON, only steady red light

[luendel]

Hi, my son TV is a SAMSUNG LN40B750U1FXZA (PICTURE 1), is not turning ON.

He said the night before was working perfect, he just turn off and the next day is not working anymore.

I start reading some threads; learning terminology and some tips to find the problem and this is what I find...

The only signal of life is the steady red light in front (PICTURE 3), this means some board still working. I touch the power up sensor next to the red light, nothing happened but if I tray turning on with the remote control the red light GOES OFF and nothing happened. NO PICTURE, NO SOUND.

Now, NO LIGHTS on back T-Con board (PICTURES 5, 6, 7), it is supposed to see a red light on the DIGITAL AUDIO OUT port and amber or green in the ETHERNET port.

I just open the back cover (PICTURE 2) and inspect the cap's on the power board (PICTURE 4), they looks good.

I disconnect the ribbon cable from the power board (PICTURES 8, 8a, 9, 9a, 10) and connected the power cable and BACLIGHT IS ON in the TV.

I take volt measurements on the pins (PICTURE 11) in the terminal connector according the table and I got good 5.1 Volts on 5.1 pins, 5.1 on SB but on 13V pins the measurement was 12.6V almost 13V.

I check the removable Fuses (2) on the power board, they are good. I don't see any other fuse on the other boards.

I was thinking the T-Con board is the problem but I'm confuse because the red light in front is connected to this board. Why if no lights on this board still red light in front ON?

I inspect the main board (Center Picture 2) for burns but looks good.

Which board is the problem?

Thanks for your help Forum.

[budm]

Re: SAMSUNG LN40B750U1FXZA, is not turning ON, only steady red light

We need the DCV readings of all the pin (except the GND pins) on that white connector on the power supply/inverter combo board, Samsung BN44-00284A Power Supply / Backlight Inverter: http://www.shopjimmy.com/samsung-bn4...wer-supply.htm, when Power switch is activated.
You MUST provide the pin number, follow by names, and follow by the DCV reading so we can see what Voltages and signals are present or not present and go from there. Notes: you have to have all the connector back in places when making the measurement.
Black probe of the meter to the chassis.
BTW. 12.6V on the 13V pin is fine.

[luendel]

Re: SAMSUNG LN40B750U1FXZA, is not turning ON, only steady red light

Ok thanks I'm going to do that!!

[CapLeaker]

Re: SAMSUNG LN40B750U1FXZA, is not turning ON, only steady red light

I am guessing the isn't any PSON / BLon coming back from the main board.

[luendel]

Re: SAMSUNG LN40B750U1FXZA, is not turning ON, only steady red light

1) Readings WITHOUT white connector:

PIN EQUAL READINGS
-------------------------------------
1-------PS--------- 0.794 VDC
2-------SYNC------- 6.8 mVDC
3-------SB----------5.2 VDC
4-------GND-----------------
5-------GND-----------------
6-------GND-----------------
7-------13 V-------12.38 VDC
8-------13 V-------12.38 VDC
9-------GND-----------------
10-------GND-----------------
11-------GND-----------------
12-------GND-----------------
13-------5.1 V-------5.2 VDC
14-------5.1 V-------5.2 VDC
15-------5.1 V-------5.2 VDC
16-------5.1 V-------5.2 VDC
17-------GND-----------------
18-------GND-----------------
19-------13 V-------12.38 VDC
20-------GND-----------------
21-------13 V-------12.38 VDC
22-------13 V-------12.38 VDC
23-------INV-ON-------3.298 VDC
24-------I-PWM-------7.9 mVDC
25-------E-PWM-------4.84 VDC
26-------GND-----------------
27-------5V DET-------5.2 VDC
28-------GND-----------------
29-------GND-----------------
30-------GND-----------------

2) Readings WITH white connector but NO press power in remote control, just connect power cord and red steady light:

PIN EQUAL READINGS
-------------------------------------
1 PS-------20.2 mVDC
2 SYNC-------21.2 mVDC
3 SB-------5.2 VDC
4 GND-----------------
5 GND-----------------
6 GND-----------------
7 13 V-------0.6 mVDC
8 13 V-------0.7 mVDC
9 GND-----------------
10 GND-----------------
11 GND-----------------
12 GND-----------------
13 5.1 V-------0 VDC
14 5.1 V-------0 VDC
15 5.1 V-------0 VDC
16 5.1 V-------0 VDC
17 GND-----------------
18 GND-----------------
19 13 V-------0.6 VDC
20 GND-----------------
21 13 V-------0.7 mVDC
22 13 V-------0.7 mVDC
23 INV-ON-------0 VDC
24 I-PWM-------10.3 mVDC
25 E-PWM-------0 VDC
26 GND-----------------
27 5V DET-------0 VDC
28 GND-----------------
29 GND-----------------
30 GND-----------------

3) Readings WITH white connector AND press power in remote control, NO red light in front:
I can't read constant value number on the voltmeter.

PIN EQUAL READINGS
-------------------------------------
1 PS-------goes High to low....High to low....High to low
2 SYNC-------goes High to low....High to low....High to low
3 SB-------goes High to low....High to low....High to low
4 GND-----------------
5 GND-----------------
6 GND-----------------
7 13 V-------goes High to low....High to low....High to low
8 13 V-------goes High to low....High to low....High to low
9 GND-----------------
10 GND-----------------
11 GND-----------------
12 GND-----------------
13 5.1 V-------goes High to low....High to low....High to low
14 5.1 V-------goes High to low....High to low....High to low
15 5.1 V-------goes High to low....High to low....High to low
16 5.1 V-------goes High to low....High to low....High to low
17 GND-----------------
18 GND-----------------
19 13 V-------goes High to low....High to low....High to low
20 GND -----------------
21 13 V-------goes High to low....High to low....High to low
22 13 V-------goes High to low....High to low....High to low
23 INV-ON-------goes High to low....High to low....High to low
24 I-PWM-------goes High to low....High to low....High to low
25 E-PWM-------goes High to low....High to low....High to low
26 GND-----------------
27 5V DET-------goes High to low....High to low....High to low
28 GND-----------------
29 GND-----------------
30 GND-----------------

[budm]

Re: SAMSUNG LN40B750U1FXZA, is not turning ON, only steady red light

The Standby Voltage (SB) is fluctuating when connected to the main board so it can be dues to bad caps in the SB power supply or the main board is loading the SB supply too much so it goes into shutdown/restart cycle.
BTW, this power supply board has pull-up resistors for doing the board self test mode which you did when you disconnect cable between the Power supply and the main board then the backlights come on.
Also the PS-ON signal for this board is >0.7VDC = ON, <.1v = OFF.
It can still be weak SB power supply since in self test mode the loads are very light, but when main board is connected then it will see heavier loads. You can use dummy load to draw about 1A as a start on the SB supply to see if it can handle that or not.

[luendel]

Re: SAMSUNG LN40B750U1FXZA, is not turning ON, only steady red light

Hi Budm,
How I can perform the dummy test?

[budm]

Re: SAMSUNG LN40B750U1FXZA, is not turning ON, only steady red light

You can use two 10 Ohms 5W resistors connected in parallel to make 5 Ohms 10W resistor, that will draw about 1A of current from SB power supply, just connect the resistors between SB pin and GND pin.