Samsung LA55B650T1F 55" TV Won't power on

[simmiv]

Hi, I have a Samsung LA55B650T1F tv which died i.e. the red standby light went off when turning it on and it never came on again. I've disconnected the AC and left it overnight and reconnected it. Upon pwr on, it doesn't come up. I can sometimes see the leds on the ethernet connector flicker faintly on/off and then nothing.
So, after opening up the tv, I did a visual inspection of the power board, connectors etc. All looked ok. I have checked all the electrolytes with an ESR meter and checked their capacity out of circuit and they are fine.
When disconnecting CN801 between power board (BN44- 00268A) and the main logic board and then plugging in power, the back leds flash on/off twice and then stay on permanently.
Measuring voltages on CN801 is as follows NOTE (Missing pins are not connected):

AC PWR Plugged In
1 - PS = 0v
2 - Sync = 0v
3 - SB = 5.22v
5 - GND = 0v
7 - 18v = 0v
9 - GND = 0v
11 - GND = 0v
13 - 5.1v = 0v
15 - 5.1v = 0v
17 - GND = 0v
19- 13v = 0v
21 = 13v = 0v
23 - INV-ON = 0v
25 - E-PWM = 0v
27 - 5V-DET = 0v
29 - GND = 0v
30 - GND = 0v

AC PWR Plugged In with CN801 Disconnected and led lights on (Led light flashes twice on/off and then stays on permanently.)
1 - PS = 0.78v
2 - Sync = 0v
3 - SB = 5.22v
5 - GND = 0v
7 - 18v = 18v
9 - GND = 0v
11 - GND = 0v
13 - 5.1v = 5.22v
15 - 5.1v = 5.22v
17 - GND = 0v
19- 13v = 12.43v
21 = 13v = 12.43v
23 - INV-ON = 3.8v
25 - E-PWM = 4.95v
27 - 5V-DET = 5.21v
29 - GND = 0v
30 - GND = 0v

AC PWR Plugged In, all cables plugged and pwr on via remote
1 - PS = fluctuating 0.5 - 0.7v
2 - Sync = 0v
3 - SB = fluctuating 0.6 - 2.4v
5 - GND = 0v
7 - 18v = fluctuating 5 - 12v
9 - GND = 0v
11 - GND = 0v
13 - 5.1v = fluctuating 0.6 - 2.2v
15 - 5.1v = fluctuating 0.6 - 2.2v
17 - GND = 0v
19- 13v = fluctuating 1.90 - 5.80v
21 = 13v = fluctuating 1.90 - 5.80v
23 - INV-ON = fluctuating 0.1 - 1v
25 - E-PWM = fluctuating 0.2 - 2.3v
27 - 5V-DET = 0v
29 - GND = 0v
30 - GND = 0v

I've also tried disconnecting everything from the main board, except the CN801 pwr cable and it still causes the fluctuation of the voltages.

I'm thinking the power board is ok, but the main board may have a problem. It would appear it's loading one of the supplies off the power board and then retrying to power it up again in a cyclic way. Has anyone seen this type of symptom. The main board has three smd caps and the ESR measures them in circuit within specs based on their capacity and voltage. They also look ok and no leakage to be seen.
Any thoughts?
Simmi

[budm]

Re: Samsung LA55B650T1F 55" TV Won't power on

At this point it looks to be the main board is loading down the SB.
Try checking the resistance between SB 5V pin of the main board and the Ground pin of the main board to see what kind of resistance it has.

[simmiv]

Re: Samsung LA55B650T1F 55" TV Won't power on

Hi, thanks..
I have measured the resistance on all the voltages to the main board and none of them are shorted. Most are more than 200ohms..
I then used a current limited power supply and checked each voltage to the main board separately..
The 5.2v Stby should not draw more than 0.2a. It drew about 30-100ma
While feeding the 5.2v stby I used the remote to power on and measure the PS voltage. It goes from 0v to just over 2v and then down again.. Seems to be fine..
Then I tried the 5.1v current limited to 4Amps as per spec. It drew less than 4amps..
Next came the 18v current limited to 0.3Amps as per spec. It drew less than 0.3Amps
Lastly I tried the 13v current limited to 4.2Amp as per spec. Then it drew to the max. I increased the current limite to 4.5Amp and it maxed that out as well. So based on that I would assume the 13v are the problem. As I don't have any schematics of the main board I don't know what the 13v are used for.. Would anyone know if I could disconnect it to try out. I can always feed it higher current and see what heats up, but prefer not to do that..

[budm]

Re: Samsung LA55B650T1F 55" TV Won't power on

The 13V is used for the T-CON board and may be for the Class D audio power amp section also, so try having the T-CON LVDS cable disconnected to see what happen.
But one thing about the 13V, the T-CON gets the Voltage from switched MOSFET, so if you only feed the 13V pin only and nothing else on the main is fed with other Voltages then the T-CON will not be fed by the switched 12V.
You can also verify if the 13V is also used by Class D power amp IC by looking up the spec sheet of the IC to get the VCC pin and see if it is connected to the 13V line or not.

[simmiv]

Re: Samsung LA55B650T1F 55" TV Won't power on

It lives again!
After I measured the current drawn by the main board via a current limited power supply, I must have made a mistake. I repeated the measurements and checked again. This time, non of the voltages fed to the main board drew more than they should. Not sure how I goofed up the first time. This time they were all well within limits (5.3v stby 30ma, 18v 0ma, 5.3v 800ma and 13v 630ma). Thus I began to suspect that the main board wasn't the cause of this current overload on the power supply. I could not find a circuit diagram for this power supply (BN44-00268A), but I did find one for BN44-00264C) Although not identical, they both have ICE3BR1065JF SMPS Current Mode Controller. I began to suspect it might be misbehaving. What controls it is the current sense resistor RM801, which on my board is 0.4 Ohms. Visual inspection didn't show anything wrong, but when measuring in circuit I couldn't get a stable measurement. i.e. it was rocking up and down. So, I pulled it out and measured it out of circuit. I then discovered it was still rocking up and down in resistance. From about 0.4 Ohms up to several ohms. I put it under the microscope and lo and behold, two hairline cracks on either side! I didn't have a direct replacement, but put in two 0.15 Ohms in series. Now the TV works fine. So in summary, if you have these symptoms, make sure you check RM801 and/or the controller. See photos as well..

[budm]

Re: Samsung LA55B650T1F 55" TV Won't power on

Good find.