Jan 22, 2018, 4:05 PM
Post #1 of 5
Hey everyone, I'm fixing this TV for a friend and need a bit of advice. It quit working after a lightning strike. It is a Samsung 55" LED, LTJ series. The standby red LED will come on and blink when you plug in the TV, but it will not power on. I noticed it would make a clicking noise when you plugged it in. There was a pattern.. click, one second pause, click, 8 second pause, repeat.
Samsung 55" LED, no power on, clicking
My suspect going in was the power supply board. The model number of this board is BN44-00428b. Luckily the schematic and service manual for this board were readily available online. It seems this power supply works very much like one for a PC. When plugged in it produces a 5V standby which is sent to the mainboard through the 22 pin connector. When the mainboard wants to power on the TV it outputs 5V to the PS_ON input (pin 20). When the board receives this input it turns on relay RL801S (the source of the click), which powers up the 5V, 13V, Vamp, and Vdrv voltages. Seems easy enough.
I check the PS_ON input from the mainboard. It has a constant 5.3 volts as soon as the AC cord is plugged in. It never loses this voltage, even while the board is power cycling. So the mainboard is correctly signaling the power supply to come on.
And it does.. for 1 seconds. While it is on, the red LED in the optical port on the mainboard lights up. The B5V output is 5.3 V, the B13V output is 12.6V, and Vamp is 12.6V. All within specs. Then the board just shuts down. All outputs go to 0 (except the standby A5V and PS_ON, both stay at 5V). It stays off for 8 seconds and the cycle starts again.
Seemed pretty simple to me.. the power supply is receiving the input to stay on, and it isn't staying on. I know these supplies self-regulate, so if there's an overvoltage situation they shut down. While I didn't see any overvoltages, I know the detection circuit could have easily been damaged in the strike.
After looking at the schematic, PS_ON basically goes directly to the transistor that turns on the relay (QS582), only passing through RS862. PS_ON can be pulled low after that resistor by transistor QM853 (POINT B), which would shut down the supply. Three things can turn on QM853 and shut down the supply: if IC9151 or IC9152 activated their FLAG output, B13V exceeds a certain voltage, or A5V (standby) exceeds a certain voltage.
I wanted to directly test this, but all the components in these circuit are surface mounted on the bottom of the board and would be difficult to access with the board powered up. Since the board had been hit by lightning I opted to just replace the power supply board rather than try to fix it. I read there are many problems with bad caps causing the power cycling, but I didn't see any failed ones (and I know they can be bad with no obvious outside signs). I ordered a replacement from ebay, a used board supposedly pulled from a working TV. I confidently put the board in, plugged it up, and expected the TV to power up.
It didn't. Same issue. Click, click, click.. Same pattern and timing. The board I replaced this one with was a revision A board. It should work just fine.. the connectors were the same, and revision B was just a design improvement with no changes to functionality, according to the manual.
Now I'm doubting my testing process. After tossing it around a bit the only other thing I can think of that's external to the power supply that would shut it down is a shorted output triggering the over current protection. It seems unlikely given the correct voltages for the brief second the supply turns on, but it's the only option outside of just receiving a bad board. I tried unplugging the two LED power connectors at the top of the board, just to see if those were shorted, but it made no difference. So tomorrow I'm going to take my inductive current probe and test the current draw of the B5V, B13V, and Vamp circuits. If they are within specs, did I just get a bad board? Am I missing something simple? Any help would be greatly appreciated!