TV Forums

Free Games Games Forums Music Forums TV Forums

  Main Index FORUM
HOME
Search Posts SEARCH
POSTS
Who's Online WHO'S
ONLINE
Log in LOG
IN
Rules & FAQ RULES
F.A.Q.

Crumbtrail TV Forums: TV Equipment: TV Repair Forum:
Samsung 55" LED, no power on, clicking

 

 


CraigParton
New User

Jan 22, 2018, 4:05 PM

Post #1 of 5 (420 views)
Shortcut
Samsung 55" LED, no power on, clicking Can't Post

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.

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!



jts1957
Veteran


Jan 22, 2018, 7:55 PM

Post #2 of 5 (414 views)
Shortcut
Re: [CraigParton] Samsung 55" LED, no power on, clicking [In reply to] Can't Post


Quote
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.


What else is wrong with the Main Board besides it never allowing the PS_ON to go low to turn Off whatever Model TV?


-------------------------
Location: Far, Far Away


CraigParton
New User

Jan 23, 2018, 10:42 PM

Post #3 of 5 (402 views)
Shortcut
Re: [jts1957] Samsung 55" LED, no power on, clicking [In reply to] Can't Post


In Reply To
What else is wrong with the Main Board besides it never allowing the PS_ON to go low to turn Off whatever Model TV?


That's a good question! That's actually the reason I made this post, to hopefully get some insight into that. Do you have anything you'd like to contribute?

I'm quite aware that PS_ON should only go high when you press the power button, however not being an expert at TV repair I'm not sure of the operating strategy of the TV. Maybe there's an initial power-up phase when the TV is first connected to AC? And since the power supply isn't staying on for more than 1 second the mainboard never completes this and keeps PS_ON high? I don't know.. that's why I'm asking. Outside of the PS_ON input, what else could the mainboard do to make the power supply cycle?

I don't have the TV at my house which is why I didn't post the entire model number. Since the problem seems to be with the power supply I thought the number of that board would be enough, but I can get the model number if needed.


jts1957
Veteran


Jan 24, 2018, 1:14 AM

Post #4 of 5 (394 views)
Shortcut
Re: [CraigParton] Samsung 55" LED, no power on, clicking [In reply to] Can't Post

PS_ON High = power supply voltages are up?
1K resistor from ground to PS_ON pin = Do power supply voltages go down?

Model - You do that.


-------------------------
Location: Far, Far Away


MikeMasonUK
New User

Jan 26, 2018, 11:56 PM

Post #5 of 5 (367 views)
Shortcut
Re: [CraigParton] Samsung 55" LED, no power on, clicking [In reply to] Can't Post

I've came across a problem like this once with a Samsung. It would click and then nothing else would happen.

I replaced the power board and main board (got them both from the same seller for cheap). Replaced them both and the TV is still working 2 years later. So I can't tell you which one was the faulty one.

However the LED at the front never came back to life so it must have blown along with the board(s). It's not effected the TV though which works perfect.

I'm sorry I can't help you anymore than that. I'm not an expert and have only repaired a dozen TV's.

 
 
 


Search for (options) - Page loaded in: 0.26 s on (CGI/1.1)