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Old 11-26-2020, 07:55 PM   #1
waakye
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Default Samsung 50uhd8550 standby led pressing power button does not turn TV on

Hello everyone,
First of all I want to thank all the good professional people at badcaps. I have repair several tv by following all your great technical advises.
Below is my issue:
I have a Samsung 50uhd8550. The standby led is on but pressing the power does not turn it on. I disconnected the connector from the power supply board to main board and the backlights did not come on. I measured the pins on the connector CNM803 and I only gets the following:
PS ON/OFF = 3.5V
A5V = 5V
ALL other pins are showing no volts.
Any help will be appreciated. Thanks in advance.
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File Type: jpg 16064418683833519390477948432141.jpg (1.05 MB, 10 views)
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Old 11-27-2020, 03:57 PM   #2
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

Any help guys. Thanks
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Old 11-27-2020, 04:08 PM   #3
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

Since you have standby voltage, check if the PS-ON/OFF changes when the power button is pressed, If it does not change it is likely the problem is with the main board.
to turn on the power supply without the main board connected, check the voltage on the PS ON/OFF pin of the power supply connector, if it is at 3.3v, then short it to ground should turn on the power supply, and give you B13V, this will not likely turn on the backlights on this power supply.

Last edited by R_J; 11-27-2020 at 04:12 PM..
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Old 11-28-2020, 12:54 AM   #4
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

Hi RJ,
Thanks for replying and willing to help.
I verified that am getting 3.34volts on the PS -On/OFF pin.
With the main board and backlights disconnected, I short the 3.34v to ground and I heard the main relay clicked. I verified that all B13 pins on the CNM803 and there was 0 volts. Anything I can try next.
Appreciate your help.
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Old 11-30-2020, 03:52 PM   #5
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

Does any has or can point me to where I can get a schematic of the power supply board for the TV. Thanks in advance.
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Old 11-30-2020, 04:12 PM   #6
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

I doubt you will find a schematic for this board. Can you provide the number on ICP801, this is the main ic and I need to know the pin out to trace the circuit, There is a cap marked CP820, check the voltage Across it, this is the ic's vcc voltage
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Old 11-30-2020, 04:27 PM   #7
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

Hi R_J,
The voltage across the capacitor CP820 reads 1.760v dc. This is the number on ICP801: 44A6XDM SPC1012T. Btw are you located in the west of Canada?. Thanks for the help.
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Old 11-30-2020, 04:31 PM   #8
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

Yes I am out west, Is the voltage across cp820 with the power supply forced on? it needs to be forced on to check this voltage.
Check the voltages on QS801, this is the vcc regulator/switch. The collector should have the vcc supply, the base will have a zener diode to set the voltage and turn on the transistor, the output is on the emitter.
JP1028 & JP1031 should be hot ground, JP1021 should be VCC supply (around 20~30vdc)

Last edited by R_J; 11-30-2020 at 04:39 PM..
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Old 11-30-2020, 05:03 PM   #9
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

This is the reading so far. When the power supply is forced on CP820 reads .399volts dc. QS801 reads collector is 18.1v, emitter is 1.774v and base is 2.2vs. when the power supply is forced on QS801 reads collector 29.6v, base ~ .4volts and Emitter .4volts.
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Old 11-30-2020, 05:15 PM   #10
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

JP1021 is showing around 18.9v to 19.1volts
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Old 11-30-2020, 05:47 PM   #11
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

The collector voltage seems ok, the base is not correct, There should be a zener diode (ZDS802) and another diode (DS811) in series between base and ground, then there should be a resistor from the base that connects to pin3 of an optocoupler (PC802S) and the other pin4 will connect back to the QS801 collector (or vcc source)

I traced out the circuit the best I can, here is a basic circuit that is similar.
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File Type: jpg reg-sw.JPG (28.5 KB, 13 views)

Last edited by R_J; 11-30-2020 at 06:47 PM..
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Old 11-30-2020, 06:10 PM   #12
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

Hi R_J,
I have attached an image of the the QS801. Where exactly do you want me to get the readings.
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Old 11-30-2020, 06:36 PM   #13
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

I tried to trace out the circuit, Red should be raw vcc source, Yellow is the VCC to supply the main/pfc ic. Green is the base and control circuit, when the optocoupler led is turned on, pins 1 & 2, the pins 3 & 4 are basically connected together. I added a basic circuit so you can get the idea of how it works.
When the power supply is forced on, opto pins 3 & 4 should be close to the same value, if not it could be that the opto is not being turned on or it is bad.

Last edited by R_J; 11-30-2020 at 06:49 PM..
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Old 11-30-2020, 06:49 PM   #14
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

This is the readings I am getting with the set not forced on. (p_son not activated)
QS801 reads base= 2.24v
Collector= 18.6v
Emitter = 1.765v
After the set is forced on
QS801 reads base= 100.1mv
Collector= 21.2v
Emitter = 0.390v
Am getting same reading at the lines you traced out.
I really appreciate your help in this.
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Old 11-30-2020, 06:51 PM   #15
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

Check that ZDS802 is not shorted, also check the voltage on pins 3 & 4 of the opto when the opto is turned ON, it should supply 18v to one end of RS808, the other end should connect to the transistor base and apply about 15~18v (depending on the zener value)

Last edited by R_J; 11-30-2020 at 06:56 PM..
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Old 11-30-2020, 07:10 PM   #16
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

Check the pin 3 and 4 of PCS802S. Pins 4 shows 21.4v and pin 3 shows 100.1mv when the power supply is forced on.
When it's not forced pin 4 of PCS802S shows 18.6 and pin 3 shows 3.56v.
The end of the resistor RS808 is showing the voltage on pin 3 of the PCS802S.
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Old 11-30-2020, 07:18 PM   #17
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

So either the optocoupler is not turning ON, OR the zener diode is shorted
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Last edited by R_J; 11-30-2020 at 07:29 PM..
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Old 11-30-2020, 07:23 PM   #18
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

Could you tell me how to measure the zener diode?.
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Old 11-30-2020, 07:32 PM   #19
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

Using your multimeter in DIODE test, check the resistance from the single lead end to the double lead end, then reverse the leads and check again, one way it should be around .6xx the other way should read open circuit or close to it.
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Old 11-30-2020, 07:38 PM   #20
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Default Re: Samsung 50uhd8550 standby led pressing power button does not turn TV on

One way reads .376 on both ends and when reverse reads .470.
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