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Old 05-21-2022, 03:53 PM   #1
Gjackson
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Exclamation Strange Power Supply Behavior on Skyworth 55U5A14G

Hi, I have a 55 inch Skyworth tv that has a visible standby light but won't power on, Usually this fault is indicative of a bad main board but I went to test the power supply just to make sure of that. The power supply output has 14 pins (see image attached), here are my readings when the main board is connected to the power supply (PSU model: L5R021 ,I cannot find much info about it as it is not popular):

22/12v - 12v
22/12v - 12v
GND - GND
GND - GND
12V - 12V
12V - 12V
GND - GND
GND - GND
NC
NC
STB - 4.66V
NC
ADJ - 4.98V
ENA - 0.05V

This is already a little confusing to me, first off I do not understand how 12v is at the output, isn't the enable pin practically at 0 volts? What is ADJ? and The STB seems a little low to me.

The following are the readings I get with the main board and backlight connector disconnected from the PSU.

22/12v - 12v
22/12v - 12v
GND - GND
GND - GND
12V - 12V
12V - 12V
GND - GND
GND - GND
NC
NC
STB - 0V
NC
ADJ - 0V
ENA - 0V

How come STB is now completely gone but the 12v remains still? STB should always maintain a constant voltage even when the main board is disconnected correct?

If I have 12v at the output always, is the missing STB when it is disconnected an issue? Could I just buy a replacement main board and it would work?

Forgive my inexperience but this all seems a little too confusing for me, any help would be very much appreciated!!
Attached Images
File Type: png PSU.png (1.26 MB, 11 views)
File Type: jpg Standby LED.jpg (88.2 KB, 6 views)

Last edited by Gjackson; 05-21-2022 at 04:23 PM..
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Old 05-21-2022, 04:55 PM   #2
R_J
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Default Re: Strange Power Supply Behavior on Skyworth 55U5A14G

This is an always on power supply that outputs 12 volts at low current, The stb pin voltage is supplied by the main board and turns on the pfc circuit etc on the power supply so it will operate at a higher current to operate the tv/backlights etc.
The main board will likely have a 12 volt to 5 volt regulator.
The STB pin is the same as a PS_ON pin.

Last edited by R_J; 05-21-2022 at 05:07 PM..
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Old 05-21-2022, 05:14 PM   #3
Gjackson
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Talking Re: Strange Power Supply Behavior on Skyworth 55U5A14G

Thank you!
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Old 05-21-2022, 05:17 PM   #4
dskall
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Default Re: Strange Power Supply Behavior on Skyworth 55U5A14G

So then STB should change state when you push power button? What is ENA for?
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Old 05-21-2022, 05:34 PM   #5
lotas
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Default Re: Strange Power Supply Behavior on Skyworth 55U5A14G

ENA - enable backlight
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Old 05-21-2022, 05:36 PM   #6
Gjackson
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Default Re: Strange Power Supply Behavior on Skyworth 55U5A14G

I see, I had them mixed up. I thought ENA was for the PSU enable not backlight, i will buy a new main board and replace it. The PSU seems fine at this stage
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