Repair And Test Of LED TV Power Supply Board Without Connecting To A TV
In the E-book written by Kent Liew – Smart-OLED/LED/LCD TV Repair Tips – V6, chapter: “Another New Tool to easily to do the PSU self-test with monitoring feature”, guides the reader on how testing can be performed on a PSU without having to connect to the TV. This test can confirm the condition of the PSU on whether it is working or it is faulty. With the knowledge gained from Jestine Yong to repair the PSU, I took on this challenge to begin the repair. I decided to repair a faulty Power Supply Unit (PSU) on hand to test the method specified in the E-Book.
I perused the web and managed to obtain the circuit diagram for the PSU as depicted in Figure A.
This test board as shown in Figure B, can be bought from AliExpress.
The PSU tester has built-in load resistors (3 X 5W) to simulate connection to LCD/LED TV, which will switch the PSU on if connected.
Visual checks confirmed that there were no burnt parts. When the PSU was connected to the tester, the tester did not turn on, indicating that the PSU was faulty. According to the label in the PSU output side, the stand-by volt is 5VDC marked as 5VSB (Voltage Standby). This voltage is supplied to the mainboard of the TV. Without this voltage the TV will not turn on.
Upon checking the primary supply towards Pin 1 and 3 of T401-A (Circle in RED), 330V was noted and this proof that this voltage was not boosted. This confirmed that the primary side of the PSU was working well except that the 330 VDC was not boosted. This means the PFC circuit was not working due to the missing 12 volt DC (12 VSB) from the main power supply (look at figure E). When checking the Schottky Barrier Diodes, it came to my attention that the bottom diode (D403 – Circle in ORANGE) was shorted as displayed in Figure C.
After replacing the faulty diode, the fault remained. The ring tester confirmed that the transformer is in working order (Full lights). This test was performed at pin 1 and 3 of T401-A without applying power to the PSU as shown in Figure D.
Examining the PFC circuit, I realized the PFC IC U201 pin 8 (VCC) had no voltage supply as shown in Figure E.
Following the VCC_PFC supply line, the VCC_PFC supply to U201 as portrayed in Figure F, was noted that it was coming from the below circuit:
The Drain (D) pin receiving 330 VDC steps down the voltage through built in regulation circuit (5.75 volt) and this means this IC does not need a start up resistor. This is depicted in Figure G.
Suspecting a faulty IC U401 (no switching) -figure H, I replaced the faulty LNK6767 with a LNK6777V as it was the direct substitute for LNK6767 available for replacement in the market. This replacement IC resulted in the 12 volt DC output (12 VSB) return.
However, there was still no standby voltage of 5VSB. Upon tracing the voltage lines of the DC-to-DC converting circuit, it came to my knowledge that the 5VSB is produced by IC U501 (MP1584EN), a high frequency step-down switching regulator with an integrated internal high-side high voltage power MOSFET. This can be observed in Figure I as shown below.
When probing, I realized that Pin 7 of U501 was receiving the 12VSB but there was no voltage output at pin 1. I proceeded to replace the U501 which resulted in the return of the 5VSB.
Finally, I tested the repaired board with load using the PSU tester as shown in Figure I. Connecting the repaired board to the tester showed that the PSU is fully functional.
With the guidance of Jestine Yong, I was able to complete this repair within a week, a record timing for me! I hope it was an informative read for you. Thank you for reading thus far. Do try this approach should you encounter a power supply unit without a TV for testing.
This article was prepared for you by Michael Selvam from Singapore, a 57 year old zealous electronic enthusiast. As a passionate electronic repair hobbyist, he tinkers with home entertainment systems and any faulty electronic equipment he gets his hands on. He obtained his knowledge to repair by reading books and took up the electronic courses I conducted. At present, he is employed as a Cluster Manager (Property Management) with CapitaLand and does his repair whenever time permits.
From Jestine: For your information, Michael was my training student and you can check out his training photo HERE.
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You may check on his previous repair article below:
https://jestineyong.com/repair-of-philips-led-smart-tv-55pft6100-98/
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Robert Calk Jr.
November 20, 2019 at 1:07 pm
Good job, Michael!
Shaylyn
November 20, 2019 at 1:37 pm
Wow a very well explained process! Nice work!
Vic
November 20, 2019 at 2:41 pm
Can you give more information about the PSU tester.
I would like the brand and model number. I was not able to find this on the Alibaba website.
Thank you for your help.
Michael
November 21, 2019 at 3:04 pm
You can visit Aliexpress at
https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20191120230216&SearchText=Power+Supply+Board+Tester+Repair+Tool+
Albert van Bemmelen
November 20, 2019 at 2:44 pm
Superb first-class repair with all steps in clear detail explained! And on top of that also an excellent read Michael! You probably did not really needed that new PSU test board at all, since you could have done it with the Blue Ring tester and a Digital meter all the same.
Milan
November 20, 2019 at 5:54 pm
Very good explanation
Shawn
November 20, 2019 at 7:30 pm
A Brilliant Read with extremely wonderful detailing. Keep up the Good Work 🙂
Yogesh Panchal
November 20, 2019 at 10:05 pm
Good job! Michael
Lynn Blakely
November 21, 2019 at 12:14 am
Great Job Michael: The use and knowledge to find the schemaiic, is a necessity in many cases. You really covered a lot of the parts that failed by showing portions of the schematic where we could read them and visually see what you were telling us in the explanations you were giving. Excellent job and great detailed pictures. The test equipment can tell us a part is bad. It is not always telling us the truth when it says it is good. The PSU tester serves a purpose like a volt meter. It does make it easier to plug in different cables to then show the voltages.
Devon Wilson
November 21, 2019 at 9:17 am
Greta job Michael. Thanks for sharing, for sure the details you provided are quite informative. Keep up the great work.
Waleed Rishmawi
November 21, 2019 at 3:14 pm
a job well done. it seems like you know your way around in the power supply so I totally agree with Albert, you do not need that power supply tester...you already have the skills needed for the repair. keep up the good work and have a blessed day
Mason Sarles
November 21, 2019 at 9:29 pm
Michael. Great job. Logical troubleshooting.
Thank you for Sharing,
Mason Sarles
TAYO
November 22, 2019 at 10:34 am
I wish i was this good...
Parasuraman S
November 23, 2019 at 6:26 pm
Logical solutions done in a professional way! Well done and many thanks for sharing!
Justice
November 26, 2019 at 8:21 pm
Michael this is GREAT article !!!!
Andrea Del Corso
November 29, 2019 at 2:05 am
Great article,compliment!