Phillips TV Dead model 20G X1552
Background: This TV was taken to another technician and was not able to repair. Usually equipment from other technicians are difficult to repair. Initial observations, Cover Bolts missing only two were fitted, opened the set and scanned the board and noticed the Posistor was also missing; main fuse seems to have been replaced. No sign of shorted component, what could have caused the fuse to be replaced?
After that initial test I decided to give this TV a try and I took the motherboard with me to my workshop. On my workbench I connected the board to the power supply via the series bulb, the bulb light briefly and went off, this lead me to conclude that there is no shorted component on the supply.
I used the fly-back tester to ring the S.M.P.S transformer, one probe on the main filter capacitor positive pin and the other on the collector/drain pin of the S.O.T. All the LEDs light and this confirmed to me that there is no short on the primary and also the secondary diodes are not shorted.
Also did a ring to the fly-back transformer and all the LEDs light and again no problem here. Then I applied power and did some voltage testing, first across the main capacitor and found 320vdc and the drain of the S.O.T was also 320Vdc. I checked the voltage at the gate/base of the S.O.T and got zero (0) volts. I was not happy with this reading; I was expecting around 0.6Vdc but have nothing.
I tried to check back from the base pin trying to find the start-up resistors. I could not get the path and being late at night, I decided to go to sleep and continue the following day. Come the following day at 6.00 am before going to work I picked the motherboard again and took it outside. While there I noticed a small separate circuit but still part of the power supply and I became interested with what it is doing there in the first place.
Luckily it was detachable and I easily removed it from the main board. Scanning around this small board I noticed an I.C and checking the part number of the I.C found it was IC UC3842BM. This is a very common I.C found in S.M.P.S where it is used as control I.C and therefore decided to scan the surrounding component to this I.C. There were two small electrolytic capacitors and I decided to test them using an ESR meter and the result I got shocked me.
Capacitor 47uF/25V ESR was very high and the other was quite high. After picking these two capacitors I decided to Google for this Circuit to see the important of these capacitors and this is what I got.
As you can see from the circuit diagram, Capacitor 2506(47uF/25v) is used to filter the start-up voltage which is very important for the control I.C UC3842BM and therefore once this capacitor dried the supply also died and hence the power supply appeared dead. After replacing these two capacitors, this restored back the power supply to normal.
I think the technician who was working on this set previously did not have an ESR meter otherwise he could have picked up these capacitors easily.
Let me also point out here that this capacitor has a high failure rate and therefore if you don’t have an ESR meter always replace this capacitor(start-up filter capacitor directly)
Thank you friends
Humphrey Kimathi is from Nairobi Kenya and the author of CRT Troubleshooting guide, DVD Player repair guide, CRT Television repair course and Basic Electronics course. He is also a blogger at Electronicsrepairmadeasy.com
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Note: You may check out his previous repair article below:
https://jestineyong.com/14inch-lg-television-dead-now-repaired/
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Justice (City of Durban --South Africa)
August 14, 2018 at 1:34 pm
Kimathi wow what a great job. Keep up good work, very informative.
Humphrey Mndaka
August 15, 2018 at 2:20 pm
Thank you Justice for your feedback, glad you have found the info. valuable
Regards Humphrey
James
August 14, 2018 at 2:06 pm
Good stuff yo'
Humphrey Mndaka
August 15, 2018 at 2:21 pm
Thank and welcome James.
Regards Humphrey
Albert van Bemmelen
August 14, 2018 at 2:46 pm
A pro at work! The way you explained using the fly-back tester to ring the S.M.P.S transformer was not only technical correct but also poëtic music to my ears.
Humphrey Mndaka
August 15, 2018 at 2:17 pm
Oh! Albert, This is very encouraging... Thanks you.
Regards Humphrey
Parasuraman
August 14, 2018 at 3:59 pm
Excellent and very professional and systematic approach and trouble shooting. Great job there!
Humphrey Mndaka
August 15, 2018 at 2:28 pm
Humbled by your remarks Parasuraman, Thank you and more power!
Regards Humphrey
Robert Calk Jr.
August 14, 2018 at 4:13 pm
Good job, Humphrey. I can't imagine a tech without an ESR meter.
Humphrey Mndaka
August 15, 2018 at 2:39 pm
Thanks Robert, An ESR meter is not a nice thing to have but a MUST thing to have for any technician.
Regards Humphrey
Yogesh Panchal
August 15, 2018 at 12:47 am
Good Job! sir.
Humphrey Mndaka
August 15, 2018 at 2:41 pm
Thanks Yogesh, wishing you all the best.
Regards Humphrey
Mihai
August 15, 2018 at 12:51 am
Good job sir ! Thanks for sharing your experience with us.
Humphrey Mndaka
August 15, 2018 at 2:43 pm
Thank you Mihai, keep learning!
Regards Humphrey
Humberto
August 16, 2018 at 12:11 am
Good job Humphrey. By the way, I own my own Blue ESR Meter and this tool has saved me in many repairs, an useful toll.
Humphrey Mndaka
August 16, 2018 at 4:41 pm
Thank you for your feedback Humberto, keep going!
Regards Humphrey
Hakan
August 16, 2018 at 2:22 am
What is S.O.T?
Humphrey Mndaka
August 16, 2018 at 4:38 pm
Hi Hakan, This is the transistor which switch the chopper transformer(Primary side of the S.M.P.S), also referred to as Chopper transistor.
Regard Humphrey
Francis Kuuderb
August 20, 2018 at 2:34 pm
Mr.Humphey.As an professional repairer , you definatelly need to own an ESR meter to make your easier for you.But don' t have one .How much will it cost me to own since i don't have a paypal account.
Humphrey Mndaka
August 20, 2018 at 8:12 pm
Hi Francis,
You can buy one from the link below..
https://jestineyong.com/blue-meters/
Am sure the seller accept western union or moneygram
All the best
Humphrey
Thabang
February 2, 2024 at 12:17 am
Hi I need the value of the resistor 3506 on this circuit its burned out