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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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