I customer came to my shop and asked if I can check out his television with went dead yesterday while watching news.
He went back to his house and after ten minutes he was back with the said television.
I plugged the TV via my series bulb to confirm what he said and indeed the TV was dead.
Looking at the fuse I noticed it was not there, further checking I noticed the TV was from another technician and after repairing decided to use a just wire instead of actual fuse.
This friend is bad practice and should be discouraged because a fuse in an important safety component.
Also I noticed the previous repairer has improvised the supply with a universal power supply kit.
Since I noted he/she has not removed any components from the primary supply I decided to repair the original supply and therefore I removed the improvised power supply.
The first thing I replaced was the switching transistor and with the switching transistor in circuit, I decided to apply the power again using the serried bulb and the bulb went bright briefly and then off-This is what is expected if the supply has no shorted component.
I decided to do some voltage testing on the primary side and the best place to start was the voltage across the main capacitor and I got 290 vdc, this is not bad in our country because we use 240 volts and therefore decided to continue with my troubleshooting.
I decided to start with testing for open resistors since I was not expecting any shorted component according to my initial testing using the series bulb.
From my experience the most common resistors to open are low ohm/high wattage resistors and therefore they are big in size and I decided to start with them.
As you can see these are very few on the power supply and I tested them very quickly and got one which was open.
I also noted that it was connected to the emitter of the Switching transistor. This resistor is called current sense resistor and very important component on the power supply.
Usually if you find this resistor open expect the switching transistor to have shorted also, therefore whenever you change the switching transistor be sure to change this resistor also.
On the circuit board it was referenced as R510, kindly see the diagram below.
After replacing this resistor I applied power again to see the effect it has and still the set appeared dead.
I decided to do voltage testing again and this time I decided to test the voltage on the Collector (middle) leg and the base (Left) leg of the switching transistor and I got 290 at the middle leg and zero at the base.
For this transistor to switch you must have these two voltages, the one at the base is called start up voltage… Act like the actual switch used to switch on electricity in your house.
To trace what is blocking this voltage I started testing the voltage at the middle of the start up resistors…in this TV they were two and testing at the middle of the two resistors I got 146 volts and the other side of this resistor toward the base of the switching transistor I got zero volts.
I lifted one leg of the second resistor and the voltage increased to 290 Volts.
With that I concluded that there must be a shorted component on that path which I taking that voltage to the ground.
I suspected transistor V515 and V516 and I removed them completely from the circuit and testing again I noted the same problem was still there.
With that I decided to check out the diodes which common failure mode is shorting.
Again the diode on the power supply are very few and after testing a few in circuit with my digital meter set to diode range I noted one which has reading on both sides of my meter probes.
I soldered one leg and re-tested and again got two reading, with that I confirmed the diode reference D515 was indeed shorted and needed a replaced.
I got one from a junk board around, I installed it and applied power to the TV was up and running.
see you in the next class