The important of checking errors done in the previous repair

On your line of duty as a technician sometimes you will receive equipment brought to you which have gone through another technician and was not able to repair due to lack of spare parts or he/she did not have what it takes to sort out that problem.

As a technician it is very important first to scan the whole board noting down any component which seem to have replaced and confirm indeed it was replaced with the same (exact) component.

The reason I am saying this is because sometimes most of the spare part Shop attendant don’t have good knowledge in electronics components and therefore if they find that they don’t have the exact component they usually suggest to you an equivalent which according to them they think can work but when you look up for this component datasheet you will find that even they are not related.

I had this experience one time I received a Television motherboard from a friend who lives upcountry and I decided to make a try because according to him all technicians from his home town were not able to repair his Television.

I scanned the primary power supply section and noted there was a replacement done on the switching transistor (S.O.T) but I didn’t bother to confirm if it is the exact replacement and so I continued to troubleshoot the board.

I tested all the components on the primary side and they all tested okay according to my meter but still the power supply was not starting, the voltage was reaching the switching transistor collector pin and there was also voltage at the base of this transistor but still nothing is happening.

So I decided to check on the internet the Specification of this new installed transistor “ D2499” and this was the specification” Horizontal deflection output for color Television”

So it means even if I found this transistor was once again faulty and change it with the same new one still I would not have repaired this set, so I replaced this transistor with “D1710C” and the Television started but shutdown immediately(I could hear the relay click)

When I used the dummy load (60 Watt) bulb on the B+ line I noted the bulb came up with intense light. I noted the Television was having the B+ adjust Pre-set and adjusted it slowly and I powered on again and this time the dummy load bulb was well lit and I was able to measure the B+ voltage and set it to 110 Volts according to the working B+ voltage of that Tv.

From that day I leant the hard way the important of confirming first whatever replacement done previously are actually okay and meet the specification required for that circuit.

Another common mistake is replacing the secondary diodes with ordinary diode, please note that the secondary diode are fast switching and therefore cannot be replaced with the ordinary 1Nxxxx series.

With that thank you guys

See you in the next class

Humphrey Kimathi

Author CRT television repair course.