First I checked the status of the fuse and surge limiter. The fuse passed the test with flying colors and the surge limiter did not make-it was Gone according to my resistance test meter.
Now whenever you find the fuse or the Surge limiter gone as I have said it before, let me repeat it for those who are new in my class Please never replace any of these two components without checking the cause why they had to blow unless you want to keep the manufacturer of these two safety components in the business by buying their products.
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Therefore if you find the fuse or surge limiter has gone suspect a heavy short on the primary power supply components. On the high list of suspect is the chopper transistor- So I went straight to this transistor and sure it was dead short collector-emitter junction.
I replaced the chopper transistor and the surge limiter and because I still suspected some more short on the supply I put the television on using the series current limiter bulb.( Those who have already bought my CRT TV repair course e-book knows how valuable this small gadget is in terms of saving your new replaced components from shorting immediately after replacement).
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On power ON the series bulb never lit, I checked the voltage at the collector of the chopper transistor and was present, around 300 vdc but none at the base.
If you find no Voltage at the base two things may have happened.
1. Either there is an open component on the start-up path usually the start-up resistor or dry joints or even circuit broken.
2. There is a shorted component at the base of chopper transistor which is taking the chopper transistor base directly to the ground. See diagram Transistor 2SC3807 or capacitor across this transistor
In most cases it is the transistor which gets shorted and therefore starts by testing the condition of this transistor c3807 and finally the cap517.
You can even lift up the middle leg of this transistor c3807 and if you can now get some voltage at the base of chopper transistor (v513) then you can conclude that this transistor c3807 is shorted.
In my case I set the meter on the diode test (continuity) and tested between the two legs of capacitor c517 and the meter buzzer rang indicating to me that there is a shorted component across that path.
I removed one leg of capacitor c517 off the circuit and still I had a short reading on the meter. Finally I removed transistor v512 (c3807) and the short disappeared indicating to me that this transistor was also shorted.
I replaced the transistor and powered the set and the power supply was restored.
But still there was a problem because the TV was coming up and down. I was still using the series bulb and whenever the Television was shutting down the bulbs were very bright indicating some problem on the secondary side.
Then I decided to tap the set a bit on the side with my hands to see if there were any dry joints remaining but nothing happened.
Then I decided to tap the CRT neck board circuit with the back of the screw driver and I noticed the screen kept on changing. I checked it keenly and I noticed a lot of dry joints there too.
So what has this to do with the narrow screen? This is because the RGB amplifier voltage source is coming directly from the primary supply and hence a problem on the neck board circuit can even cause the power supply to drop or even block it completely.
So a Technician who is ignorant of how that Television work may spend up to two weeks looking for a faulty components on the power supply whereas the problem is at far end on the small CRT neck board circuit and end up being frustrated enough to send the Television back to the owner with the very unpleasant word to the Customer” Not able to repair due to lack of spare parts”
After re soldering the CRT neck board the screen was now full width and there was sound but no picture…..To be continued…..
Wishing you all the best Guys
Let meet in our next class
Let meet in our next class