LG Television NO PICTURE- AUDIO OK blue background

This is a LG television made in Egypt (or is it made or assembled in Egypt) and the customer complain was No picture but audio is okay.

So I went down to his residence to check on it…remember most of my repairs are done on the site and therefore I normally do some preparation for the task ahead of the time.

In this assignment I had prepared myself with the basic equipment plus a circuit diagram of the LG television (any) because mostly TV circuit diagram for a particular model are very similar in terms of major circuit modules.

In this case from the symptoms given by the owner via the phone I was very sure the problem was likely to be on the CRT circuit board, so I went down to his place and found he was not around but the houseboy was around to assists me with what I wanted.

Asking him about the television he told me that there was another technician there and I wanted to know how far he went into the television even before I opened it and he confided with me the technician came and after a keen observation on the screen for about ten minutes without his eyes blinking he told them that the problem is new in the world and therefore he sees no hope on the television and therefore the best place for it is in the dustbin or make it a toy for his children.

My friend that is why it is very important always to get a second opinion even in the medical world. I mean if your doctor tell you that your leg need to be amputated (God forbid) please ask him/her for some times and try another doctor to hear his opinion.

For me I was very happy with this technician because he was honest with himself and never attempted to repair it because he could have done even more mess. So I opened the set and went straight to the small CRT circuit (RGB)

I did a voltage testing on the points highlighted and filled the figures (voltages) on the sheet of circuit diagram I had carried with me as shown Below.

 Now let us analyze the fact at hand…in electronics we deal with facts, in our case at hand the facts are voltage measurements and being voltages they are represented by figures and as you know figures never tell lies and therefore when need to deal with this truth.

From the figures and from what my electronics teacher told me about the RGB circuit I was not happy with the figures for the BLUE cathode at the CRT pins connector which was at 65 volts…as a rule of thumb the voltage at this point for the three cathode should be nearly the same with an margin of plus or minus 5 volts between the three cathodes and therefore the 65 volts on the blue cathode is first suspect.

So I tried to figure out why this voltage is very low compared to others , looking at the voltages on the B-C-E of the Blue transistor amplifier I noted that it has 4 Volts at the BASE and 4 volts at the EMITTER..

Again I am not happy with this because the Voltage drop across the base-emitter of a transistor should be between 0.3volts to 0.7 volts for silicon transistors but in this case was zero.

Did I say zero? Yes this means the voltage entering the base is 4 volts and coming out at emitter is 4 volts
again…how can it be?

The only reasoning is that the transistor is shorted base-emitter junction and hence behaving like a jumper across the base and emitter.

So I decided to remove this transistor out of the circuit completely and actually I was able to get a picture but missing the blue color and therefore the screen was yellowish..THE PRODUCT OF RED AND GREEN.

With that I concluded that the blue amplifier transistor is actually the culprit and having a few of these transistor at all times I rushed to the workshop and back with two pieces of the type and replaced it.

I replaced the transistor and the picture was back to normal.

Thank you guys

Let meet in the next class.