14 inch China Television sound OK but no picture

I got this phone call from a client who wanted his television checked by a technician, I passed by and I found the TV was ON but there was no picture on the screen (dark screen) but audio was present and very clear.

I opened the TV on site and the board was covered with one inch thick layer of dust and therefore decided not to disturb it lest it cause havoc in the customer house.

But I did some scanning using my set of natural tools in the name of eyes and I was able to see two big capacitor with top slightly swollen.

Usually in electronics we start from known to unknown, don’t strain a lot by thinking hard how to solve the problem but rather start with rectifying the obvious things before cracking your brain trying to figure out why the equipment is not behaving.

In most cases dark screen can be cause by screen voltage being very low, the RGB amplifier voltage being very high or there is no bias voltage for the three RGB amplifier transistors and therefore this was the place I was to concentrate first but since I have seen these two capacitors which were swollen I decided to replace them before continuing.

The two capacitors were rated 47uF/160V and there reference on the circuit board was C561 (47uF/160V) and C562 (47uF/160V).

Although one capacitor was located immediately after the secondary output and the other near the fly-back transformer I was able to notice from the bottom side that the two capacitors are related and also in parallel.

From my spare kit I was not able to get 47uF/160V capacitor for replacement and therefore decided to use one 100uF/160 volts capacitor which was in my shop.

So why use one 100uf/160Volts instead of two 47uF/160v capacitors? As I have said above the two capacitors were in parallel and from basic electronics I know that capacitors in parallel we add the capacitance and therefore the two 47uF capacitor will add up to 94uF total capacitance.

Therefore using one 100uF/160volts capacitor will serve the purpose, after replacement I decided to apply the power to see the improvement and to my surprise I got the screen was full.

I connected the Television to my DVB-T2 Set Top Box and I was not happy with the picture, the picture was reddish and lacking sharpness like when we have problem with the focus. I adjusted the focus knob on the fly-back and nothing changed.

I scanned the small RGB amplifier circuit at the back of the CRT neck board and I found lots of dry joints around the circuit and I soldered them thoroughly.

Still no much improvement and since the screen color has too much red I decided to touch the three RGB transistors with my finger with power off after applying power for a few minutes and I noted two out three RGB amplifier transistors were getting too hot and I decided to replace them directly since I had spares for the same.

After replacing the two transistors I applied the power again and now the picture was very clear and no blurred image like before and with that the Television was back to normal. 

Thank you guys

See you in the next class