I received an Aucma television assembled here in Kenya in my shop for repair, the screen was blank and therefore I decide to open it for further investigating.
The power ON light was green and therefore I expected the power supply was okay and therefore after opening the set I went straight to the RGB circuit.
Just before I was about to pull my meter to do some voltage testing on this circuit I noticed that the heater light was ON.
Once I saw the heater bulb was ON I kept aside the meter and proceeded to the fly back transformer looking for the screen POT.
I adjusted it slightly and the screen was now full of light, but there were retrace lines across the screen and I guessed I have set the screen voltage too high.
So I continued to adjust the screen voltage down and still I could not solve this problem of retrace lines.
So I decided to continue with checking the voltage at the CRT neck board, I started with the RGB voltages and I noted they are all high.
I checked the voltages at the base of the RGB transistors and I got Zero volts in all the three transistors. This explains why the RGB voltage was high, since there was no bias voltage (base) all the three transistors behaved as if they were open.
I followed back this circuit down to the jungle I.C. The TV was using I.C TDA11105, on this I.C the RGB out were labeled clearly and therefore I decided to test the voltages and still got almost zero volts.
I was not happy with this voltage and I was about to begin blaming the jungle ic as the cause of the problem.
I normally do this as the last my resort and therefore I was careful not to make quick conclusion about this i.c.
While still thinking about what next, I remembered when testing the RGB connector at the neck of the CRT tube I was expecting at the four pins at least the R to be around 2 volts,G=2 volts B=2 volts then zero ground and around 8 volts.
I remembered the 8 volts was reading 4 volts and therefore decided to do some testing on the line.
From the main I.C pin 42 was supposed to be 8 volts and when I measured it I got again 4 volts which is still suspicious.
Usually when voltage drop can be a hint for a shorted component or filter capacitor on that line has developed his ESR and therefore I tested all the Capacitor with an ESR meter and all tested good.
Then I decided to do resistance test along the 8 volts line to see what is blocking this voltage.
When testing across one diode I noted the meter beep on both sides of the diode and to me this was not normal.
Actually I was testing from the bottom side and therefore what made me become curious is while testing for continuity I noticed a beep when the meter was on the live line and when on the ground and this lead me to conclude that there is a shorted component on that line and the ground. (Tip, the best place to pick a shorted line is testing across the filter capacitor between positive (+) and negative (-) leg)… any beep means there is a component shorted on that line.
When I got the beep both sides of the diode highlighted below. I decided to lift one leg out of the circuit and re-test and indeed I still got a beep and therefore concluded that the diode was sorted.
Looking on the circuit board I noted it was not just a diode but a zener diode.
To know the voltage rating with this diode I figured out that since it was for 8 volts source then it must be rated around 8.2 volts and went to the shop to get one since I did not have this in my store.
I replaced the zener diode and applied power and I was happy with what I show on the screen…Back to normal.