Panasonic Television with intermittent single horizontal line
Intermittent problems are mostly caused by dry joints; you can confirm this by hitting the Television gently with bare hand on the sides. This I did and confirmed there were many dry joint because the screen kept on changing.
Sometimes I even managed to get a picture, which was reddish purple in color. I mean this Television has all sort of weird symptoms.
The Television was using I.C LA7837 for the vertical Drive so I decided to Google for the datasheet of this IC and I bumped on this shown in the diagram below.
You know sometimes if you fail to get the circuit diagram of the exact model you are working on you can go round this problem by getting only the data sheet of the suspect I.C and this can go a long way in assisting you to know which pin is which for proper diagnosis.
Very important is the supply (v cc pin) which is very important because a new working I.C from the factory is as good as a bad one if the V CC supply is missing, it’s like trying to drive a new car without gasoline.
That is why it is very important to make sure the supply voltage to any I.C is present and is correct value. So from the data sheet I was able to see that for this particular I.C LA7837 it is feed by three Voltage sources.
Pin 1 is feed with 12 Volt DC; pin 8 and 13 is feed with 24 volts each, Please note that the 12 volts is from the secondary of the power supply and the 24 Volts is from the secondary of the fly back transformer and therefore one can fail and the other remain but still the vertical I.C will not work.
I mean if any of the three supplies is missing the vertical I.C will not work and therefore if you find any of the supply missing be sure to do the voltage testing from that pin backward and find out what is blocking the voltage.
Look for open fusible resistor or filter capacitor for that line has developed high ESR. Usually if capacitor has high ESR causes the supply for that line to drop.
One thing I was sure about is that there was a problem with the vertical circuit. I measured all the components around the vertical I.C and they seem okay according to my meter.
The voltages were also okay to me. I.e. main supply 24volts both sides of the diode, and 12 volts for pin 1.
After doing a lot of re-soldering on all the suspect areas including the main ic which generate the vertical pulses i finally decided to change the vertical ic.
This made the Television come back to life.
Below see the actual vertical circuit of this Panasonic Television
1. Intermittent horizontal line in crt television and monitor is usually caused by dry joint on the Vertical IC or dry joints on the vertical yoke connector
2. Capacitor dried up…try touching the top of electrolytic capacitors around the vertical ic and whichever is getting hot should be replaced.
If all the supply voltage to the vertical ic are okay and still you are experiencing vertical failure symptoms consider replacing the vertical I.C directly.
Thank you guys
Let meet in our next class.