Tuesday, April 10, 2012

A question about safety capacitor replacement

Hi Humphrey
I have another problem with a Sharp CRT TV Model number 19R-M100. Every time I change the horizontal output transistor it immediately gets shorted. The voltage readings are correct at the transistor. I want to change the safety capacitor and or timing capacitor in the horizontal circuit but I don't have the exact value capacitors for replacement. 

Usually Juan when you change the H.O.T it is advisable you change the Safety capacitor also.
If the HOT is getting shorted immediately this could also point for faulty FBT or the Horizontal yoke are shorted.

Do I need to put in the exact value capacitors in this circuit or it could be a little less or it could be a little more in capacitance value?

If you check on the Circuit diagram of any TV model you will notice this component is marked with a small triangle next to it. Usually components marked by this symbol are critical for safety. And therefore for continued safety of the equipment under repair it is recommended to replace these components with only manufactures recommended parts.

This includes exact Capacitance but for the Voltage it should be always same or higher but never below.

One capacitor is 7700pF, 1.6KV; and the other capacitor is 0.47uF, 200V.

In order to see if the 7700pF capacitor can be replaced to the 0.47uF we have to do some mathematics here Juan.

1 uF=1000,000pF therefore 0.47uF=0.47 X 1000,000= 470,000pF

7700pF is too far small compared to 470,000pF.

Now let consider the working voltage of the two capacitors:

We have 1.6KV against 200V

1KV=1000Volts (K stands for Kilo which means 1000 units, in our case units is Volts)

Therefore 1.6KV= 1.6 X 1000=1600Volts

Again when you compare the 1600Volts to 200Volts is far way too small and if you replace this capacitor it will definitely explode immediately.

Finally my take on this is you salvage this component from a junk board if you lucky or you can also make up the value you want be joining two capacitors in parallel (capacitance adding up to around 7700pF) also please note that 7700pF is same as 7n7 so if you get one written 7n7 you can use it.

In order to connect the capacitor in parallel you will also need to make sure their working voltage is 1.6KV or higher for both capacitors.

Thank you very much for your help!


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