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Ceramic Disc Capacitor-How to accurately test it.
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Author: Jestine Yong
Topic: Technology
Viewed: 83 time(s)
[ Not Rated Yet ]
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The last article i mentioned about electrolytic capacitor
breakdown when under load. In this article i will talk about the
high voltage resin coated ceramic disc capacitor. In computer
monitor, it is frequent used in the screen voltage (G2) line at
the crt board. When it fail, it can cause erratic or
intermittently raster bright and dim and sometimes no display.
Adjusting the flyback transformer screen control will not solve
this problem. You can isolate the problem as whether if it cause
by a defective fly back or G2 line at crt board. Remove the G2
wire from circuit and measure the screen voltage with a digital
multimeter- it should have a constant voltage (will not vary).
If the voltage is not stable suspect the fly back. If it ok then
trace the screen voltage circuit. Usually this circuit consist
of only resistors and capacitors. Sometimes a defective crt
socket might cause this type of problem also!
Many times a tech can't solve this intermittent or no display
problem is because their digital capacitance and ohmmeter
measured ok on the capacitors. The capacitors in the screen
voltage line are usually rated of 102,103, and 472 1kv to 2kv. A
'missed' in checking this capacitors will caused the technician
can't solve the problem, unless he direct replace the capacitor.
An intermittent capacitor can pull down the screen voltage
causing the display problem. As mentioned earlier, an ohmmeter
with 12v output and a digital capacitance with 3v output are
unable to accurately check the capacitor that have the rating of
1-2 kilovolt!
If you have the analog insulation tester or meter, the panel
will show a short circuit when certain voltage are pump in to
check the high voltage capacitor. The voltages that you can
select is depends on the brand or model you buy. Some have the
range of 50v to 1000v and some have the range from 100v to 500v.
It is optional whether you should have one. If you have one then
it would be an added advantage for you. The other option that
you have is to direct replace the suspected capacitor. In my
country, you can get a new one in less than US150.00 or you may
bid a used unit from eBay.
As for me, checking the small blue resin coated ceramic disc
capacitor that have the capacitor code of 104 50v, besides using
digital capacitance meter to check for the value, i also use an
analog meter set to times 10Kohm that have the output of 12v to
check this type of capacitor. Many times it will revealed the
bad intermittent capacitor. You will be shocked to see the
capacitance meter checked ok but show a shorted reading when
check with analog multimeter. In the market there is certain
brand of analog multimeter that have the range of 100kohm. If
you open up the cover, you can't see the 9v battery in it, what
you see only the 2 pieces of 1.5v battery. This type of meter
can't accurately detect the short in the capacitor because the
output voltage is only 3volt!
For those of you who involved in repairing monitor, you will
agree with me that checking the flyback internal capacitance-
quite often you get the right value 2.7nf with digital
capacitance meter but when you switch on the monitor, you will
hear a 'tick-tick' sound from the flyback. This is because the
capacitor is breaking down when under load. If you did not do
anything with the flyback, gradually the internal capacitance
will become shorted and you will need to refurbish the flyback.
About the author:
Jestine Yong is a electronic repairer and a writer. For more
electronic repair information please visit his website at
http://www.noahtec.com/electronic-repair-articles.htm
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