electronic repair
 

Testing Ceramic Capacitor


The last article i talked about electrolytic capacitors breakdown when under full operating voltage. In this article i will mention about the resin high voltage  coated ceramic disc capacitor. In computer monitor, it is quite often used in the screen voltage (G2) line at the crt color board. When it spoil, it can cause erratic or intermittently raster bright and dim and sometimes will cause no display symptom due to G2 voltage being pulled down to very low voltage. Adjusting the flyback transformer G2 control will not solve this problem. You can solve the problem as whether if it cause by a defective flyback or G2 line at crt board. Remove the G2 cable from circuit and check the screen voltage with a digital multimeter- it should have a steady voltage (constant). If the voltage is not constant (vary) suspect the flyback. If it ok then trace the screen voltage circuit. Usually this circuit consist of only resistor and capacitors. Sometimes a defective crt socket might cause to have erratic problem.


Many times a technician can't solve this intermittent or no display problem is because their digital capacitance and ohmmeter checked ok on the capacitors. The capacitors in the screen voltage line are usually rated of 10nf, 22nf, and 47nf 1kv to 2kv. A 'missed' in testing this capacitors will caused the technician can't repair the problem, unless he direct replace the capacitor. An intermittent capacitor can pull down the screen voltage causing the no picture or erractic display problem. As mentioned earlier, an ohmmeter with 12volt output and a digital capacitance with 3volt output are unable to accurately test the capacitor that have the rating of 1-2 kilovolt! If you have the analogue insulation 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 name or model you buy. Some have the range of 100v to 5000v and some have the range from 50v to 500v. It is your choice whether you should have one. If you have one then it would be an added advantage for you when checking an intermittent capacitor. The other option that you have is to straight away replace the suspected capacitor. In my country, you can get a new insulation tester in less than US150.00 or you may buy one from Ebay. Not only ebay sells insulation tester, you will be amazed of some of the tester that they sell you've never seen before.

To 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 capacitance value, i also use an analogue meter set to the range of times 10Kohm so that it have the output of 12volt  to check this type of  capacitor. Most of the time the meter will revealed the bad intermittent capacitor. You will be shocked to see that the digital capacitance meter checked ok but showed a shorted reading when test with analogue multimeter. In the market there is certain brand of analogue multimeter that have the range of 100kiloohm. If you open up the cover, you can't see the 9v battery in it, what you see is only the 2 pieces of 1.5volt battery. This type of multimeter 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 3.0 nanofarad with digital capacitance tester but when you switch on the monitor, you will hear a 'tick-tick' sound from the flyback transformer. This is because the capacitor is breaking down when under full operating voltage. If you don't do anything about it, gradually the internal capacitance in the flyback transformer will become shorted and you will need to refurbish the flyback. Just 'play' around with some of the capacitors using different type of tester and you will definetely gain some knowledge on how to successfully test and check a ceramic capacitor.