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 fails, 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.
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