Open Circuit In Heater (Filament) Of CRT Tube
After so many CRT Monitors sent in for repair, i could say that i only came across one unit of a 14″ Acer Monitor that had an open winding in the filament (heater). The complaint was no display but have high voltage and i found that the heater was present but there was no glow in the filament. Checking the filament with an ohmmeter revealed that in fact the filament already had an open circuit internally. This is very uncommon and normally the filament is quite robust.
Do you know that our filament could take about 10 to 11 volt? A normal tube is using 6.3 volt dc and by increasing the heater voltage one can bring back a tube to life but again it depends on how severe is the tube. I had done an experiment before many years ago where i deliberately increase the heater voltage to see what voltage the filament could withstand. I tested it by using my adjustable power suppl. Increasing the voltage up to 11 volt seems to be still okay but when i set to 12 volt dc, the filament immediately break.
Now, if you ever come across any tube that have dim display and you have confirmed that the tube already weak then modify a circuit that can only go up to 10 volt. 11 volt could be marginal and the best is from 8 to 10 volt. 12 volt will surely burn up the filament. If you have the time and a bad tube (winding shorted and etc) then you can do the experiment yourself and see how is the outcome. You will see that the filament will glow extra bright before it burn out.