Filament Voltage Test.
When replacing either the flyback transformer or the video board, the filament voltage may not be correct.
Measuring the filament voltage is not accurate using a true RMS voltage meter, because of the high frequency components, which make up the filament voltage. An oscilloscope, with RMS capability, may be used to measure the filament voltage.
Although a visual check of the filament color is a indicator of the filament voltage, it is a good practice to check the filament voltage if there is any doubt about this important monitor parameter.
The following filament voltage test is an accurate method of finding the true RMS voltage to the filament. This is accomplished by comparing the light output of the filament when it is driven by the monitor to the light output of the filament with an applied DC voltage using a loaded photocell.
SETUP: |
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470Ω , CF |
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FR205 |
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.033uF, 854 | CF |
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100K, CF | 855 |
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To find the filament voltage;
1.Solder two short buss wires to the filament pins to clip on.
2.Use black tape to secure the photocell over the hole in the plastic CRT socket connector. Caution: Be sure not to move the photocell between the two tests.
3.Record the DC voltage output from the photocell with the monitor running normally. The monitor should be powered up for 10 minutes before making this measurement.
4.Turn off the monitor.
5.Connect the variable voltage, 1 amp, DC power supply to the filament. The negative lead to the filament ground at the CRT socket pin 10.
6.Adjust the power supply voltage for the photocell reading, recorded in step 3.
7.The equivalent filament RMS voltage is now recorded by measuring the DC voltage at the CRT socket pins 9 and 10.
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