CATHODE-RAY TUBE DATASHEETS

 

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How to test CRT

A CRT complete test is a hard and slow task, since a tube tester or a specially built circuit, with the many voltages and signals required for the CRT operation are necessary. Otherwise, there are some simple tests can be performed in order to evaluate a CRT shape readily.

a) There is one or more circular spots in the tube neck, usually near to the base pins. These spots are made during the tube manufacturing, they serve to indicate the internal vacuum state. The spots need to look silvered, shining or a little darkened. If they looks all white, there is air inside the tube and it is unusable.

b) The filament must to show continuity when tested with a ohmmeter or multimeter. The resistance is about 2 to 6 ohms.

c) The screen must appear with no risks, spots or failures in the phosphor. Burns in the fluorescent surface are permanent, and have a brown color.

A tube approved in these three tests has a little chance to have more defects. A last test permits to evaluate the cathode emission :

d) Supplying the filament with your nominal voltage, a little cathode-control grid voltage appears. This voltage causes a grid-cathode leak current if a micro ammeter is connected to the grid and the cathode pins. Notice that only the CRT filament is energized .

The grid-cathode leak current is cathode-emission related. The heating expels electrons from the cathode surface, creating an electron cloud around the cathode. Since the control grid is very near, the more accelerated electrons can reach it, and the grid results negative in respect to cathode. Wiring grid and cathode externally, an electron current appears, as high as the cathode emission. A good tube has a leak current in the range of 50-100 uA after some minutes of pre warm.

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