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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