The following assumes that the picture is fine but the brightness is
fixed - probably at too high a level. However, there could be several
interrelated problems if a common supply voltage were missing, for example.
If it is a knob, then it should be varying the control grid (G1) voltages
relative to the cathodes (K) of the CRT. This is not likely to be a very
complex circuit. If you do not have a schematic, I would start by tracing
from the control, check continuity and solder connections. Check the
control itself for proper operation with an ohmmeter. A power supply going
to one side of the control (negative probably) may be missing. Tbe control
grid voltage will end up on the little board on the neck of the CRT - check
there as well for bad solder connections or open resistors.
If brightness is a digital control, then you will need a schematic unless
there is an obvious bad connection.
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