"Skype_man" wrote in message
...
Bad connections in the filiament circuit feeding
the picture tube (to light it up). The tiny board
on the neck of the crt tube, or a connection, or
resistor feeding it right by the flyback transformer.



..... or the tubes filament has become intermittantly open circuit.
Heats up and goes open circuit, once open circuit the filament turns off
until it makes contact again and heats up again.

I've had this problem twice in ten years that I can remember.

Use a power supply connected directly to the filament connection on the
tubes neck. Place an ameter in series. Power the tube up and monitor it.