I had the same problem with my own bell & Howell 642.
In my projector a 5W 1000ohm resistor in the amplifier had broken.
Try first to adjust the potentiometer near one of the EL84 tubes, if that doesnt help, try to measure the 5W 1000ohm resistor. Replace it if has gone high. You may need to adjust the potentiometer to make the exciter lamp work.
Atreju
January 2009
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