[Mondial]

Terry, the RF signal is used to power the lamp in the pickup head. It can't be powered by AC because the photocell would pick up the 60 cycle hum and low voltage high current filtered DC was hard to obtain. So the solution was to power the lamp with RF from the oscillator operating from HV filtered DC. Therefore no hum from the lamp.

I believe the RF osc freq adjustment sets the output level and therefore the brightness of the lamp.