Operating and Technical Descriptions
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8.11. Magnetic Focus
A current regulator, V105, Q113, is used to supply the focus current,
the value of which is set by control RV116.
8.12. Target Discharge
Under adverse overload conditions it is possible that the tube target
may become positively charged and the tube cease to function. To
alleviate this condition a discharge sequence operates every time the
equipment is switched on. G4, and hence G3 (via D120), is held at 9V
for 5 seconds and then charges to its normal operating value with a
time constant of 270ms.
8.13. G4 Monitoring
A jack socket (P108) is wired in series with the G4 supply so that the
electrode current may be monitored when setting up the tube.
8.14. Internal Monitor
A miniature CRT is used to provide a self-contained monitor viewed
through a magnifying lens. The monitor has its own field scan circuit
while the line scan and EHT transformer circuits share drive waveforms
with the tube. A moulded Cockcroft-Walton multiplier produces 4KV
from the EHT transformer output waveform.
A secondary winding of the EHT transformer also provides low voltage
supplies for monitor field scan, line shift, alignments and battery
indicator.
The video output of the equipment is displayed on the monitor by
modulation of the control grid G1. RV113 is the gain (contrast) control
and RV107 the dc level (brightness) control. The focus is set by
RV105.
8.15. Automatic Iris Control
The circuit is designed to close the lens iris when continual overloads
appear on the video signal.
Video peaks are sensed and compared with an average video level
taken from the alternate field balancing circuit. When the differential
signal exceeds a threshold, pre-set by RV118, the aperture is reduced
by the motor driven iris until the overload is removed.
For large overloads a rapid shutdown mechanism operates on a
second threshold via Q117 and Q118 and the iris shuts down to a fully
closed position. Limit switches operate at the extremes of the iris
movement position.
8.16. Desiccator
A desiccator with an integral indicator is fitted to the back plate lying
over the video board. This is normally blue and turns pink when
moisture is present.
