Operating and Technical Descriptions
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6.
COMPLETE SET UP FOR NEW TUBE
6.l.
Replacing Tube
See Appendix 2.
6.2.
Gl and Pedestal Control Adjustments
These controls are adjusted using the values of I G4 (BEAM) and I G4
(BEAM + PEDESTAL) pre-determined for each tube during factory test
to obtain a specific value of pedestal current. Using the jack lead and a
micro-ammeter, plug the jack into the Ig4 socket (Scan board). Turn
the pedestal amplitude control (Scan board RV110), fully clockwise
[anticlockwise SMT boards] for minimum Ig4; adjust the G1 control
(Scan board RV111), for recommended I G4 (BEAM) value and then
increase the reading to the recommended I G4 (BEAM + PEDESTAL)
value using the pedestal amplitude control.
Typical Values:
I G4 (BEAM) = 4µA
I Gg4 (BEAM + PEDESTAL) = 11µA
Alternatively if relevant figures are not available for the tube, firstly
adjust IG4 current for 4µA as instructed in the previous paragraph.
Monitor the head amplifier output voltage on an oscilloscope and adjust
the pedestal control for a forward line scan pedestal of 175mV.
NB The I G4 (BEAM + PEDESTAL) value can be noted for future
reference.
6 3.
Picture Acquisition
An image will normally be obtained with the camera controls
undisturbed from the previous tube. Set the camera to 'pan' mode.
Turn the circular blanking control slightly anticlockwise to display the
entire target image. Adjust the line and field shading pre-sets for
minimum shading either by inspecting the picture or preferably by
displaying the relevant field/line video waveforms on an oscilloscope.
Adjust for minimum shading using the parabola and tilt correction
waveforms. Switch to 'chop' and cover the lens. The black level control
may now be adjusted for a signal level of 1.5V on the oscilloscope.
This provides optimum range for the output level clamp. Shading may
sometimes be further improved by re-orientating the tube for minimum
symmetrical shading. If this is necessary the picture acquisition section
will have to be repeated.
