Theory of Operation
4–18
1720/1721
has to change to brighten up the line(s). This is accomplished by increasing the
current through the current source (Q342). U341A is an open collector dual
comparator that goes low when the Line Select Blanking occurs, which allows
current in R241 to add to the current in Q342, the current source.
The Focus control operation must also control two different display criteria. In
the normal mode of operation the Focus voltage will be selected by the control
setting only, Q241 is off. When a line select unblanking pulse occurs, U341B
turns on and additional current flows through Q241. R244, the LS Focus
adjustment, is adjusted for optimum focus in Line Select at the normal display
focus setting.
Trace rotation is necessary to compensate for changes in the magnetic field
surrounding the 1720/1721. Q254 and Q256 are emitter followers that provide
the Trace Rotation current to a coil located inside the crt shield, around the tube.
Current amplitude and polarity are controlled by the front-panel ROTATE
screwdriver adjustment.
U344A is a triangle generator whose output is compared to the front-panel
SCALE control output level by U344B (a comparator). The output of U344B is
a 6.5 kHz square wave, the duty cycle of which is controlled by the front-panel
SCALE ILLUM control. U344B drives saturating switch Q246, which applies
the square wave to the graticule lights, DS402 and DS802. L146 and C147 serve
as a low-pass filter to keep noise off the +15 V supply.
The + and –15 V supplies generated on the Power Supply circuit board are
further regulated to meet the on-board needs of the 1720/1721 Main (A3) circuit
board. U164 and U172 are the post regulators for the –11.8 V and +11.8 V
supplies. R259 is the –11.8 V Adjust and R267 is the +11.8 V Adjust.
Trace Rotation
Graticule Illumination
Post Regulators
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