Tektronix 1730 Series, Instruction Manual

Page: 106 / 258

1730–Series Theory of Operation
4–22
Control Circuit
Diagram 6
Blanking signals, for video and readout, are input to an intensity switching
matrix along with a dc voltage level set by the front-panel INTENS control.
Focus level, for the CRT focus anode, is set by regulating the current through a
transistor current source. The amount of focus current through the transistor
depends on the setting of the front-panel FOCUS control. The effects of small
variations in the magnetic field surrounding the instrument are compensated for
by an adjustable magnetic field placed around the CRT bulb. Scale Illumination
for the CRT face plate is set by controlling the output amplitude of a triangle
generator that drives the scale illumination bulbs.
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 (Q242 is off). When a Line Select Un-Blanking pulse occurs,
U239B turns off and additional current flows through Q242. R245, the LS
Focus adjustment, is adjusted for optimum focus in LINE SELECT at the normal
display focus setting.
U252 is a transistor array with two of the transistors connected as a differential
current switch. The static output level (pin 8) is set by the front-panel INTENSi-
ty control using Q243 (in the Focus Control) as a current source. The Blanking
signal is input to the transistor switching array through U252B (pin 9). When it
goes high, the current output, collector of U252A (pin 8), is shut off and the
Z-Axis Amplifier (Diagram 9) blanks the CRT. See Figure 4-9.
In LINE SELECT mode, the intensity setting has to change to brighten up the
line or lines. This is accomplished by increasing the current through the current
source (Q243). U239A is an open collector, dual comparator whose output goes
low during Line Select Blanking to allow the full current available, across R238
and R241, to flow in the circuit.
Focus Control
Z-Axis Control