Circuit Theory
3–18
1740A/1750A/1760 – Series Service Manual
Diagram
11
Z-Axis & Control
The bezel controls are the five, two-section, potentiometers located below the
CRT. The outside two are dedicated controls for vertical and horizontal
positioning. The center three potentiometers are assigned by menu selection.
U3 and U4 are 8-bit switched capacitor successive approximation A-to-D
converters with serial output. Levels, from the potentiometers, are input on the
AN inputs, converted, and output as serial data that can be read by the Micropro-
cessor (Diagram 6) on the serial bus (MISO).
U49 is a 4-section, Dual In-line Package (DIP) switch. Blanking selection
signals, from the Dynamic Control (Diagram 7), going low close the switch
elements. CR4 serves as an OR gate. The output of CR4 drives Q7, which is
the current drive for the Z–Axis amplifier on Diagram 22. The higher the
collector current the greater the CRT intensity.
CR5 is also an OR gate. The OPT BLANK originates on the Component circuit
board for the 1760–Series. BLANK is from the Dynamic Control (Diagram 7)
and pulls low, to ground the base of Q7 when the CRT is blanked.
Trace rotation is controlled by an output from one of the DACs on Diagram 9.
U5A drives a coil around the CRT that is located inside the CRT shield.
U1D is an oscillator with a 600 ms period. Its output drives U1A directly and
U1B through a comparator (U1C). The output of U1A and U1B is a 50% duty
cycle, with each amplifier driving two of the four graticule light bulbs. Only two
of the bulbs are lit at one time.
The +8 V and – 8 V supplies are nearly identical. They consist of bipolar
voltage regulators (U53 and U78) with output clamping and parallel resistance
power dissipation compensation.
The VPP1 supply is used to program the Flash EPROM. For normal operation
P4 is in the 2–3 position. U20 is a voltage sensing regulator whose output
voltage is established by R46 and R47.
Bezel Controls
Blanking Logic
Trace Rotation
Graticule Lights
+8 V & –8 V Supplies and
VPP1 Supply
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Summary of Contents for 1740A Series
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