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Circuit Theory
1740A/1750A/1760 – Series Service Manual
3–13
The Horizontal Signal input selection consists of dual-in-line package (DIP)
switch segments (U59, U63, U70, and U94) and an 3-to-8 line decoder (U50).
The decoder is controlled by 3 synchronous outputs from the Line Rate
Controller (Diagram 7). It is permanently enabled (pins 4, 5, and 6) so that any
change in state of the control lines (pins 1, 2, and 3) will pull one of the six
outputs (Y0–Y6) low and close the appropriate DIP switch segment.
The Horizontal Amplifier, U56, contains two independent amplifiers. The
external gain controlled Auxiliary Amplifier is used as a single ended luminance
amplifier. The output of the Luminance Filter Amplifier (Diagram 3) is input to
the + input of the Aux amplifier. Its gain is controlled by the PIX CONTRAST
level from a DAC shown on Diagram 9. The single ended output drives an
inverting operational amplifier, U73B. The minus input of U47B is a summing
junction for the PIX BK LVL (black level) and the luminance from U56. The
output of U47B is the picture monitor intensity signal to the Z–Axis control
circuit.
The main Horizontal Amplifier, which has inputs for horizontal signals and the
readout signal, converts the single ended input from the Horizontal MUX to a
differential output. In addition, it amplifies the differential input of the readout
signal. U47D is an inverter to generate the –H RO SIG. Output is controlled by
the H RO SEL signal from the Line Rate Controller.
Horizontal levels, such as Gain and Position from the DACs (Diagram 9), and
control signals, such as Mag from the Line Rate Controller (Diagram 7), are
input through U56, the Horizontal Amplifier. Gain and frequency response
characteristics of the CRT are compensated for by a network between the
VOUT+ and VOUT– terminals. The + and –H signals from the VOUT terminals
are also supplied to the Vertical Deflection Amplifier (Diagram 6) for ortho-
gonality adjustment (Y-Align).
The Horizontal Deflection circuit consists of seven discrete transistors to drive
the horizontal deflection plates of the CRT with a differential signal.
Q28 is the current source for this paraphrase amplifier. The amplifier itself is
driven from inputs Q12 and Q13. Their bases are a summing junction for the
input signal and compensated feedback. Q11 and Q13 are common base
amplifiers with the bases held at –3 V. Q8 and Q15 are driven independently.
Shunting resistors across Q8 and Q15 lessen power dissipation in the current
source (Q28).
CR8 is a boot strap circuit to divert current to the negative-going side when the
amplifier is slewing rapidly.
Horizontal Signal MUX
Horizontal Amplifier
Horizontal Deflection
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Содержание 1740A Series
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