Tektronix 1740A Series, User Manual

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Circuit Theory
3–36
1740A/1750A/1760 – Series Service Manual
C25 and Q8 are a start delay circuit that holds the error amplifier output low,
through CR12, until C25 is charged. Delaying the start of the high voltage
oscillator allows the Low Voltage Power Supply to start, unencumbered by the
load from the high voltage oscillator.
CR7 is the high voltage rectifier. Filter capacitors C6 and C7 work with CR7 to
provide –2750 V to the CRT cathode. U1 is a four-times multiplier providing
+11 kV to the CRT anode.
Q1 and Q2 form an operational amplifier that sets the voltage at the bottom of
the focus divider. The front-panel FOCUS pot determines what that voltage will
be. The Center Focus control, R9, is set for optimum beam focus, as viewed on
the CRT, with the front-panel FOCUS control set to mid range. Once the Center
Focus adjustment has been set, adjusting the front-panel FOCUS control changes
the voltage at the bottom end of the divider and, consequently, the voltage on the
CRT focus anode.
The cathode of the CRT is at a –2750 V potential with the grid coupled to the
Z–Axis Amplifier by the grid drive circuit. The grid is approximately 75 V
negative with respect to the cathode. The 200 V p-to-p sine wave present at the
cathode of CR11 is input to the grid drive circuit where it is clipped for use as
CRT control grid bias.
The sine wave from the cathode of CR11 is coupled through R41 to a clipping
circuit consisting of CR8 and CR9. Clipping level for the positive excursion of
the sine wave is set by the CRT Bias adjustment, R53. The negative clipping
level is set by the front-panel INTENSITY control through the Z–Axis Amplifi-
er. The clipped sine wave is coupled through C12 to a rectifier made up of CR5
and CR6. The rectified, clipped sine wave is the CRT control grid bias voltage.
C8 couples the blanking signal from the Z–Axis Amplifier to the CRT control
grid. DS1 and DS2 limit the CRT grid to cathode voltage at instrument turn on
or off. DS4 limits the CRT heater to cathode voltage.
The junction of R10 and R5 is the summing junction for the amplifier. It is at +5
Vdc. R6 and R17 are a voltage level shifter to bias the base of Q3 at 0 V, when
the summing junction is at +5 V. R5 is the feedback resistor, which sets the
overall amplifier gain at 36 V/mA of input current. Q3 is an emitter follower
that drives Q4, a common emitter amplifier. Q6 is a common base stage driven
from Q4. The collector of Q6 is the output of the amplifier. Q5 is a constant
current source that is the collector load for Q6. C11 is a speedup capacitor that
modulates the constant current source to increase amplifier rise time.
Power Supply Outputs
Focus Amplifier
Grid Drive Circuit
Z-Axis Amplifier
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