Z-Axis, Crt, Probe Adjust, And Control Mux (Diagram 7) - Tektronix 2246 1Y Service Manual

Theory of Operation—2246 1Y and 2246 Mod A Service
Driver Amplifiers
The differential output current signal from U802
passes through com m on-base current amplifiers
Q809 and Q810. These transistors drive cu rre n t-to -
voltage converters Q803-Q804 and Q807-Q808.
Emitter followers Q804 and Q803 convert the current
signal to a voltage
complementary-FET output amplifiers, Q801 and
Q802, to produce the negative-going deflection
voltage. Emitter followers Q808 and Q807 convert
the other side of the differential current to drive
Q805 and Q806 to produce the positive-going hori­
zontal deflection voltage.
The circuit of Q804 and Q803 is configured to
respond rapidly to a
signal; the circuit of Q807 and Q808 is configured to
respond quickly to the positive-going feedback
signal. Zener diode VR802 and associated resistors
R843 and R844 maintain the collector bias of Q803
and Q808 at 24 V.
Magnifier registration and horizontal readout cen­
tering is set by MAG REG potentiometer R809.
Adjustment of R809 is done to balance the currents
into the emitters of Q809 and Q810 to obtain the
correct horizontal position of the readout within the
graticule display area.
Output Amplifier
The differential circuitry of both sides of the Hori­
zontal Output Amplifier is similar; operation of only
one side of the
Complementary-FET amplifiers
produce the negative-going horizontal signal to drive
the left deflection plate. Two transistors are used to
provide adequate power handling. Since the two
gates are at different bias levels, signal voltage is
applied to the gate of transistor Q801 via C803.
Resistor R828, connected between the source and
drain of Q801, is a parallel current path around Q801
that balances the power handling requirements of
the two FETs. The amplifier FETs are high gain
devices, and the overall gain must be reduced to
maintain circuit stability at the faster sweep rates. To
provide the high-frequency gain reduction, resistor
R850 is in series with C802, from the source of Q802
to the drain of Q801, to damp the driving-energy to
Q801 supplied by C802 during the sweep retrace
transitions. Feedback resistor R806 provides positive
feedback and sets the overall gain of the output
amplifier stage. A parallel trim m er capacitor across
the feedback resistor, C807, adjusts the 2 ns sweep
timing for its best linearity. Impedance matching to
the deflection plate and additional signal damping is
provided by R802.
3-38
signal to drive the
negative-going feedback
amplifier is described.
Q801 and Q802
As the gate voltage of Q802 increases to follow the
input ramp signal, the drain voltage goes negative
from about 87 V toward the 15 V source voltage. At
the same time, the
complementary-FET Q801 is reducing the current
through Q801, thereby allowing its source voltage to
fall. At the end of the ramp signal, the input voltage
falls, and through the positive feedback, Q802 is
rapidly biased off. That also biases Q801 on, and the
energy stored in C802 quickly returns the deflection
plate voltage back to its starting point.
Common-Mode Stabilizer
Operation amplifier U801A compares the node
voltage at the junction of R820, R821, and R822 to
ground. Its output drives
com m on-m ode point (at the junction of R811 and
R812). The purpose of this dc feedback circuit is to
keep the average voltage level on the right and left
horizontal deflection plates set to the center of the
amplifier' s dynamic operating range (about 70 V).
Z-AXIS, CRT, PROBE ADJUST, AND
CONTROL MUX (Diagram 7)
Z-Axis and Auto Focus Amplifiers
The Z-Axis and Auto
operate on the same principle and both get their
drive signal from the Z-Axis Focus Driver. However,
the differences are enough that both circuits are
described.
Z-AXIS AMPLIFIER. Intensity control signal current
from the Z-Axis/Focus driver is applied to the Z-Axis
amplifier via Q2707. That transistor acts as a current
buffer amplifier. The input signal line is clamped at
5.4 V by Q2715 to prevent an overdrive of the Z -
Axis circuit. The Z-Axis Amplifier output transistors
consist of Q2701 and Q2702 on one side of the
com plem entary-sym m etry totem -pole output ampli­
fier and Q2703 and Q2704 on the other side. Two
transistors are used on each side to divide the
power handling requirements needed to drive the crt
control grid. The crt grid capacity is large, and
requires a relatively large amount of power to
change the intensity level quickly.
In the base circuit of Q2704, CR2705 prevents the
base-to-em itter voltage from exceeding 0.6 V.
Zener diode VR2701 dc level-shifts the signal
voltage level at the emitter of Q2705 for proper
biasing of Q2704. The ac signal components are
bypassed around VR2701 by C2703. Base biasing
for Q2702 and Q2703 is taken from a series-
resistance divider formed by R2711, R2712, R2713,
and R2714 between ground and the +130 V supply.
signal on the gate of
the amplifier input
Focus Amplifiers circuit