Tektronix 485, Instruction Manual

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Circuit Description—485/R485 Service
the subsequent A sweep (B display). The first segment of
non-linear shaping is done in U1710 by the biasing, which
gives no output for an INTENSITY control setting between
ground and —3.8 V. No change in Focus voltage is required
over this low-intensity range.
The MAX Focus adjustment is used to set the limiting in
this stage to coincide with maximum Z limiting. The
resistor, pot, and diode network between pin 8 of U1710
and the base of Q1716 serve to provide the last two
segment of non-linear shaping and focus gain setting. A t the
low-intensity condition, the base of Q1716 is at +10 V and
CR1710 is conducting. A t higher intensity, as current
passes into pin 8 of U1710, the voltage drops and CR1710
disconnects as the voltage reaches +8.5 V. When the
maximum (lim it value) current into pin 8 is reached, this
voltage is typically +5 V. The Focus Gain adjustment
provides for optimizing the high intensity focus.
The Auto-Focus amplifier provides a stablized voltage
gain of about +35 from the Focus Gain control to the
output at Q1728 collector. At maximum intensity level,
this output voltage can be adjusted from about zero to
+70 V and is typically in the +20 V to +50 V range, after
proper Focus Gain adjustment.
The network at the base of Q1736 causes its conduction
to be proportional to the amplitude and frequency of the
Auto-Focus output voltage. This arrangement permits the
power transistors Q1728 and Q1732 to operate at a
minimum power level consistent with the displacement
current output required.
A DC-Restorer network provides for coupling the output
of the Auto-Focus amplifier to the focus electrode of the
CRT. This consists of four diodes, six capacitors and four
resistors including C1651 and C1681. The low-intensity
focus is controlled by the high voltage focus pot, which is
located for operator adjustment at the rear of the instru­
ment.
C A LIB R A TO R
Q2106, Q2112, and associated circuitry compose an
emitter coupled multivibrator. The frequency of operation,
1 kHz or 1 MHz, is selectable from the front panel by
switch S2105. R2109 and R2112 are the current sources
for the emitter coupled multivibrator. Accurate 50% duty
cycle is obtained by close match of these resistors. R2106
and R2111 are used to protect Q2106 and Q2112 from
damage if they are plugged in with power on. C2111 is a
speed up capacitor.
Q2114 and Q2124 compose a current mode switch.
When Q2124 is on, CR2133 is o ff and the calibrator voltage
(at J2134) is virtually zero. When Q2114 turns on, the
current in Q2124 is diverted through Q2114 and the
calibrator output voltage is determined solely by the
current through R2131 and R2132. R2133 keeps the
output impedance at 450
i l when CR2133 is off. R2124
damps the resonant circuit created by the inductance of
CR2133 and the collector capacitance of Q2124. Schottky
diodes, CR2124, and CR2133, are used for low capacitance
fast switching. CR2124 keeps Q2124 from saturation.
Risetime of the calibrator is less than 1 ns only when
terminated in a 50 load. The negative-going edge is not
suitable for checking amplifier transient response.
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