Tektronix 585 - Page 41

Circuit Description — Type 585
The quiescent voltage at the junction of D142 and C154
is approximately — 3 volts. The quiescent voltage on the
other side of D 1 42 is about — 3.1 volts. As the Sweep-Gating
Multivibrator switches and V 1 45 plate drops to about — 7.5
volts, D142 conducts. The plug-in side of C154 then drops
about — 2 volts (less than the — 4.5 volts possible due to
additional circuit capacitance loading the output of C 1 54).
The charge on C154 rapidly stabilizes at — 7.5 volts, and
remains there until the Sweep-Gating Multivibrator ter 
minates the sweep. At this time, the plate of V 1 45 rises
very rapidly momentarily cutting D 1 42 off. Cutting off D 1 42
disconnects the capacitance of the dual trace switching cir 
cuitry at the time that any loading capacitance would slow
the action of Disconnect Diodes V152A and B. The positive
pulse from C154-R154 switches the alternate-trace plug-in
unit from one trace to the next trace. (See the instruction
manual for the alternate-trace plug-in unit in use.)
Alternate-Trace Sync B
Synchronizing pulses for alternate-trace plug-in preampli 
fiers are supplied from the screen grid circuit of V245. As the
plate current of V245 increases or decreases, the screen
voltage as developed across L249 decreases or increases
accordingly. Only the positive pulse is used by the
alternate-trace plug-in unit.
At the end of each Time-Base B sweep, a positive pulse of
about 15 volts is developed across R254 and sent directly
to the HORIZONTAL DISPLAY switch. The pulse from Time-
Base B is presented to the plug-in interconnecting plug only
when the HORIZONTAL DISPLAY switch is in the Time-Base B
or B' INTENSIFIED BY 'A' positions.
HORIZONTAL AMPLIFIER
General
The input to the Horizontal Amplifier is selected from
waveforms applied from either Time-Base Generator A,
Time-Base Generator B or the External Horizontal Ampli 
fier.
The dc-coupled Horizontal Amplifier consists of the Input
Cathode-Follower (CF ) stage followed by the Driver CF
stage that drives the paraphase Output Amplifier. The
Output Amplifier then drives separate Output CF ’ s that
drive the crt horizontal deflection plates. The CF Capaci 
tance Driver tube in the cathode circuit of the left-hand
deflection plate Output CF pulls the cathode of the Output
CF rapidly down during a sweep. The gain of the Output
Amplifier is controlled by negative feedback applied from
the left Output CF to a frequency compensated network
between the Input CF and the Driver CF. The negative
feedback is modified for 5X sweep magnification.
Input Circuit
The positive-going sawtooth voltages produced by the
Time-Base Generator circuits are coupled through a
frequency-compensated voltage divider to the grid of the
Input CF V343A. The input divider has a 1.5 to 1 attenua 
tion ratio. The small time-constant network C340-R340 im 
proves the start of the waveform at the fastest sweep rates.
The two positioning controls, HORIZONTAL POSITION
R333 and VERNIER R338, affect the beam position by alter 
ing the d c level at the grid of V343A. The VERNIER posi 
tion control can move the spot about 2 centimeters while
the HORIZONTAL POSITION control can move the spot
from about the center of the crt screen to well off the left
edge. These limits apply when the HORIZONTAL DISPLAY
switch is in the INTERNAL SWEEP position. Because of their
low resistance, an adjustment of the positioning controls
does not alter the attenuation of the divider network.
Brightened Area “ A ” Sweep Unblanking Signal
Fig. 4-9. *B ’ INTENSIFIED BY ‘ A ’ operation.
Driver Cathode Follower
Signals from the cathode of V343A pass either through the
input portion of the feedback circuit or straight into the
grid of the Driver CF V343B. The Driver CF eliminates pos 
sible loading of the feedback network by the grid of V364A
during high positive sawtooth voltages. The low output im 
pedance of the Driver CF drives the grid of V364A assur 
ing no distortion of the linear sawtooth sweep voltage.
With the 5X MAGNIFIER switch SW347B in the OFF posi 
tion, the grid of the Driver CF V343B receives the positive
going sawtooth voltage from the center of feedback divider
network R348, R349, R355 and R356. The amplitude of the
sawtooth voltage as it leaves the cathode of V343A is re 
duced by a factor of five by the negative going sawtooth
voltage applied at the top of R355. With the 5X MAGNIFIER
switch SW347B in the ON position, or with the HORI 
ZONTAL DISPLAY switch in either external horizontal input
position, R348 and R349 are switched out of the circuit, and
the total signal voltage from the cathode of V343A is
applied to the grid of the Driver CF. Part of the feedback
circuit remains connected between the left deflection plate
Output CF cathode and the grid of the Driver CF, but
essentially no feedback action occurs due to the low im
pedance of the Input CF cathode.
The NORM/MAG. REGIS, control, R358, is used to elim 
inate any voltage drop across R348 and R349 (5X MAGNI 
FIER OFF) when the crt beam is at the center of the screen.
It has no appreciable effect with the 5X MAGNIFIER ON.
Thus when the two centimeters of display at the center of
the screen are to be magnified, the display will be expanded
symmetrically about the center of the crt when the 5X
MAGNIFIER switch is turned ON.
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