Tektronix 442, Instruction Manual

Page: 39 / 170

Theory of Operation—442 Service
If the cathode voltage at the —2000 V point starts to go
positive (less negative), this positive-going change is
applied to the base of Q446. Q446 conducts harder, which
in turn causes Q454 to conduct harder. This results in
greater bias current to the base of Q458 through the
feedback winding of T460. NowQ458is biased closer to its
conduction level so that it comes into conduction sooner
to produce a larger induced voltage in the secondary of
T460. This increased voltage appears as a more negative
voltage at the crt cathode to correct the original positive­
going change. By sampling the output from the crt
cathode supply in this manner, the total output of the high-
voltage supply is held relatively constant.
Components, R445and C445, dampthe response of the
regulator against fast changes in the load such as when
the crt is unblanked at the beginning of the sweep.
Resistors R446, R453, and R457, help prevent in­
stabilities in the high-voltage oscillator.
High Voltage Rectifiers and Output. High-voltage
transformer T460 has three output windings. One winding
provides about 6.3 V for the crt filament. The crt filament is
referenced to the cathode voltage (= -2 kV), preventing
cathode-to-filament breakdown. A second winding
provides high ac voltage to multiplier U460 to produce a
10 kV crt anode accelerating voltage. Thesamewindingis
tapped and rectified by CR465 to produce the dc voltage
for the crt cathode. Components C465, R465, and C466
filter the dc voltage.
The third winding is used to control the crt intensity.
Components CR463, C462, C463, C464, R462, and R463
rectify and filter the secondary voltage to provide ap­
proximately -2100 V which is applied to the crt grid. The
entire winding is referenced to the output of the Z axis
amplifier whose output voltage variations are used to
control the crt intensity by varying the grid to cathode
voltage. The dc path for the Z axis signal to the grid is
through R462, CR463, R463, and thetransformer winding.
Resistor R462 isolates the transformer capacitance from
the Z axis Amplifier. Capacitors C463 and C464 provide a
path for fast changes in the Z axis output to the crt grid.
Resistor R464 provides a discharge path for C462, C463,
and C464. Glow lamps DS463 and DS465 prevent the grid-
to-cathode voltage from rising high enough to cause
breakdown within the crt during turn-on or when the
cathode or grid is shorted to ground.
Crt Control Circuits. Crt display focus is controlled by
FOCUS control R468. ASTIG adjustment R477, which is
used in conjunction with the FOCUS control to provide a
well-def i ned display, varies the voltage on the astigmatism
grid. Geometry adjustment R473 varies the voltage on the
horizontal deflection plate shields to control the overall
geometry of the display.
Two adjustments control the trace alignment by vary­
ing the magnetic field around the crt. Y axis adjustment
R474 controls the current through L470, which affects the
crt beam after vertical deflection but before horizontal
deflection. Therefore, it affects only the vertical (Y)
components of the display. Trace Rotation (TRACE ROT)
adjustment R472 controls the current through L472 and
affects both vertical and horizontal rotation of the beam.
R475 and R476 provide the proper voltage for the
vertical plate shield, and R478 and C478 decouplethefirst
accelerator electrode from the +32 V supply.
Probe Adjust
The PROBE ADJust circuit provides a square-wave
output of approximately 0.5 V peak-to-peak negative from
ground at 1 kHz.
U24 is an operational amplifier that functions as a free-
running multivibrator. The frequency determining ele­
ments are as follows:
When the output (pin 1 or 6 depending on serial number
of the 442) of U24 is positive, voltage divider R22-R23 sets
pin 3 at a positive voltage. Feedback through R24 charges
C24 until the pin 2 level reaches the same positive voltage
as pin 3. When pin 3 and pin 2 are at the same voltage, U24
output switches from positive to negative. The output of
U24 is about 7.2 V either positive or negative. Then C24
starts charging negative. When pin 2 and pin 3 are at the
same voltage again, U24 output switches positive, andthe
cycle repeats.
During the positive half cycle, CR26 is forward biased
and CR27 is reverse biased, keeping the output at ground
level. During the negative half cycle, CR26 is reverse
biased and CR27 is forward biased, causing current to
flow from ground through R27, CR27 and R26 to - 8 V.
This sets the output level to approximately —0.5 V.
TRIGGER
(Diagram 4)
Digital logic devices are used to perform some of the
functions in this instrument. LO and HI designations are
used in this circuit description to indicate the state of the
digital circuit. HI indicates the more positive of the two
levels. The specific voltages that constitute a LO and HI
logic state may vary between individual devices.
Input and Switching
SOURCE Switch. SOURCE switch (S2010) selects
trigger signals from COMP (composite), CH 1/CH 2
(internal), LINE, EXT, EXT/10, and X-Y sources.
3-6
R E V A M A Y 1980