Tektronix 442, Instruction Manual

Page: 35 / 170

Theory of Operation—442 Service
function. The trigger SOURCE, EXT/10, uses the same
signal as EXT but the signal is attenuated by a factor of ten
for user convenience.
A selected trigger signal is amplified and inverted by
the Trigger Input Amplifier. The trigger signal passes
through a resistor network to the Trigger Level Com­
parator, which determines the voltagelevel (onthetrigger
waveform) at which triggering occurs. The SLOPE switch
selects the slope or sync (TV) polarity. The Trigger Level
Comparator also supplies a signal to the TV Sync
Separator circuit.
Two Schmitt trigger circuits produce a logic trigger
signal: one isfor normal trigger signalsandtheotherisfor
TV signals. The MODE switch selects which Schmitt
trigger circuit will operate.
SWEEP AND HORIZONTAL AMPLIFIER
The Sweep circuit, when triggered by the Trigger
circuit, produces a linear sawtooth output signal for the
Horizontal Amplifier to produce a sweep. The slope of the
sawtooth signal is controlled by the SEC/DIV switch,
which determines the time period and frequency. When
the sawtooth output reaches a predetermined level, the
Holdoff circuit resets the Sweep circuit, blanks the crt
(through the Z Axis Amplifier) and prevents subsequent
triggers from initiating another sweep until sweep reset is
completed. The HOLDOFF control can be adjustedfor low
frequency stability.
CRT CIRCUIT
The Z Axis Amplifier determines the crt intensity and
blanking. It sums the current inputs from several sources:
INTENSITY control; X-Y intensity limit; unblanking
signals from the sweep circuit; chop blanking signalsfrom
the Vertical Switching circuit; and signals from the
External Z AXIS INPUT connector, J419.
Output from the Z Axis circuit controls the trace
intensity through the HV circuit. The HV circuit provides
the voltages (greater than 100 V) necessary for operation
of the crt.
PROBE ADJ
The Probe Adjust Generator provides a square-wave
voltage output for checking and adjusting the compensa­
tion of voltage probes.
POWER SUPPLY
The Power Supply circuits provide low-voltage power
necessary for operation of the instrument.
DETAILED CIRCUIT DESCRIPTION
The operation of circuits unique to this instrument is
described in detail in this discussion. Circuits commonly
used in the electronics industry are not described in detail.
The following circuit analysis gives names of individual
stages, and describes how they are connected to form
major circuits. The detailed circuit diagrams are shown in
Section 7.
VERTICAL INPUT
(Diagram 1)
Since the Channel 1 and Channel 2 input circuits are
electrically nearly identical, only Channel 1 is discussed in
detail. The 4100 series circuit numbers identify the
Channel 1 components and 4200 series numbers identify
the Channel 2 components.
Input Switching
Input Coupling Switch—AC-GND-DC. The vertical
input signal is ac-coupled, dc-coupled, or grounded by
S4100. With the coupling switch in the DC position, input
signals are coupled directly to the VOLTS/DIV switch
attenuator. With the coupling switch in the AC position,
input signals pass through C4102 to the attenuator. With
the coupling switch in the GND position, the input to the
attenuator is grounded. This provides a ground reference
without disconnecting the signal from the input connec­
tor. In the GND position, C4102 is charged to the average
signal level through R4102 so that a trace remains on the
crt when S4100 is changed to the AC position.
VOLTS/DIV Switch. The VOLTS/DIV switch selects
the attenuator ratio and the preamplifier gain to determine
the deflection factor. The basic deflection factor of the
vertical deflection system is 2 mV/division. At this setting,
no attenuators are switched into the system and the gain
switch circuit sets the preamplifier gain to maximum.
Precision attenuators are switched in and out of the
attenuator and gain switching circuits to provide a com­
plete range of deflection factors as indicated on thefront
panel.
Attenuators are frequency compensated voltage
dividers that provide constant attenuation at all frequen­
cies within the bandwidth of the 442. Input RC
characteristics (approximately 1 M fi times approximately
3-2
R E V A M A Y 1980