Circuit Description— Type TU-7
scope vertical amplifier. When using the LOW LOAD or
HIGH LOAD positions, an external signal applied to the
Type TU-7 INPUT connector is ac coupled through C22 to
pin 1 of the interconnecting plug. Since the signal is applied
to pin 1 only, it provides single-ended drive to the vertical
amplifier. Front-panel VARIABLE control R ll, connected be
tween the junction of RIO/Cl0 and ground, controls the
amount of input signal applied to pin 1. RIO and CIO pro
vide some input signal frequency compensation. Vertical
positioning of the trace is provided by the voltage applied
to pin 3 of the interconnecting plug through the vertical
position network.
An internal trigger signal is applied from the junction of
R12 and R14 to pin 5 of the interconnecting plug. For
oscilloscopes that can trigger directly from the plug-in unit,
this signal simulates the internal trigger signal from a dual
trace piug-in preamplifier, such as the Tektronix Type 1A1.
In all positions of the TEST FUNCTION switch except
COMMON MODE, -{-PULSE, and —PULSE, two separate
voltage dividers set the dc voltages at pins 1 and 3 of the
interconnecting plug at +67.5 volts with the trace centered
and no signal applied. This voltage, which simulates the
nominal output voltage of a plug-in preamplifier, is essential
at the input of the oscilloscope for linear operation of the
vertical amplifier. The voltage at pin 1 is set by voltage
divider R16, R17, R18 and R19. The voltage at pin 3 is set
by voltage divider R84, R85, R86, vertical positioning net
work R80A, B, C, D, R81, and VERTICAL POSITION control
R82. The VERTICAL POSITION control varies the divider
voltage applied to pin 3 of the interconnecting plug and
thus controls the vertical position of the display in all posi
tions of the TEST FUNCTION switch except COMMON
MODE.
in the GAIN SET position of the TEST FUNCTION switch,
a precision 250-to-l divider consisting of R17 in series with
parallel resistors R18 and R19 sets the amount of input cali
brator signal applied to the oscilloscope vertical amplifier.
Thus, if a 100-volt peak-to-peak calibrator signal is used,
the divider dc couples a 0.4-volt signal to pin 1 of the inter
connecting plug. This signal results in a 4-cm deflection
on the crt if the gain of the oscilloscope vertical amplifier
is set correctly.
In the COMMON MODE position of the TEST FUNC
TION switch, a signal applied to the INPUT connector is
ac coupled through C22 and applied equally through R25
and R26 to pins 1 and 3 of the interconnecting plug. The
TEST FUNCTION switch disconnects the AMPLITUDE and
VERTICAL POSITION controls. In this position, voltage di
vider R1
6,
R17, R18 and R19, sets the dc voltage at both
pin 1 and pin 3. Since the same signal is applied in phase
to both sides of the oscilloscope vertical amplifier, the
signals will cancel if the rejection ratio of the amplifier
is high. The position of the trace on the crt is the dc bal
ance point of the oscilloscope vertical amplifier, whether
or not a signal is applied to the input.
DUAL-TRACE SWITCHING CIRCUIT
General Operation
The Dual-Trace Switching Circuit consists of plate-coupled
switching multivibrator V95A/V95B and steering diodes
V102A/V102B, Under normal conditions, the circuit performs
five general functions:
1. When the TEST FUNCTION switch is set to ALTERNATE,
the switching multivibrator operates in a bistable configura
tion. An alternate-trace sync pulse from the oscilloscope
at the end of each sweep is applied via pin 16 on the inter
connecting plug, switching the multivibrator from one state
to the other by turning on the steering diode that was cut
off. The output of the multivibrator is a sequence of two
dc levels for each complete cycle of operation. The two
levels produce two alternate traces on the crt. A signal ap
plied to the Type TU-7 EXT INPUT connector also appears
at pin 1 of the interconnecting plug, and is displayed by
the lower trace, simulating the Channel 1 operation of a
dual-trace preamplifier plug-in unit. The upper trace, which
simulates Channel 2, displays essentially no signal.
2. In the alternate mode of operation, a portion of the
applied signal is picked off and used as an internal trigger
signal at pin 5 of the interconnecting plug. If fhe oscillo
scope is capable of internally triggering on the signal at pin
5, a stable display of the applied signal can be obtained.
3. In the alternate mode of operation, if the oscilloscope has
two time bases that can be operated alternately, a "slave
pulse" signal is applied from the oscilloscope via pin 7 of
the interconnecting plug through the steering diode stage to
the switching multivibrator. The slave pulse sets the state of
the multivibrator so the upper trace w ill be displayed when
Time Base B generates its sweep. At the end of the Time
Base B sweep, a sync pulse applied through pin 16 and the
steering diode causes the multivibrator to switch states
so the lower trace will be displayed while Time Base A
generates its sweep.
4. When the TEST FUNCTION switch is set to the CHOPPED
position, the switching multivibrator is in an astable configu
ration. The free-running rate of the circuit is approximately
100 kc. The output of the circuit is a sequence of two dc
levels which produces a display of fwo traces chopped into
off-on segments at the 100 kc rate. A signal applied to the
INPUT of the Type TU-7 is ac coupled to pin 1 of the inter
connecting plug and is displayed by the "on” segments of
the lower trace. The upper and lower traces simulate the
operation of Channels 2 and 1 respectively of a dual-trace
preamplifier operated in the chopped mode.
5. With the Type TU-7 set for chopped mode, the switching
multivibrator produces blanking pulses that are applied
through pin 16 of the interconnecting plug to the oscillo
scope blanking circuit. The blanking pulses cause the crt
beam to be blanked during the time the beam is being
switched from one trace to the other.
Detailed Operation
When the TEST FUNCTION switch is set to the ALTER
NATE position, the voltage in the grid circuit of the switch
ing multivibrator is set at a level that causes the circuit to
operate as a bistable multivibrator. Basic operation of the
circuit is illustrated in Fig. 3-2. To show the operation of
the circuit, assume that V95A is initially conducting and V95B
is cut off. With current through V95A, V102A is forward
biased and conducting due to the voltage drop across R94.
D89 and D90 are forward biased by current through V95A.
(A)
3-2
