TM 9-6625-1753-14
C3
(2) Let's discuss examples of how the various networks combine in the input voltage attenuator to produce a
Section III. FUNCTIONAL THEORY - DUAL TRACE
particular calibrated input range. For instance, setting the VOLTS/DIV switch to .05 volts per division, the input signal goes
directly to the preamplifier circuit. It is then attenuated by the 10:1 interstage attenuator net-work.
PLUG-IN TYPE 76-02A (Y-DEFLECTION)
(3) In another case the VOLTS/DIV switch is set to 2 volts per division. With the switch in this position, the input
signal passes through the 4:1 attenuator and the 10:1 attenuator. These two attenuators reduce the input signal by a ratio of
3-6. Signal Coupling and Voltage Attenuation (Fig. 4-4)
40 to 1. The input signal then goes to the preamplifier and the interstage attenuator, where the signal is attenuated by an
The dual trace plug-in consists of two identical channels (channel A and channel B). In this section we will describe only
additional factor of 10:1 resulting in an overall attenuation of 400 to 1.
channel A.
(4) The VOLTS/DIV switch also provides a CAL position, where a 5:1 attenuator is inserted to automatically switch
a. Signal Coupling.
out the input signal and replace it with an internally generated calibrated signal (square wave). The peak to peak value of the
(1) Each channel has a one-megohm input resistance shunted by an input capacitance. At high frequencies this
calibrated signal produces four divisions of vertical deflection on the CRT screen.
results in a low input impedance. However, the input impedance can be raised and the loading effect lessened by using the
(nine-megohm) 10:1 probe (zone A1). The probe has an adjustable compensating capacitor which shunts the probe, to keep
3-7. Preamplifier
the input impedance constant as a function of frequency.
(2) Using one of five positions on NORM-INVERT switch S911, you can select the polarity and type of coupling
a. Paraphase Amplifier. After passing through the input voltage attenuation network, the input signal goes to paraphase
through which the input signal passes. With the NORM-INVERT switch set to the NORM section of the switch, the displayed
amplifier V920, V921 (zones C1, D1) via high-frequency compensating network R9201, R9206, C9201, C9202 (zone C1.)
waveform will have the same polarity as the input signal. Setting the NORM-INVERT switch to the INVERT section of the
This amplifier stage converts the single-ended input signal into a push-pull output. DC BAL (fine) variable resistor R9202, and
switch reverses the polarity of the displayed waveform.
DC BAL (coarse) variable resistor R9200 control the operating level of the paraphase amplifier. VARIABLE resistor R9213, in
(3) The AC position of the NORM-INVERT switch is used for input signals greater than 10 cps. With the NORM-
the common cathode circuit, regulates the gain of the stage. The DC BAL resistors are properly set, when without a signal
INVERT switch at AC, capacitor C9115 (zone A2) is inserted in the input circuit so that it blocks the dc component of the input
you can vary R9213 throughout its entire range without causing a repositioning of the trace on the CRT. Diode clamp
signal.
CR9200 protects the amplifier tubes when power is initially applied to the circuit.
(4) The DC position of the NORM-IN-VERT switch is normally used for dc signals, although it may also be used for
b. Preamplification. The push-pull output of the paraphase amplifier goes to the amplifier-emitter follower combinations
ac signals up Ito 25 MHz. With the switch set at DC, the input signal retains its dc component, and is coupled directly to the
Q9200, Q9202, and Q9201, Q9203. These stages are dc coupled, providing a good low-frequency response, and assuring
vertical amplifier. The GND position of the switch makes dc balancing of the input amplifier much easier.
frequency stability and a low signal-to-noise ratio. In the control circuit of the preamplifier there are three adjustments. Front-
b. Input Voltage Attenuation.
panel GAIN ADJ variable resistor R9212 sets the on-screen amplitude of the signal to the appropriate relative scale of the
(1) After passing through the NORM-INVERT switch, the input signal goes to the input voltage attenuation circuitry
graticule. ATTEN BAL variable resistor R9234R balances the bias on Q9202 and Q9203, assuring that each stage is
(zone B3-B5). By using VOLTS/DIV switch S910 (zone A4), you can select, either individually or in various combinations, four
referenced to the same voltage level (this also prevents the no-signal trace from repositioning when the load changes).
input attenuator networks. These operate in conjunction with the interstage attenuator (zones C4, C5) which are covered in
Capacitor C9209 adjusts the bandwidth of the stage to permit a sharp rise time.
paragraph 3-7c. The combined input and interstage attenuators provide ten calibrated input ranges of .01, .02, .05, 0.1, 0.2,
c. Interstage Voltage Attenuation.
0.5, 1, 2, 5, and 10 volts per division. Without attenuation, that is, with the VOLTS/DIV switch set to X1, the sensitivity of the
(1) After passing through the preamplifier circuit and NORM-INVERT switch S920, the input signal goes to the
vertical amplifier is 0.005 volts per division. Table 3-1 shows the individual input ranges, and the input and interstage
interstage voltage attenuator. This attenuator reduces the preamplifier output before it goes to the delay line drivers (par.
attenuator network combinations that provide the various attenuation ratios.
(2) Figure 3-2A shows the 10:1 interstage resistance attenuation network formed when we set the VOLTS/DIV switch
Table 3-1. Voltage Attenuation Factor
to any position between 0.05 and 10. Using the computations shown, you can see that the ratio between total network
resistance and output resistance is approximately 10 to 1. Figure 3-2B through D shows the resistance attenuation networks
formed when the VOLTS/DIV switch is set to any other position between 0.005 and 0.02.
Input range
Input
Interstage
Attenuation
VOLTS/DIV
attenuators
attenuators
ratio
3-8. Channel Switching
Not
CAL
X5
applicable
5:1
a. Channel A.
10
X2, X100
X10
2000:1
(1) Amplifiers Q9204 and Q9205 (zones A8, B8), which comprise the output stages of the channel A amplifiers,
5
X100
X10
1000:1
receive the pushpull input signal from the interstage voltage attenuator. POSITION control R9234 (zone B8), in the common
2
X4, X10
X10
400:1
emitter circuit of the amplifiers, regulates the dc level of the amplifiers which, in turn, determines the vertical position of the
1
X2, X10
X10
200:1
trace that is displayed on the CRT. Also in the emitter circuit is a compensating network which improves the high frequency
0.5
X10
X10
100:1
response of the amplifiers. These amplifiers are switched on or off by the channel switching circuit, depending on the position
0.2
X4
X10
40:1
of the MODE switch (zones B7-D7).
0.1
X2
X10
20:1
(2) Electronic switch Q9600, Q9601 (zone B8), a bistable multivibrator controls the operation of the chann el A and
0.05
X1
X10
10:1
channel B amplifiers. The electronic switching circuit can allow either one or both of them to operate normally. When the
0.02
X1
X4
4:1
MODE switch is set to A, a constant negative voltage is applied to the base of Q9601, cutting it off. With Q9601 cut off, a
0.01
X1
X2
2:1
positive voltage forward biases diode CR9301 (zone A7), which disables the channel B amplifiers. At the same time, the
0.005
X1
X1
1:1
negative-going output of Q9600 (now conducting) reverse-biases diode CR9201 (zone A8), which allows channel A amplifiers
Q9204 and Q9205 to operate normally.
3-4.2
