Tektronix 516 series, Instruction Manual

Page: 22 / 132

Circuit Description — Type 5 1 6
The basic sensitivity of the Vertical Amplifier is 0.05 volt
per divison. In other words, 0.05 volt of signal at the grids
of the Input Cathode Followers will produce one division of
deflection on the crt. Changes in sensitivity of the oscillo­
scope are accomplished by attenuation of the input signal
before it reaches the Input Cathode Followers. A “ straight-
through" position and eight different values of attenuation
may be selected by the VOLTS/DIV. control to provide nine
fixed calibrated sensitivities of 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10,
and 20 volts per division. The attenuators are resistance
and capacitance dividers which provide constant attenua­
tion throughout the frequency range of the instrument while
maintaining a constant input impedance of 1 megohm and
20 picofarads at the input connectors.
The Input Amplifier, V434, is a cathode-coupled paraphase
amplifier which converts the single-ended input from the In­
put Cathode Follower to a push-pull output. The gain of this
stage can be adjusted by means of the VARIABLE VOLTS/
DIV. control which varies the coupling resistance between the
two cathodes. This control permits a continuous range of
uncalibrated sensitivity values between the steps selected by
the VOLTS/DIV. control and up to about 50 volts per division
of deflection. Switch SW442, which switches the UNCALI­
BRATED lamp, B442, in and out of the circuit, is linked
mechanically to the VARIABLE VOLTS/DIV. control. Thus',
when the VARIABLE VOLTS/DIV. control is moved away from
the CALIBRATED position (fully clockwise), the UNCALIBRA­
TED lamp is energized to indicate that the vertical sensitivity
is not calibrated.
The CHANNEL A GAIN SET is used to set the gain of
the A Channel equal to that of the B Channel. This provides
equal deflection with equal signals through both channels,
provided both VOLTS/DIV. controls are set to the same po­
sition and both VARIABLE controls are in the CALIBRATED
position.
The DC BAL. control in the cathode circuit of V423B is
adjusted to place both cathodes of the Input Amplifier at
the same potential so that under no-signal conditions there
is no current flowing through R440 and R441. This provides
vertical stability of the trace as the value of R441 is changed
with the VARIABLE VOLTS/DIV. control.
The POSITION control varies the average dc level about
which the signal moves at the grids of the Switched Ampli­
fier, V454. Since the entire Vertical Amplifier is dc coupled,
this varies the position of the trace on the screen. The PO­
SITION control also affects the average dc plate voltage of
the Input Amplifier and, therefore, can disturb the dc balance
of that circuit. For this reason, a small portion of the posi­
tioning voltage at each plate is fed back to the opposite
cathode (through R443 and R444) to counteract this effect.
The turning off and on of the two channels is accomplished
in the Switched Amplifiers by the operation of the Switching
Circuit, which will be discussed later in this section.
Output Amplifier
The push-pull output of the Switched Amplifier is applied
to the Driver Cathode Followers, V463, which, in turn, drive
the Output Amplifier, V464 and V474. The output of the
Output Amplifier is connected through the Delay Line to the
vertical deflection plates of the crt. The Delay Line delays
the application of the signal to the deflection plates until
the crt has been unblanked and the horizontal sweep started.
This delay allows the leading edge of fast-rising pulses to be
displayed.
The overall gain of the Vertical Amplifier is adjusted by
varying the degeneration between the two cathodes of the
Output Amplifier. This is done with the GAIN ADJ. potentio­
meter, R478.
The parallel network of D477, D478, and R477 lowers the
effective resistance between the two cathodes as the dc po­
tential between them exceeds about 0.2 volt. The purpose
of this network is to compensate the compression of the crt
display which might otherwise occur when the POSITION
control is near the end of its range.
Trigger Pickoff
The output from V464 is also applied through two cathode
followers, V493A and V493B, to the TRIGGER SELECTOR
switch, SW10A. Thus, when SW10A is in either of the INT.
positions, a portion of the output of the Output Amplifier
is applied to the Time-Base Trigger to start the horizontal
sweep.
B493 and B494 protect V493 against cathode-to-grid break­
down when the instrument is first turned on by holding the
cathodes within a safe operating level of the grids until the
heater has warmed up enough to allow the tube to conduct.
After V493 starts conducting, B493 and B494 stop conducting
and are therefore effectively removed from the circuit.
Switching Circuit
Selection of the input channel whose output is to be ap­
plied to the Driver Cathode Followers is accomplished by
means of the Switching Circuit. A block diagram of the
Switching Circuit is shown in Fig. 4-3.
The Switching Circuit may be operated in any one of four
modes, depending upon the setting of the MODE switch,
SW585. When the MODE switch is in the A ONLY position,
the grid of V594A (one side of the Switching Amplifier) is
at about ground potential and the grid of V594B is at about
—96 volts. Therefore, V594A is conducting heavily, and
V594B is cut off. The plate of V594A and the cathodes of
the A Channel Switched Amplifier are at about +57 volts.
The plate of V594B and the cathodes of the B Channel
Switched Amplifier are at about + 63 volts (they are pre­
vented from going further positive by conduction through
D554). The average dc potential of the grids of both Switch­
ed Amplifiers is about + 56 volts. Therefore, with only about
one volt of bias, the A Channel Switched Amplifier will con­
duct and amplify any signal applied to its grids. Meanwhile,
with about 7 volts of bias, the B Channel Switched Amplifier
will be cut off. So the signal in the A Channel is applied
through the Driver Cathode Followers and the Output Am­
plifier to the crt deflection plates, and the signal in the B
Channel is blocked at the B Channel Switched Amplifier.
When the MODE switch is in the B ONLY position, just
the opposite set of conditions exists; the B Channel Switched
Amplifier conducts and the A Channel Switched Amplifier
is cut off. So the B Channel signal is displayed on the crt.
When the MODE switch is in the ALTERNATE position,
the crt displays the signal in one channel for one sweep of
4-3