Tektronix Type 82 PLUG-IN Instruction Manual page 22

When the MODE switch is placed to the A ONLY position,
the collector of Q575 is at +8 volts and collector of @565
is at +6 volts (see Fig. 3-3). The +8 volts reverse bicses
shunt diodes D501 and D502 in the channel A gate. Series
diodes D503 and D504 are then forward biased and they
feed the channel A signal to the Output Amplifier. The
current for diodes D503 and D504 is 10 ma through R516.
The current then divides equally—5 ma flows through each
of the grid lines in the Output Amplifier, through the series
diodes and through R501 and R502.
The current through the channel B Diode Gate is 20 ma
through R565, 16ma_ through shunt diodes D506 and D507,
and 12.8ma through R506 and R507. Fig. 3-3 shows how
the current paths divide through the channel B diodes.
"Clamp-diode"" D562 conducts if the collector of Q565 drops
below 6 volts and during the time interval that the multivi-
brator switches to the A ONLY state. Divider R564 and R573
sets the base level of Q575.
When the MODE switch is set to B ONLY, the Switching
Multivibrator reverses states and flips the +8-volt point to
+6 volts and the +6-volt point to +8 volts. The Switching
Multivibrator drives through the channel A shunt diodes
against the line impedance to provide enough diode reverse
current for rapid turn-off of diodes D503 and D!04.
Simultaneously, the Switching Multivibrator reverse biases
the channel B shunt diodes and the signal passes through
series diodes D508 and D509 to the Output Amplifier.
During dual-trace operation when the Mode switch is set
to either the ALTER. or CHOP position, the total transition
time in switching from one channel to the other is about 100
nanoseconds. The keep-alive current for the channel A
Diode Gate is supplied by resistors R501 and R502.
The Output Amplifier consisting of V524, V534, and V44
with its associated circuitry is a three-section distributed
amplifier. It is similar to the distributed amplifier section
used in the Type 580-Series Oscilloscope. All triode secti:ns
are neutralized to minimize grid capacitance. Capacitors
C528 and C538 bypass the cathodes from one to the other.
The grid and plate lines are 186 ohms push-pull and ure
at +7 volts and +50 volts respectively.
The cathodes all return to ground through the Xl GAIN
ADJ. R549 and R550 (if used). These components set ihe
current through the tubes and consequently the G,,. Resistor
R550 may be removed in some units to provide sufficient gain-
adjust range for R549. Zener D554 regulates the plate line
voltage against any current changes resulting from adjusting
the X1 GAIN ADJ.
The output of the plate line drives the oscilloscope grid
line through pins 9 and 11 of the interconnecting plug.
The termination for the grid line is R551 and R552 connecied
to pins 14 and 16 at the interconnecting plug. Capacitor
C552 is the ac ground point for the termination junction.
The termination for the grid line of the Output Amplifier is
R513 and R514. The junction of this termination is connected
the diode gate current. Capacitor C516 is the ac ground for
the R513/R514 termination.
Circuit Description—Type 82
SN 3000-Up
Two time-constant networks are incorporated at the input
to the grid line of the Output Amplifier. One network is
C510, R510, C511 and R511; the other is R512 and C512.
These networks aid in offsetting any time constants remain-
ing in the Type 82 circuitry.
SN 101-2999
Two time-constant networks are incorporated at the input
to the grid line of the Output Amplifier. One network is
C510 and R510; the other is R511 and C511. These networks
aid in offsetting any time constants remaining in the Type
All the coaxial systems used in the Type 82 are doubly
terminated to minimize reflection.
Selection of the input channel whose output is to be
applied to the Output Amplifier is accomplished by means of
The Switching Circuit consists of the following three stages
in the order described: Switching Multivibrator, Q565 and
Bias EF, Q583. These stages operate in any of four modes as
determined by the setting of MODE switch SW590. These
modes are A ONLY, ALTER., CHOP., and B ONLY. Since
A ONLY and B ONLY modes of operation are basically the
same, they are described together.
The Switching Multivibrator Q565 and Q575 is a bistable
circuit that switches channels A and B in the Type 82. When
Q575 conducts, the channel A signal or trace is displayed.
When @565 conducts, the channel B signal or trace is dis-
played. The setting of the MODE switch determines whether
the Switching Multivibrator rests in one of its stable states or
(1) A Only, B Only, Assume that the MODE switch is set
to A ONLY. In this position the switch end of R567 connects
to ground and becomes part of a divider circuit with R563,
Channel A gate diodes D501 and D502 {as shown in Fig. 3-3).
Series diodes D503 and D504 in the Channel A Diode Gate
are then forward biased and they permit the Channel A sig-
nal to pass through them to the grid line of the Output
Amplifier. The +6 volts at the collector of Q565 forward
biases D506 and D507 in the Channel B Diode Gate. Series
diodes D508 and D509 in the Channel B Diode Gate be-
come reverse biased and block the Channel B_ signals
from getting to the grid line of the Output Amplifier.
3-5