Tektronix 1A5 Instruction Manual page 30

Introduction
A detailed description of the
tained in this section.
block diagram description
of operation of
specific components within
is intended to provide enough information
1A5 to enable a technician
Simplified drawings are provided where necessary for
effective circuit explanation. No attempt is made to explain
basic operations of components except for those that are
not general knowledge. Some additional information regard ­
ing components is included in the Maintenance Section of
this manual.
Resistors on schematics are valued in ohms unless other ­
wise indicated. Capacitor values which are not assigned the
symbol "µF" are indicated as follows: pF — whole numbers;
µF — decimal numbers. Example: 0.1 is in µF; 33 is in pF.
BLOCK DIAGRAM DESCRIPTION
The Operating Instructions secion of this manual pro ­
vides a block diagram and a basic description of the Type
1A5 circuitry, with emphasis on front panel controls and
adjustments. It is suggested that it be reviewed before con ­
tinuing in this section.
Refer to the Block Diagram in Section 9. The A channel
and В channel make up most of the Type 1A5 circuitry.
These two channels are identical except that a signal applied
to the A channel will cause a displayed signal of the same
polarity as the input signal, and a signal applied to the В
channel will cause an inverted display signal.
channels are distinct until after they enter the Comparator
circuit. From there on they operate in push-pull, providing
outputs to be applied to the oscilloscope vertical amplifier.
If desired, signals can be accepted from an active differ ­
ential probe instead of from the A INPUT and В INPUT
circuitry.
Four operating voltages are developed within the Type
1A5: +20.7 and + 60 volts for circuit operation; + 6 . 2 V
for reed switch and active probe operation; and a variable
5 volts for input voltage comparison purposes.
tion, + 50 volts and — 50 volts are developed within the
Type 1 A5 for Differential Probe operation.
Assume that A-Vc has been selected by the DISPLAY
switch and a Comparison Voltage of 0 is applied to the
В channel.
A signal applied to the A INPUT connector passes through
the Input Coupling selector switch to the Input Attenuator
circuit where is is attenuated by a factor of 1, 10, 100 or
1000. The output of the selected attenuator is applied to
a contact of the DISPLAY switch. From the DISPLAY switch
the signal goes to the gate of field effect transistor (FET)
http://manoman.sqhi I l.com
CIRCUIT DESCRIPTION
Type 1A5 circuitry is con ­
The scope of coverage includes
of overall operation, the theory
individual circuits, and the functions of
the circuits. This description
about the Type
to troubleshoot any
SECTION 3
Q123A. The
and passes through emitter follower
a relay controlled
a
button. Transistors Q 1 33 and
of the FET circuit.
If Differential
a relay routes
cuit. The Comparator
part of it.
provides a
either the A channel or the В channel.
applied, Q 1 64 provides an inverted output through V 1 64
and emitter-follower Q183
Q314-Q334.
the other half of the Comparator (Q264, V264 and Q283)
provides Q324-Q344 with a signal having a
site to that applied to Q314.
The amount of amplification provided by the Gain Switch ­
ing Amplifier (Q314-Q324) is controlled by the VOLTS/CM
control. The Output Amplifier further amplifies the push-
pull signal from Q314-Q324 and provides an output which
is impedance-matched to the oscilloscope's vertical ampli ­
fier circuit.
Returning to the Comparator circuit, if the
receives signals simultaneously from the two input channels,
its output will be an amplification of the difference between
the two. Therefore, if the two signals are equal in ampli ­
tude and phase (common-mode), the Comparator will have
no signal output. In A -В mode, this "differential amplifier"
action occurs and provides an output which is representative
of the difference between the signals applied at
В INPUT connectors.
With the DISPLAY switch in A-Vc or in Vc-B
The two
the COMPARISON VOLTAGE (Vc) network
to supply a voltage to one of the signal channels.
parator will accept this as a DC signal, and will cancel an
equal amount from the signal in the other channel.
When the Differential Probe signal is selected by depress ­
ing the PUSH ON/OFF button, a push-pull signal goes
from the probe to the Comparator. The Comparator
acts as a conventional push-pull amplifier.
Power Supplies.
In addi ­
voltages from the oscilloscope: + 225 V, + 100 V, + 75 V,
— 150V DC, and 6.3 V AC.
provide four additional regulated voltages for Type 1A5
operation. The + 20.7 and + 6.2 volt supplies are located
on the Input Amplifier board and use the + 75-V input as
their source. An adjustable + 60-V supply, located on the
Output Amplifier board, is powered by the + 100-V input.
A COMPARISON VOLTAGE (Vc) supply is derived from the
+ 100-V or
as determined by the POLARITY control.
Prob e is powered by the + 6.2/ + 20.7-V supply
+ 50-V and — 50-V supplies derived from the + 60-V and
— 150-V supplies respectively.
output is taken from the source of Q123A
Q 1 43 to a contact of
by the Differential Probe PUSH ON/OFF
Q 1 28 aid in the operation
Probe operation has not been selected,
the signal to Q 1 64 in the Comparator cir ­
acts as a paraphase amplifier which
push-pull output in response
to gain switching
Responding to the same signal into
The Type 1A5 receives the following
Circuits within the Type 1 A5
— 150 V input and can be either + or — 5 V
Type 1A5
to signals from
With a signal
amplifier
Q 1 64,
polarity oppo ­
Comparator
the A and
position,
can be used
The Com ­
then
The Differential
and by
3- 1