Tektronix 1A5 Instruction Manual page 36

The MONITOR jack is connected to the amplifier circuit
whenever the DISPLAY switch is in A-Vc or Vc-B position, re ­
gardless of the POLARITY switch setting.
cables should be connected to it when waveform obser ­
vations are being made.
Operating Voltages
The Type 1 A5 receives — 150 V, +100 V, +225 V and
+75 V DC, and 6.3 V AC from the oscilloscope, as indicated
on the Output Amplifier schematic. The +100-V and +225-V
supplies are shunted together through R396 to avoid over ­
loading of the +100-V supply by the Type 1 A5 circuitry.
These voltage sources are equipped with decoupling networks
in the oscilloscope and in the Type 1A5.
lower values will be read at the Type 1A5, as indicated by
the blue figures on the Input Amplifier and the Output
Amplifier schematics.
Internal Power Supplies
A +60-V supply is created within the Type 1A5 by in ­
serting series regulator Q393 in the +100-V supply circuit.
Refer to Fig. 3-3. Output DC Level (R390) adjusts the base
voltage of Q393 to set the voltage at P 1 1 terminals 1 and
3 to 67.5% of the actual value of the +100-V supply. This
insures compatibility between the Type 1A5 and the oscillo ­
scope.
creases its emitter-base forward bias and the demanded
Only shielded
rent is provided. The increase in current is accompanied
a decrease in trans-resistance of Q393, allowing the
supply to maintain its 60-V level. A decrease
mand causes the reverse effect.
A regulated +20.7-V supply (illustrated on the Input Ampli ­
fier schematic) is provided by parallel regulator transistor
Q297. The base of Q297 is referenced to +20-V by Zener
diode D292. This effectively clamps the emitter to +20.7-V
and the resistance of Q297 is automatically regulated to
maintain this level.
Current passing through Q297 develops
across Zener diode D296
Therefore, slightly
the reed switches in the gain switching amplifier and
parison Voltages circuits. This 6.2-V source
load changes when VOLTS/CM control
R296
demands.
The +20.7-V supply is returned to +75
relay K295
ation is selected.
which provides
circuit. Differential Probe operation is
ON/OFF button assembly
ical construction
its last
Fig. 3-3. +60V Power Supply, simplified.
Circuit Description — Type 1A5
If the circuit requires a current increase, Q393 in ­
in the collector circuit
shunts Q297 to provide sufficient
and resistor R295 when Differential
Otherwise it is returned
a resistance equivalent to
which contains
of SW297 keeps it electrically
selected position. However,
cur ­
by
power
in current de ­
a 6.2-V supply
for activating
Com ­
will change with
settings are changed.
current for circuit
V through 12-V
Probe oper ­
through R294,
that of the relay
selected by the PUSH
SW297. Mechan ­
connected in
relay K295 is bypassed
3-7