Tektronix 310A, Instruction Manual, Page: 40 / 132

Circuit Description-Type 310A,
binding post. In the fu lly counterclockwise position o f the
CALIBRATOR switch, the C AL OUT binding post is dis­
connected from the divider (Calibrator circuit inoperative;
see Calibrator M ultivibrator discussion).
LV POWER SUPPLY
General
The LV Power Supply circuit provides the operating
power for the instrument from three regulated supplies and
one unregulated supply. Electronic regulation is used to
provide stable, low-ripple output voltages. Fig. 3-8 shows a
detailed block diagram of the LV Power Supply circuit.
Power Input
Power is applied to the primary o f transformer T600
through line fuse F600, POWER switch SW600 and thermal
cutout TK600. The primary o f T600 is composed o f two
main windings w ith four taps on each winding. The main
windings are connected in parallel for 117-volt nominal
operation or in series fo r 234-volt nominal operation. The
taps on each main winding can be connected to either aid
or oppose the main windings. This provides compensation
fo r line voltages slightly higher or lower than normal.
Thermal cutout TK600 provides thermal protection fo r
this instrument. If the internal temperature exceeds a safe
operating level, TK600 opens to interupt the applied
power. When the temperature returns to a safe level, TK600
automatically closes to re-apply the power.
—150-Volt Supply
The follow ing discussion includes the description o f the
— 1 5 0 -V o lt R e c t if ie r , —1 5 0 -V o lt Series Regulator,
-1 5 0 -V o lt Error A m plifier and the Reference Voltage
stages. Since these stages are closely related in the produc
tion o f the -1 5 0 -V o lt regulated o u tp u t voltage, their opera­
tion is most easily understood when discussed as a unit.
The -1 5 0 -V o lt Rectifier stage, D601A-D, rectifies the
output at the secondary o f T600 to provide the unregulated
voltage source o f this supply. Rectifier D601A is connected
as a bridge rectifier and the output voltage is filtered by
C601A-C before it is applied to —150-Volt Series Regulator
stage V607. The Series Regulator can be compared to a
variable resistor whose value is automatically changed to
maintain a constant o utput voltage. The Error A m plifier
stage provides this change by varying the conduction o f the
Series Regulator stage to provide regulated output voltage.
— 150-Volt Error A m p lifie r V602 compares the output
voltage o f this supply against a reference voltage to main­
tain a constant output. Reference voltage is provided by
voltage-reference tube V613, which sets the cathode of
V602 at about —66 volts. The control grid level of V602 is
determined by voltage divider R609-R610-R611 connected
between the output o f this supply and ground. R610 is
adjustable to set the output voltage o f this supply to —150
volts. The output current of the Error A m plifier controls
the conduction of the -1 5 0 -V o lt Series Regulator stage.
This output current changes to provide a constant, low-
ripple — 150-volt output level. This occurs as follows: The
difference in voltage between the cathode and control grid
of V602 determines its conduction. For example, if -1 5 0 V
Adj R610 is turned clockwise toward a more negative
potential, the control grid o f V602 goes more negative also
to reduce the conduction o f V602. This allows its plate to
go more positive along w ith the grid of - 1 50-Volt Series Reg­
ulator V607. When the grid of V607 goes positive, this tube
is forward biased and more current flows through it and
through the external load o f the supply. More current
through the load increases the output voltage from this
supply. Resistor R608 in parallel w ith series regulator tube
V607 allows some current to bypass the tube so that all of
the current fo r the supply does not go through V607. How­
ever, the current through R608 remains constant and only
the current through V607 is changed to provide a change at
the output. When R610 is turned counterclockwise, a
similar but opposite action takes place to reduce the output
voltage o f this supply. —150V Adj R610 is set to provide a
-1 5 0 -v o lt level at the output o f this supply.
The output voltage o f this supply is regulated to provide
a constant voltage to the load by feeding a sample o f the
o utput voltage back to -1 5 0 -V o lt Series Regulator V607.
For example, assume that the output voltage increases (more
negative) because o f a change in load or an increase in line
voltage. This negative-going change at the o utput appears at
the cathode o f —150-Volt Error A m p lifie r V602 since a DC
level change at the cathode o f voltage reference tube V613
produces a similar change at its plate. A more negative v o lt­
age also is applied to the control grid and screen grid of
V602 through dividers R609-R610-R611 and R602-R603.
However, the voltage change at the grids is less than at the
cathode so that the overall effect is to forward bias V602.
More current flows through V602 and its plate goes
negative to reduce the conduction o f -1 5 0 -V o lt Series
Regulator V607. With less current flowing through V607,
the voltage across the load decreases and the o utput voltage
returns to - 1 5 0 volts. To regulate the output voltage of
this supply to eliminate changes due to ripple, a sample o f
the unregulated voltage from the -1 5 0 -V o lt Rectifier is
connected to the screen grid o f V602 through divider
R603-R604. The voltage at the cathode o f V602 remains
constant, since voltage reference V613 is bypassed by C613
to eliminate voltage changes at this point due to ripple in
the output. The change at the screen grid is in a direction to
oppose the ripple voltage in the output.
+100-V olt Supply
Rectified voltage fo r operation of th e +100-Volt Supply
is provided by D630A-D. This voltage is filtered by C630
and connected to + 100-Volt Series Regulator V633 and to
the negative side o f the +300-Volt Rectifier stage. Refer­
ence voltage for this supply is provided by voltage divider
R637-R638 between the regulated -150-volts and the o u t­
put o f this supply. The —150 volts is held stable by the
-1 5 0 -V o lt Supply as discussed previously. If th e +100-volt
output changes, a sample of this change appears at the grid
3-12