Overload protection is provided by fuse F900 ond F902.
+10 Volt Supply and Regulator. Error sensing far the
Thermal cutout TK902 in the primary circuit of T900 opens
+10 volt supply is accomplished by the amplifier Q930.
the transformer primary circuit if the temperature inside the
A sample of the +10volt supply is applied from the voltage
analyzer rises above a safe level. TK902 resets automatically
divider, R934-R935 in series with the regulator Q920, to the
when the temperature returns to normal.
base of Q930. This voltage sample is compared with the
-10 volt emitter reference voltage. D932 provides thermal
-10 Volt Supply. This is the reference voltage for the
compensation for Q930.
other voltage supplies and the comparator circuits in the
The amplified error signal from Q930 is emitter-coupled
Type 491. Reference voltage for the -10 volt supply is
through Q922 to the base of the regulator Q920. Q921 is
set by zener diode D964, to approximately -0.9 V on one
connected in cascade with Q922 and isolates the collector
side of a comparator Q960-Q961. The voltage to the other
of Q922 from the rectifier output. This reduces the amount
half of the comparator is obtained from the voltage divider
of the power supply ripple couple into the regulator circuit.
R967, R968 (-10 volts), R969 and diodes D967-D968. When
R968 is properly adjusted the output voltage from the regula-
The +10 volt supply provides power for the POWER
tor is exactly -10 volts.
indicating light B948. Current is also supplied from the
+10 V unregulated supply, through transistor Q940 to the
Error voltage signal is sensed by the comparator Q960-
scale illumination lights. T h e S C A L E I L L U M c o n t r o l R 9 4 0
Q961 and applied as a correction signal through the com-
sets the current through the illumination circuit and controls
plementary amplifier Q952-Q951 to the base of the regula-
lamp brightness.
tor Q960. For example, an increased current demand by
Line signal for the LINE position of the SOURCE switch
the -10 volt supply load would tend to develop a negative-
is provided from pin 17 of the secondary winding of T900.
going error signal on the error side of the comparator. This
produces a positive-going correction signal to the base of
+150 Volt Regulated Supply. Error sensing for the +150
Q950 and the extra current demand of the load is supplied.
volt supply is provided by Q911. Error signal voltage in
the +150 volt supply is amplified by the cascode amplifier
The regulator circuit can never completely compensate
Q911-Q910 and applied through the emitter follower Q900
for changes in output voltage, because there must be an
to the base of the regulator Q901 as a corrective signal.
error input for the circuit to operate. However, any error
Diodes D904 and D905 protect Q900 from excessive volt-
in the output is reduced by a factor equal to the loop gain
of the regulator circuit.
age transients between the collector and emitter of Q900.
3-15
