Keithley 610B Instruction Manual page 31

MODELS 6LOB, 610BR ELECTROMETERS
CIRCUIT DESCRIPTION
d.
Transistor 4201 is an emitter follower whose function is to increase the
current gain of the series transistor, 4202, forming a Darlington pair.
Resistors R208 and R209 and diode D208 provide current overload protection.
Excessive current drawn from the power supply causes an increased voltage
drop across R208, which forward biases diode D208, thus preventing the
collector of Q203 from going more negative. Since the collector voltage
cannot rise, further amplification is prevented and, therefore, further
current increase is prevented.
e. The ~250 volts and -265 volts for the output stage are obtained from
a single winding on transformer T20L. 210 volts from the transformer is
half-wave rectified by diodes D205 and D206, and filtered by capacitors
C206 and C207, respectively. Resistor R203 provides short-circuit protection;
resistor R204 discharges the capacitors when the instrument is turned off.
1064R