The output from Q103, which is proportional to the difference between the
two inputs, is applied to a second state of differential amplification, Q104
and Q105.
The output from Q104 is applied to the base of series pass
transistor Q101. The differential amplifier adjusts the voltage drop across
the series pass transistor so as to maintain a constant output voltage. The
-18 volt provides operating current for the chopper drive multivibrator, and
supplies a constant current through R116 and R117 to its own zener reference
diodes CR103 and CR104.
(2) If the instrument is turned on with the battery voltage below
about 5 volts, there is a possibility that transistor Q101 may not begin
conduction.
Thus, when the power switch is set to BAT CHECK, the meter
would indicate an adequate battery charge, because all of the voltage drop
appears across Q101.
When the instrument is first turned on, the base-
emitter junction of Q102 is forward biased, and Q102 conducts, which causes
transistor Q101 to conduct and become saturated. As the output voltage of
the -18-volt supply rises above -11 volts, transistor Q102 becomes biased
off, and the differential amplifier controls the conductance of Q101.
(3) For instrument serial numbers 618-659, 691 and above, zener
diodes CR104 and CR105 are enclosed in a proportionally-controlled oven,
Q111, Q112, Q113, and associated components.
The oven heater is R147.
Transistors Q112 and Q113 are connected as a differential amplifier, with
the base voltage of Q113 fixed by R153 and R154. The base voltage of Q112
is set by R150 and R155.
Since R155 is temperature-sensitive, the base
voltage of Q112 varies inversely with temperature.
The output from the
collector of Q112, which is proportional to the difference between the base
voltages of Q112 and Q113, is applied to the base of Q111 and controls the
conduction of Q111, which controls heater current. For example, as the oven
temperature increases, the resistance of R155 decreases. This causes a more
positive output from the collector of Q112, which reduces the conduction of
Q111, thus reducing current through the heater R147, and decreasing heating
of R147. C108 eliminates oscillations in control circuit.
(4) In the 1000- and 10-volt dc range, the Zener reference diode is
connected directly to the Kelvin-Varley divider through resistors R119 and
R120 by means of range switch sections S2J and S2I. The voltage drop across
R119 and R120 reduces the Zener reference voltage to 11 volts at the input
of the Kelvin-Varley divider.
In the 100- and 1-volt dc range, range
resistors (R121, R122, and R123) selected by range switch sections S2J and
S2I reduce the voltage to 1.1 volts at the input to the Kelvin-Varley
divider. With the ac-dc connection to the range resistors that divide the
reference voltage to 1.1 volts.
This 1.1 volts is then passed to the
Kelvin-Varley divider by ac-dc switch section S4G. The voltage applied to
the Kelvin-Varley divider is always 1.1 volts for ac because the maximum
output of the ac to dc converter is always 1.1 volts.
m. The five Kelvin-Varley decades (fig 5-15), composed of resistors
R301 and R366, and associated voltage dials A through E, provide a means of
making the two precision voltages (11 and 1.1 volts) adjustable. The first
decade has twelve 5K resistors (a 4,999.2 ohm resistor and a 2-ohm trimmer).
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