HP 400D Operating And Service Manual page 15

T.O. 33A1-12-349-1
to within +1 volt de as the V7 plate voltage is varied +10%,
with about 60 ma of load current. The response of the
regulating circuits is fast enough to reduce ripple in
the output voltage to less than 1 millivolt, supplementing
the filtering action of C30. C36 couples the ripple com-
ponent in the regulated output directly to V8B to,avoid
attenuation in R62. R57 shunts a small portion of the
load current around V7 to prevent excessive V7 plate
dissipation at high line voltages. R63 and C35 constitute
a low-pass filter which prevents noise generated in V9
from reaching V8B.
4-17. The heater supply for the voltmeter tubes is
divided into two sections. One section supplies d-c
voltage for the tubes in the input cathode follower and
00102-2
Section IV
Paragraph 4-17
the amplifier. The other section supplies a-c voltage
for the tubes in the power supply. The voltage required
for the heaters of tubes V1 through V4 is obtained from
6.3V and 7.3V secondary windings of transformer T1,
which are series connected. The voltage developed
across the two series-connected windings is rectified
by full-wave rectifier CR3, reduced to 12.6 volts by
R66 and R68 in parallel, and applied to the series-
parallel-connected heaters of V1 through V4, as shown
in figure 5-10. The series-parallel connection of the
four heaters establishes a voltage of 6.3V for each.
The heater of V5 receives 6.3V ac from one of the wind-
ings which drives CR3. The heaters of V6, V7, and
V8 receive 6.3V ac from a separate 6.3V secondary
winding on T1.
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