Power Supply - HP 400D Organizational, Ds, Gs, And Depot Maintenance Manual
Vacuum tube voltmeter
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Also See for 400D:
- Operating and servicing manual (50 pages) ,
- Operating and service manual (30 pages) ,
- Service note (11 pages)
Table of Contents
TM 11-6625-1514-15
Section IV
Paragraphs 4-12 to 4-16
4-12. During the negative-going half cycle of the plate
voltage of V5, rectifier CR2 conducts electrons back to
both C32 and C33 from the plate of V5. That portion
of electrons going back to C32 flows through the meter
on the way (in the same direction that the electrons
flowed in the first, positive, half cycle). At this point
in the cycle, both C32 and C33 are discharged. The
pulsating current through the meter is smoothed by
C34 to prevent meter pointer vibration when measuring
low-frequency signals. The current is proportional to
the arithmetic average value of the waveform ampli-
tude of the signal. Meter calibration in rms volts is
based on the mathematical ratio between the average and
rms values of true sine wave current.
4-13. In addition, the bridge serves as a segment of a
voltage divider (in series with L11 and R108) connected
across the output of the amplifier. The negative feedback
voltage fed to the input of the amplifier is obtained across
L11 and R108. The alternating charge and discharge of
C32 and C33 produce at their junction with L11 an al-
ternating current of the same phase and waveform as
that at the plate of V5. This phase is negative with
respect to the input signal applied to the first stage of
the amplifier (V2), and drives the negative feedback
network.
4-14. POWER SUPPLY.
4-15. The power supply consists of tubes V6 through V8
and the associated circuits, as shown in the complete
4-2
Figure 4-2. Simplified Schematic of Meter Bridge Circuit
schematic diagram, figure 5-10. The power supply
furnishes regulated +250V d-c voltage for the grid and
plate bias circuits of tubes V1 through V5, unregulated
12.6V d-c voltage for the heater supply of tubes V1
through V4, and 6.3V a-c voltage for the heater supply
of tubes V5 through V8. The power supply is designed
to operate from either a 115-volt (±10%) or a 230-volt
(±10%) a-c power source of 50 to 1000 cps. The primary
winding of power transformer T1 is arranged in two
sections, which can be strapped either in parallel or in
series, to permit operation on 115V or 230V, respectively.
4-16. The output of rectifier V6 is applied to the voltage
regulator circuit consisting of V7 through V9 which
supplies a constant, +250 volts dc to the stabilized ampli-
fier circuit of the voltmeter. Tube V7 is the series
regulator tube, and V9 provides a fixed reference voltage
drop, with which the output voltage is compared in ampli-
fier V8B. V8A is a cathode follower which couples the
reference voltage from V9 to V8B without loading V9.
The regulated output voltage is applied to the control
grid of V8B, while the reference voltage is applied to
its cathode. The difference between the control grid
and cathode voltages controls the operating point of
V8B and thus its plate voltage, which in turn supplies the
grid voltage for regulator V7. Any change in the regu-
lated output of V7 produces a correcting change in the
grid bias of V7 through the action of V8B, thus maintaining
an essentially constant output voltage despite changes in
line voltage or load on the supply. The gain of V8B is
high enough to keep the output at the V7 cathode regulated
00102-2
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