TM 9-4935-282-34
803D
f. The filament of V202 and the chopper drive unit
reduces the voltage to 5 volts. The Kelvin-Varley attenuator
require a stable supply. This is obtained by using a filtered
divides its input voltage (500, 50, 5, or 0.5 volts) into 50,000
half-wave rectifier (CR203 and C214) that drives a Darlington
equal increments any number of which may be selected by
connection of Q201 and Q202. A divider string R234 and
setting the five decades with the voltage readout dials. The
R235 uses the B+ supply to obtain approximately 11.8 volts for
output of the Kelvin-Varley Attenuator, therefore, provides an
pin-5 of V202 while pin-4 is maintained approximately 0 volts
extremely accurate reference voltage.
through the emitter-base junctions of Q201 and Q202. Any
b. 500 VOLT POWER SUPPLY. The 500 volt power
change in the half-wave rectifier voltage causes a
supply uses three diodes (CR101, CR102, and CR103) and a
corresponding change in the voltage across the emitter-
filter network (R109, R110, C101, and C102) to supply
collector junction of Q201 so that the filament and chopper
unregulated DC voltage to series passing tube V101. The
voltage remain stable. The filament supply for V203 obtained
voltage is regulated by comparing a sample of the output
from an ordinary 6.3 volt AC winding.
voltage tapped off a divider string (R123, R124, R121, and R2)
g. EFFECT OF AC COMPONENTS. The only AC
with the voltage from reference tube V102 in differential
voltage component that will reduce the accuracy of the 803D is
amplifier V104. The output of V104 is fed to differential
one that either saturates the chopper amplifier or one that
amplifier V105 which drives the grid of series passing tube
beats with the chopper frequency. Since the voltage required
V101.
The control action of the differential amplifiers
for saturation is greater than that required for beating, the null
continuously adjusts the voltage drop across the series
detector is most sensitive to an AC component with a
passing tube so as to keep the sample voltage equal to the
frequency that is a submultiple or a low multiple of the chopper
voltage of the reference tube. Any difference in voltage, the
frequency. However, this is easy to detect because the meter
error, is amplified by V104 and V105 and thus changes the
will beat at the difference frequency. The low pass filter at the
voltage drop across V101 to maintain the output at 500 volts.
input of the chopper amplifier will attenuate any AC
c. For proper operation a highly stable and accurate
component. The magnitude of the AC voltage appearing at the
balance of amplification must be maintained between the two
output of the filter depends on both is amplitude and frequency
halves of differential amplifier V104. The filament supply for
before filtering. For all practical purposes, one should never
V104 must be regulated to maintain this balance. Regulation
encounter any trouble above a hundred cycles. Below this, the
is provided by a transistor regulator which supplies constant
filter may not attenuate the AC component enough. However,
voltage to the filament of V104. The filaments of the AC to DC
this is not as bad as it appears. A 60 cycle AC voltage that is
converter tubes (V501 and V502) are also supplied by this
10% of the input voltage will cause an error of approximately
regulator. A filtered full wave rectifier (CR104, CR105, and
0.01% which is well within specifications. If AC components
C103) which is regulated by a three transistor network (Q1,
that affect accuracy are ever encountered, additional filtering
Q2, and Q3) supplies the regulated DC filament voltage. One
as set forth in paragraph 2-11 will eliminate the problem.
side of the filaments are connected to the reference supply
h. GAIN AND ZERO ADJUSTMENTS. Variable resistor
common (0 volts) while the other side is maintained at
R232 in the null detector power supply provides a means of
approximately 5.9 volts through the emitter-collector junction of
adjusting the output current of the amplifier to zero when there
Q3. The output of Q1 drives a Darlington connection of Q2
is no input signal. The gain of the amplifier is adjusted by
and Q3. Any variation in the unregulated supply causes a
means of R222 in the feedback circuit.
corresponding change in the voltage across the emitter-
i. RECORDER OUTPUT. The recorder output is picked
collector junction of Q3 so that the filament voltage of V104,
off divider string R225, R1, and R226. Output level control R1
V501, and V502 remains stable.
provides a means of adjusting the output voltage up to a
d. RANGE DIVIDER. In the 500 volt DC range the
maximum of at least 18 millivolts at full scale deflection. The
output of the 500 volt supply is passed directly to the Kelvin-
voltage at the output terminals is proportional to the meter
Varley attenuator by range section S2F. In the 50, 5, and 0.5
reading.
volt DC positions, range resistors (R320 through R330)
selected by section S2E divide the reference voltage to 50, 5,
3-3. 0 TO 500 VOLT REFERENCE SUPPLY
and 0.5 volts before it is switched to the Kelvin-Varley
attenuator by section S2F. With the AC - DC switch set to AC,
a. GENERAL. When the 803D is used for differential
AC - DC switch section S5E provides connection to the range
voltage measurements, an internal voltage is nulled or
resistors that divide the reference to 5 volts. This 5 volts is
matched against the unknown voltage. An extremely accurate
then passed to the Kevin-Varley attenuator by section S5D.
reference voltage is therefore required. This is obtained from
The voltage applied to the Kelvin-Varley attenuator is always 5
the 0 to 500 volt reference supply. The 0 to 500 volt reference
volts for AC because the AC to DC converter always supplies
supply is composed of a well regulated 500 volt power supply,
up to a maximum of 5 volts to the vtvm attenuator.
a range divider and a Kelvin-Varley five decade attenuator.
e. KELVIN-VARLEY ATTENUATOR. The five Kelvin-
The output of the 500 volt power supply is applied directly to
Varley decade resistor strings R401 through R449 and
the Kelvin-Varley attenuator for the 500 volt DC range. In the
associated voltage dials A through E (S6 through S10)
50, 5, and 0.5 volt DC ranges, the range divider reduces the
voltage to 50, 5, and 0.5 volts before it is applied to the Kelvin-
Varley attenuator. For any AC range, the range divider always
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