TM 11-6625-1614-15
Model 410C
Section IV
Paragraph 4-1 to 4-18
4-1.
4-2.
OVERALL DESCRIPTION.
The Model 410C includes
SECTION IV
THEORY OF OPERATION
an input network, a
modulator - amplifier- demodulator, and a
meter
circuit. A block diagram of the Model 410C is shown
in Figure 4-1.
4-3. Signals to be measured are applied through the
appropriate input lead to the input network. AC volt-
ages are detected in the AC probe, and therefore all
signals to the input network are DC. The input net-
work attenuates the DC signal to a level determined
by RANGE and FUNCTION SELECTOR settings. The
attenuated DC voltage is applied to the modulator which
converts the DC to AC for amplification. The ampli-
fied AC signal is converted back to DC voltage inthe
demodulator and coupled to cathode follower VIB. The
cathode follower output to the DC AMPLIFIER OUT-
PUT connector and meter circuit is a DC voltage
proportional to the amplitude of the signal applied to
the input. A portion of the voltage to the meter circuit
is returned to the modulator as feedback. When the
feedback voltage and attenuated DC voltage are nearly
equal, the meter stabilizes.
4-4. CIRCUIT DESCRIPTION.
4-5. INPUT NETWORK.
4-6. The input network includes a precision voltage
divider, which by means of the FUNCTION SELECTOR
and RANGE switches, providesa maximum of 15 milli-
volts at the modulator input regardless of the range
set and signal applied. The ± DCA, ±DCV, OHMS, and
ACV modes of operation are discussed below.
4-7. DC CURRENT MEASUREMENTS: Refer to Fig-
ure 4-3, throughout this explanation. The purpose of
the input network is to provide proper attenuation of
currents applied.
Currents from 1.5 µa to 150 ma
full scale are applied with input impedance decreasing
from 9K ohms on the 1.5 µa range to approximately
0.3 ohms on the 150 ma range.
4-6. Tbe change in input impedance is varied by using
DC current shunts in conjunction with RANGE switch
A2S2. The DC voltage developed across these shunt
resistors, when applied through the modulator-am-
plifier-demodulator network to the meter, provide a
deflection on the meter proportional to the DC current
being measured.
4-9. DC VOLTAGE MEASUREMENTS. Refer to
Figure 4-4 throughout this explanation. The purpose
of the input network is to accurately attenuate the in-
put signal to a maximum of 15 millivolts at the modu-
01556-2
later input. The network presents an input impedance
of 10 megohms on the three most sensitive ranges and
100 megohms on all other ranges.
4-10. The resistor R1 (located in the DCV probe) in
conjunction with resistors A2R10 through A2R26, pro-
vides the 10 megohm input impedance required for the
three most sensitive DCV ranges. Resistors A2R4
and A3R30 are shunted out of the circuit by the RANGE
switch on the three most sensitive DCV ranges.
4-11. When using the eight less sensitive ranges,
A2R4 and A3R30 are placed in series with Rl and
A2R10 through A2R26 to present more than 100 meg-
ohm impedance to the input.
4-12. A3R30 is used to calibrate full scale on the
1500 volt range. (See Paragraph 5-35. )
4-13. RESISTANCE MEASUREMENTS. The purpose
of the input network shown in Figure 4-5 is to place
approximately 0. 6 volt DC source in series with a
known (reference) resistance. The resistance to be
measured is ptaced in parallel with the known resis-
tance, which changes the voltage proportionally. The
maximum changes in voltage applied to the modulator
is 15 mv because of attenuation provided by A2R4,
A3R30, and A1R2.
4-14. A DC current of approximately 60 ma is
supplied at the junction of A2R22 and A2R23 through
A7R10, R2, A2R2 and A2R1 to the input network. The
OHMS ADJ.,
R3, sets the meter for full scale
Resistor A2R1 is shorted out in the XIM position of
the RANGE switch; resistors A2R1 and A2R2 are
shorted out in the X10M range. The resistors A2R2
and/or A2R1 are electrically removed from the cir-
cuit to increase the voltage at the junction of A2R22
and A2R23. This is done to compensate for tbe load-
ing of the attenuator (A2R4, A3R30, and A1R2) on
these ranges.
4-15.
AC VOLTAGE MEASUREMENTS. Refer to
Figure 4-6 throughout this explanation. Voltage at
the AC probe is converted to DC and applied to the in-
put network. The input signal is attenuated to produce
a maximum of about 15 millivolts at the modulator in-
put .
AC zero adjustment of meter pointer is made
with the AC ‘ZERO control.
4-16. MODULATOR-DEMODULATOR.
4-17. Refer to the Amplifier Schematic, Figure 5-10 ,
and to the Mechanical Analogy Schematic,
Figure
4-2 throughout this explanation.
4-18. The input network applies approximately 15
millivolts DC, for full scale meter deflection (posi-
tive or negative, depending on the polarity of the
4-1
