IET Labs, Inc. 1864-1644, User And Service Manual

Page: 30 / 46

THEORY
1864-1644 Positive Polarity Megohmmeter
20
A unity-gain FET-input ampli fi er (+1) follows the
standard resistors in the circuit con fi guration. R210
and C203 comprise a low-pass fi lter input to FET
Q204. The ampli fi er components include a differ-
ential ampli fi er (Q202, Q203), a coarse ∞ control
(R244), the SET ∞ control (R242) and an output
transistor (Q201). The signal then enters the series
combination of R135 and R134 back to the GUARD
terminal.
Resistors R221 through R223 (R
B
) are meter-sensi-
tivity resistors that are ganged to the voltage resistors
R124 through R127 (R
A
). R222 is used for both the
50 V and 500 V ranges, while the 200 V range uses
the circuit resistance and has no added resistor. The
remaining two resistors, R221 and R223, are used
for the 250 and 100V ranges, respectively. Potenti-
ometer R243 is an adjustable control in the meter
sensitivity circuit.
5.2.3 Type 1864 Megohmmeter (Figure 7.9)
The circuit of the 1864 Megohmmeter is basically
the same as that of the 1863 (paragraph 5.2.2). The
exceptions are explained in the following paragraphs.
In the 1864 the second dc power supply is a qua-
drupler. This supply establishes the plate voltage of
V101 with the use of resistors R109 through R114.
The regulator circuit has a slightly different input
when the
TEST VOLTAGE switch is switched
from V (1) to 0 V (10). Resistors R124 and R125 are
switched out of the circuit in the 0V (10) position.
Voltage-selection resistors for the 1864 are R126
through R133 and the meter sensitivity resistors are
R221 through R228. An additional range resistor,
R220, is in the 1864.
Figure 5-1 Megohmmeter block diagram