When the ‘TEST’ switch S3 is pressed, the high voltage is no
longer shorted out and it will be outputted at the ‘TEST’ jack.
Current will then flow through the resistor under test to the
SIG binding post and then to the wiper of S2A, the OHMS
RANGE selector switch. This current then passes through the
selected resistor, R16 through R21. The IR drop generated
shows up as a negative voltage at the output, pin 6, of U3.
U3 is an extremely low bias current amplifier, so that less
than 1 pA is diverted into its input. Also, the protection
diodes, Q5 and Q6, are low leakage transistors. Thus, even
currents as low as 1 nA, generated by a 100 V test voltage
and a 100 GΩ resistor, are passed with essentially no loss.
NOTE: D5 and D6 on the display board parts layout (Figure
8) correspond to Q5 and Q6 on the display board schematic
(Figure 7a). This is due to the fact that transistors are being
used as diodes.
NOTE: The action of D3 is to create a virtual ground at the
input (pin 2) and at the ‘SIG’ binding post. This has the
advantage that the full value of the test voltage is developed
across the resistor under test, with no error caused by the
current monitoring resistor, R16 through R21. A second
advantage is that the power common may then be used as a
guard voltage because it is essentially at the same potential
as the ‘SIG’ binding post.
The signal current through the resistor under test increases
with increasing test voltage. Thus, the IR drop at the output
of U3 varies with the test voltage. The signal current with 50
V test voltage and a 1 MΩ test resistor is 50 µA. With S2A
set to the 1 MΩ range, this current flows through R21, 499
Ω. Thus, the voltage at the output of U3 is 25 mV. For higher
resistance ranges, S2A selects higher values of resistance,
R20 (4.99 kΩ), R19 (49.9 kΩ), etc. to compensate for
decreasing values of signal current. Thus, the output of U3
remains at 25 mV for full-scale test resistors for all ranges,
with 50 V test voltage.
However, if the test voltage is increased, the signal current
and the voltage output of U3 will increase. To compensate
for this, the amplifier section of U1 having pins 1, 2, and 3
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