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OPERATION
MODEL 169
b.
Select the range from the five ranges avail
able.
The decimal point is positioned by the range
pushbutton.
The "u" annunciator will appear if
200yA range was selected; otherwise, the "m" annun
ciator will appear.
It should be noted that the
voltage drop across the current shunt in the instru
ment (called voltage burden) can affect the circuit
under test if it has a low source voltage.
This
voltage burden can be reduced by selecting the
highest current range that will resolve the desired
measurement.
c. Connect the signal to be measured to the INPUT
HI and LO jacks; observe the current reading on the
displ ay.
2-26. RESISTANCE (q) MEASUREMENT.
2-27.
The Model 169 reads resistance from lOOmSl/
digit to 20 megohms.
The maximum reading is 1999
with appropriate decimal point.
Overrange is indi
cated by a "1" followed by three blanked digits.
Maximum Allowable Input:
300V dc or rms.
Use the
Model 169 to measure resistance as follows.
A
CAUTION
Turn the test circuit off and discharge
all
capacitors
before
attempting
in-
circuit measurements.
A voltage may be
present in the circuit if either the minus
sign is displayed, along with a steady
displayed number other than zero, or the
reading changes more than two digits when
the leads are reversed.
a.
Turn on power and depress the n pushbutton.
The "n" annunciator should appear.
b.
Connect the circuit to be measured to INPUT HI
and LO and select the desired range from the six
ranges available.
The decimal point is positioned
by the range pushbutton and the applicable range
annunciator should appear.
c.
Read the resistance on the display.
If the
200n range is selected, disconnect the test leads at
the test circuit, short them together and subtract
the reading obtained from the reading obtained
above.
NOTE
As
previously
mentioned
for
in-circuit
measurements, the presence of a voltage
will cause a change in the reading when
the leads are reversed. The presence of a
forward biased semiconductor junction will
also cause a significant error, and there
fore, it is a good practice to reverse the
leads and make comparative measurements of
in-circuit resistances.
In addition, it
should be noted that the bottom two re
sistance
ranges
will
not turn
on
most
semiconductors, except
in
an
overrange
condition.
d.
will
Diode Tests.
Any of the top four ohms ranges
semiconductor
junctions,
with
an
turn
on
on-scale reading, but the 20Kn range is recommended
for out-of-circuit tests of semiconductors because
its test current is higher.
To test a junction,
first forward bias it and then reverse bias it while
observing the display reading for both conditions.
The HI jack is positive.
Compare the results as
fol lows:
JUNCTION
INDICATION
CONDITION
1. Overrange, both Polarities
OPEN
2. Low reading, both polarities
SHORTED
3. Overrange when reverse biased
PROBABLY
and an on-range reading when
GOOD
forward biased
2-6
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