TM 11-5826-225-35
chassis ground and points indicated in the Test
(25) If audio output is low or missing,
connection column of the following chart. Ad-
check condition of switch contacts on
just signal output put to 400 cps at a level to ob-
wafers S1F and S1G. Check RF
tain the voltage indication specified in the
transformer T1 and associated cir-
Input column. Set vtvm for measuring ac volt-
cuit. Check RF resolver B3.
ages in the appropriate voltage range and
(26) Setting control unit and signal gen-
Connect ac probe of vtvm between the points
erator controls accordingly, listen for
indicated and chassis ground. Compare the
audio on the other two frequency
indications obtained on the vtvm with those
ranges. If weak or missing, check RF
listed in the output column, If gain differs
transformer T2 or T3, and resolver
abnormally from that listed in the chart, use
B3.
further isolating techniques (para 28) to de-
(27) Remove ground from agc test point
termine the defective component within the
TP3. Adjust variable resistor R90 as
stage.
outlined in paragraph 317.
Note. Turn variable resistor R82 fully clockwise.
After making gain tests, adjust R82 (para 3-17).
2-7. Stage-Gain Measurements
Use the techniques given in a through j be-
low when either the receiver audio is abnor-
mally low, or the bearing indicator has a slow
response.
a General Instructions. Connect the receiver
and control unit to Test Set, Direction Finder
Set AN/ARM-93 as shown in figure 2-2 and
outlined in paragraph 26b (1). Unless other-
wise instructed, do not connect RF Signal
G e n e r a t o r AN/URM-25 to the test set. In
addition to the test set and RF signal genera-
Set vtvm for measuring dc voltages from 0 to
tor, Multitester ME26/U and Audio Oscilla-
5 volts. Connect positive test lead to the cathode
tor TS382/U are also required for these tests.
of audio detector CR1 and negative test lead
Verify operation of the test equipment by per-
to the chassis. Turn variable resistor R90 fully
forming the steps outlined in the operation
clockwise. Using a jumper wire, ground agc
and maintenance sections of the technical
testpoint TP3. Adjust RF signal generator for
manuals covering the test, equipment being
a 142.5-kc output signal, modulated 30 percent
used. Record the output level of the signal gen-
at 400 cps. Set output level to approximately
erator for each test and determine the gain
2.5 volts. Using the 50-ohm probe, connect
of each stage. Stage gain is computed by divid-
output signal to the junction connection of net-
ing the input signal voltage applied to the
work Z1 and the collector of fifth IF amplifier
following stage by the input voltage applied
Q11 (labeled testpoint TP2 in fig. 510 or
to the stage under test. Some transistor stages
5-10.1).
Ad-
in various circuits of the receiver contribute
just output level of signal generator until vtvm
little if any gain and in some instances a loss
indicates 5 volts dc. For this reference voltage
in gain. These stage gains or losses are shown
level the signal generator RF output should
in charts included in the following stage-gain
not exceed 2.1 volts rms. When performing the
measurement procedures. Use the main sche-
stage gain tests in the following chart, start
matic diagram (fig. 510 or 5-10.1)
wit h the last IF amplifier stage and work hack
in conjunction with
toward the mixer stage. Connect signal gen-
the parts location illustrations referred to.
erator 50-ohm probe between chassis ground
and points indicated in the Test connection col-
b. Audio Amplifier Stages (figs. 2-3 a n d
umn. For each stage-gain test, adjust signal
2-4). Using a 10-microfarad coupling capaci-
generator output to the voltage specified in the
tor, connect the audio oscillator output between
2-25
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