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|  load connected between the terminals listed
between the HEADSET and AF OUTPUT
below:
RETURN terminals of the audio test box
(1) HEADSET and AF OUTPUT RE-
and repeat the measurements made in e
TURN on the test box (the test box
above. Record the db indications. The db
is connected to the C-1439/U
relationships should be the same as in e
AUDIO jack for this test only).
above.
(2) AUXILIARY jack J1403-R (AN/
j. Connect the 600-ohm load and ME-
30B/U between terminals D and E of
T R C - 6 8 ) or terminal board
TB1702-23 (AN/VRC-24) and
BROAD BAND jack J704. Turn the NOR-
BB switch (TM 11-5820-222-20) to BB.
chassis ground.
(3) Terminals D and E of BROAD
k. Measure and record the audio output
BAND jack J704.
voltage at modulation frequencies of 100,
250, 400, 1,000, 4,000, 10,000, 15,000,
20,000, and 25,000 cps. The db indications
153. If. Bandwidth
of the ME-30B/U from 100 to 10,000 cps
Connect Radio Set AN/VRC-24 or AN/
should be within 3 db of the indication at
TRC-68 and the test equipment as shown
1,000 cps. The db indications at 15,000,
in figure 117. Operate the radio set for
20,000, and 25,000 cps should be not more
normal receive.
than -7 db from the db indication at 1,000
a. Set the AN/URM-25D to 3.5 mc.
cps.
b. Adjust the AN/URM-25D output level
for -1.0 volt on the ME-26/U.
c. Increase the AN/URM-25D output
level to 6 db above (twice) the level set in
Connect the AN/VRC-24 or AN/TRC-68
as shown in figure 116. The AN/URM-25D,
b above.
the ME-26/U, and the AN/USM-50 are not
and the frequency below 3.5 mc (indicated
used in this test. Connect the TS-382/U to
on the AN/USM-26) which provide -1.0 volt
the EXT MOD jack on the TS-497/URR to
on the ME-26/U. The difference between
provide external modulating frequencies.
the two frequencies (6-db bandwidth) must
a. Tune the rt unit and TS-497/URR to
be not less than 80 kc.
any convenient frequency.
e. Increase the AN/URM-25D output
b. Adjust the TS-497/URR for a 1,000-
level (at 3.5 mc) to 60 db above (1,000
microvolt signal modulated 30 percent with
times) the level set in b above.
a 1,000-cycle audio signal.
c. Set the front panel METER switch to
and the frequency below 3.5 mc (indicated
S-METER and adjust the TS-497/URR for
on the AN/USM-26) which provide -1.0 volt
a maximum S-METER indication.
on the ME-26/U. The difference between
d. Connect the TS-723A/U and the 600-
the two frequencies (60-db bandwidth) must
ohm load between audio test box terminals
be not greater than 150 kc.
SPEAKER and AF OUTPUT RETURN.
g. Determine the 6-db and 60-db half-
Note. To perform the harmonic distortion meas-
bandwidth difference percentages by using
urements in this procedure, refer to TM 11-5097.
the following formula:
e. Set the TS-382A/U modulating fre-
quency to 300 cycles and measure the
deviation (plus) - deviation (minus) x 100 =
harmonic distortion with the TS-723A/U.
average deviation
Record the percentage of harmonic distor-
Half-bandwidth difference percentage
tion. Repeat this procedure for modulating
frequencies of 500, 1,000, 2,000, and 3,000
difference (in megacycles) above
cps. The percentage of harmonic distortion
3.5 mc (d or f above).
for these measurements must not exceed
deviation (minus) is the frequency
10 percent.
difference (in megacycles) below
f. Repeat the procedure detailed in e
3.5 mc (d or f above).
above with the TS-723A/U and the 600-ohm
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