Service manual

STP 11-25R13-SM-TG
3 - 46
REPAIR A RADIO TRANSMITTER
113-575-0058
Conditions: In a tactical or garrison environment, given an inoperable or malfunctioning radio transmitter
with an accompanying DA Form 2407, dual-trace oscilloscope with 10X test probes, multimeter with test
leads, ATSG, electronic equipment tool kit, soldering tool kit (light to medium duty), bench stock, Federal
Communications Commission (FCC) and local-country modulation (LCM) standards, manufacturers'
manuals, DA Pam 738-750, and another person for safety.
Standards: The standards are met when the transmitter is repaired and operating within FCC and LCM
standards.
Performance Steps
WARNING: For safety, always work on transmitters with another person present.
1. Review the DA Form 2407.
2. Sectionalize the trouble to a defective stage.
a. Visually inspect for physical damage, missing controls, broken switches, and other
discrepancies.
b. Turn ON transmitter power and allow for a 5-minute warm-up time.
c. Check specifications for an AM transmitter.
(1) Maintain input power between 90 percent and 105 percent of the authorized antenna input
power.
(2) Maintain carrier frequency for monophonic transmissions or the center frequency for
stereo transmissions within +/- 20 Hz of the assigned frequency.
(3) Do not exceed 100 percent modulation of the carrier on negative peaks or 125 percent on
positive peaks.
(4) Check that total harmonic distortion (THD) at 0 to 84 percent is less than or equal to 5
percent; at 85 to 95 percent, THD is less than or equal to 7.5 percent.
(5) Maintain intermodulation distortion (IMD) 30 dB below the level of either of the two
modulating tones.
(6) With a 1 kHz reference level, check that frequency response is within +/- 2 dB from 100 Hz
to 5 kHz at 25, 75, and 100 percent modulation.
(7) Check that carrier shift does not exceed 5 percent at any percent of modulation.
(8) Check that unweighted carrier hum and extraneous noise level over a band of 30 Hz to 20
kHz are 45 dB below a 400 Hz, 100 percent modulation reference.
(9) Check spurious emissions from the carrier using the unmodulated carrier as a reference.
( a) Attenuate emissions between 15 kHz and 30 kHz at least 25 dB.
( b) Attenuate emissions between 30 kHz and 75 kHz at least 35 dB.
( c) Attenuate emissions more than 75 kHz at least 80 dB.
d. Check specifications for an FM transmitter.
(1) Maintain input power between 90 percent and 105 percent of the authorized antenna input
power for an FM station authorized more than 10 watts (W) output power. Stations
operating on authorized output power of 10 W or less may operate at less than the
authorized power, but not more than 105 percent of authorized power.
(2) Operate carrier frequency within +/- 2 kHz of the assigned frequency. For transmitters of
10 W or less, operate at +/- 3 kHz of the assigned frequency.
(3) Do not exceed 100 percent total modulation, referenced to 75 kHz deviation. However,
stereo stations simultaneously providing subsidiary communication services on
subcarriers may increase the total peak modulation 0.5 percent for each 1 percent of
subcarrier injection modulation; however, the total carrier modulation must not exceed 110
percent (82.5 kHz peak deviation).
(4) Check the system audio THD for the modulating frequency range at 25, 50, and 100
percent of modulation.
( a) From 50 Hz to 100 Hz, THD is 3.5 percent.