Service manual

STP 11-25R13-SM-TG
3 - 41
REPAIR A TELEVISION TRANSMITTER
113-575-0057
Conditions: In a tactical or garrison environment, given a TV transmitter with a distorted signal, video
audio test signal generators, dual-trace oscilloscope with 10X test probes, multimeter with test leads,
electronic equipment tool kit, soldering tool kit (light to medium duty), bench stock, DA Form 2407, DA
Pam 738-750, manufacturers' manuals, Federal Communications Commission (FCC) and local-country
modulation (LCM) standards, and another person for safety.
Standards: The standards are met when you can troubleshoot to the faulty component and repair to
restore the TV transmitter to peak power output.
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 the transmitter and allow for a 5-minute warm-up time.
c. Check TV broadcasting transmission standards.
(1) Maintain visual output power as near as practicable to the authorized transmitter output
power, but not less than 80 percent or more than 110 percent of the authorized power.
(2) Operate the visual carrier frequency at 1.25 MHz above the lower boundary of the channel
and within +/- 1 kHz of the assigned frequency.
(3) Operate the aural carrier frequency 4.5 MHz +/- 1 kHz above the actual visual carrier
frequency.
(4) Check that the total modulation of the aural carrier does not exceed 25 kHz deviation
(monaural). Check that the total modulation of the visual carrier does not exceed 87.5
percent.
(5) With a reference visual carrier of 200 kHz, check visual frequency responses:
( a) +/- 2 dB at -500 kHz.
( b) +/- 20 dB at -1.25 MHz.
( c) +/- 42 dB at -3.58 MHz.
( d) +/- 2 dB at 500 kHz.
( e) +/- 2 dB at 1.25 MHz.
( f) +/- 2 dB at 3.58 MHz.
( g) +2 dB/-4 dB of 3.58 MHz level at 4.18 MHz.
( h) -20 dB at 4.75 MHz.
d. Check TV audio mono specifications.
(1) Operate audio frequency response (with a 1 kHz reference level) within +/- 2 dB from 50
Hz to 15 kHz.
(2) Check that the effective radiated power of the aural transmitter does not exceed 22
percent of the peak radiated power of the visual transmitter. Nominally, operate the
effective radiated power at 10 percent of the peak-radiated power.
(3) Operate blanking level at 75 +/- 2.5 percent of the peak-carrier level.
(4) Operate the reference white level of the luminance signal at 12.5 +/- 2.5 percent of the
peak-carrier level.
(5) Check that the audio signal to noise (S/N) ratio is greater than 55 dB from 50 Hz to 15
kHz. Perform the S/N measurement with the exciter not in operation.
(6) Check system audio total harmonic distortion (THD) through the modulating frequency
range at 25, 50, and 100 percent modulation.
( a) From 50 Hz to 100 Hz, THD is 3.5 percent.
( b) From 100 Hz to 7.5 kHz, THD is 2.5 percent.
( c) From 7.5 kHz to 15 kHz, THD is 3 percent.