Service manual

STP 11-25R13-SM-TG
3 - 44
Performance Steps
4. Isolate the fault to a component.
a. Measure amplitude linearity with the VTSG stair-step signal.
b. Check visual modulator.
(1) Look for transfer characteristics inherently compressed in the white direction (+100 IRE)
on the WFM.
(2) Look for characteristics sometimes simultaneously compressed in the sync direction (-40
IRE) on the WFM.
c. For noise level problems, check preamplifier (preamp) input stages, the most common sources.
(1) Connect an oscilloscope to the preamp to determine if the noise is caused by a hum
component.
(2) Connect the oscilloscope to the power supply to determine if the filter circuit causes the
noise.
(3) Adjust the preamplifier power supply humpots while observing the noise measurement.
d. Check for improper frequency problems.
(1) Remove the oscillator after establishing the reference level.
(2) Leave all the gains as originally set.
(3) Place the noise and distortion meter in the noise position and increase the sensitivity to
obtain noise level in the unmodulated carrier.
(4) Replace the crystal oscillator if the above procedure does not eliminate noise from the
unmodulated carrier.
e. Check differential phase for 3.58 Hz with the oscilloscope.
5. Replace, repair, or adjust faulty component.
6. Operations check by verifying specifications to ascertain that no problems exist; readjust or align as
necessary.
7. Complete the DA Form 2407 (Blocks 27, 28, and 36), using the appropriate codes listed in DA Pam
738-750.
8. Turn the DA Form 2407 into the supervisor to inspect the work.
Performance Measures Results
1. Review DA Form 2407. P F
2. Sectionalize the trouble to a defective stage. P F
a. Visually inspect for discrepancies. P F
b. Operations check the transmitter. P F
c. Check TV broadcasting transmission standards. P F
d. Check TV audio mono specifications. P F
e. Check MTS. P F
3. Localize the trouble to a specific circuit. P F
4. Isolate the fault to a component. P F
a. Measure amplitude linearity. P F
b. Check visual modulator. P F
c. For noise level problems, check preamplifier input stages. P F
d. Check for improper frequency problems. P F
e. Check differential phase for 3.58 Hz. P F
5. Replace, repair, or adjust faulty component. P F
6. Operations check; readjust or align as necessary. P F
7. Complete the DA Form 2407. P F