Service manual

STP 11-25R13-SM-TG
3 - 36
Performance Steps
4. Isolate the fault to a component.
a. Troubleshoot power supply problems.
(1) Thermal relay shutdown occurs when the power supply output current exceeds the current
rating specified for the operating temperature. When the temperature decreases to
normal, the thermostat resets automatically. If thermal relay shutdown occurs often,
forced air-cooling may be required.
(2) Determine whether the fault is internal to the power supply or in the load.
( a) With a dummy load substituted for the normal load, an internal fault causes the lamps
to burn brighter than the normal load and the supply shuts down.
( b) If the lamps light at the normal output voltage and remain lit, the problem is in the
load.
b. Troubleshoot sweep problems.
(1) If deflection fails entirely, the pickup tubes will be biased off and no video will be obtained.
(2) If a steady positive voltage exists, sweep failure has occurred unless there is a shorted
transistor in the protection circuit.
c. Troubleshoot video circuit problems.
(1) Check erratic video levels with test pulses.
NOTE: If the pulse inserted at the preamplifier input is erratic, then the trouble is in the preamplifier,
video amplifier, or processing amplifier (proc amp) in the camera head.
(2) Stable pulses but erratic pickup tube video indicates a problem in the pickup tube circuitry.
(3) In a multiple-channel color camera, erratic video usually exists in one channel. If all
channels exhibit erratic video, the trouble occurs at a point after the combination of all
channels in the encoder. RGB monitoring points will show no level changes on an
individual channel at the encoder input.
(4) Erratic video often indicates a problem in the remotely controlled video gain-control stage.
(5) For erratic black level (with peak-to-peak video level constant), check the test pulses; then
check video of varying amplitude.
NOTE: Sometimes, erratic video results when a clamp-pulse width or timing adjustment is on the edge of
the proper setting.
d. Troubleshoot Gamma circuitry.
(1) If the Gamma correction range of any one channel is low, the color balance with other
channels cannot be obtained.
(2) Check the Gamma circuitry with the camera focused on the crossed gray scale.
(3) With an oscilloscope, check the range of control against the specification sheet for the
camera.
NOTE: A troublesome Gamma stage is most often the result of a faulty transistor or diode.
5. Replace, repair, or adjust faulty component.
6. Operations check by performing the procedures listed in performance steps 2c (1) through (10) to
verify operational readiness of the camera; readjust or align as necessary.
7. Complete DA Form 2407 (Blocks 27, 28, and 36).
8. Turn the DA Form 2407 into the supervisor to inspect work.