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MAINTENANCE MANUAL
H. Do not use the rack connector when you do a cable test of the video antenna cables. This is because a hybrid coupler is installed at FSS 295 in the vertical stabilizer cavity. The TDR procedure to find failures only does a test of each cable segment which connects the FSS 295 coupler to the receiver rack(s) and the video antennas (Fig. 201).
TASK 20-10-32-762-001
2. Coaxial Cable Test (Fig. 201)
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A. General
(1)
You can use the DISTANCE switch and FEET/DIV control used to do tests for cables with a maximum length of 2000 feet. You can show the full length directly on the CRT.
(2)
To test the cables, you must set the FEET/DIV control and X1/X.1 control until the CRT display is longer than the cable. For example, if the cable is 150 feet, set the FEET/DIV to 200 and the multiplier at X.1.
(3)
If the FEET/DIV control is set like this, it makes sure that the reflected signal shows on the display. Measure the distance between the incident pulse rise and the reflected pulse rise. Also, if the cable is less than 2 feet in length, the distance from the sampling bridge to the CABLE connector can have an effect.
(4)
To decrease the effects of jitter, use the X.1 multiplier if possible.
(5)
To more accurately find a discontinuity, set the FEET/DIV control to a lower value. (It is not necessary that the reflected pulse be in the display window.) Push the ZERO REF CHECK switch and adjust the ZERO REF SET control so that the incident pulse rise is set at a vertical reference line. The ZERO REF SET control possibly must be adjusted again when the FEET/DIV (METERS/DIV) control changes. Always set the incident and reflected pulse to the 10% points of their amplitude.
(6)
The CABLE DIELECTRIC switches let the TDR 1502 find problems in cables with different relative propagation-velocity constants (Vp). The SOLID POLY switch is calibrated for solid polyethylene dielectrics, which have a Vp of 0.66. The SOLID PTFE switch is calibrated for solid polytetrafluoroethylene (Teflon), which has a Vp of 0.70. The OTHER switch has a range from 0.55 to 1.00. It is controlled by the screwdriver adjustment control, VAR. When this screwdriver control is turned to the fully clockwise position, it is calibrated for air dielectrics. Air dielectrics have a Vp of 1.00. If all three of the CABLE DIELECTRIC switches are released, the TDR 1502 is automatically calibrated for air dielectrics (Vp=1.00).
B. Equipment
(1) Time Domain Reflectometer System - Tektronix
1502
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20-10-32
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A
757
MAINTENANCE MANUAL
(2) Breakout box A34011-1 (preferred)
A34011-112 (optional)
