Field Installation
23
MB-750D-H and MB-750D-H/40 Installation Manual
resulting measurement will be flat. Figure 6 in Section 3, Preinstallation Procedures, provides a
comparison of cable loss versus slope for three different operating frequencies.
Open the housing and check the ac and dc voltages to assure that both are within the required
limits. The ac voltage should be between 38 Vac and 60 Vac as measured with a true rms
voltmeter. The dc voltage should be 24V ± 0.2 volts.
Manual Thermal Gain Control
If the unit has been bench aligned, as described earlier, only minor changes should be expected
at this point. The following steps are to be used in making final manual thermal gain control
adjustments:
Connect a signal level meter to the output test point and tune the meter to the high end
channel nearest 750 MHz.
Turn the
MAN
control to maximum (fully clockwise) then reduce the output according to the
information provided in Table 2 in Section 3, Preinstallation Procedures.
The output level of the high end channel should be within 0.5 dB of the desired output signal
level. Minor adjustments of less than 0.5 dB can be made by using the
MAN
control.
If the output signal level is greater than the required level after performing step 3, the input
pad must be changed. Subtract the desired signal level from the measured level and the
result is the required pad value. Remove the module cover and install the proper input pad.
Replace the module cover and repeat step 3.
Output tilt, i.e., the difference in signal level between channel 2 and the high end, can be
measured through the specially prepared test lead as described in the preceding topic,
Balancing the Amplifier. The difference in signal level between channel 2 and the high end
should be negligible when measured through the test cable if the proper equalizer has been
installed. If the channel 2 signal level is greater than the high end, replace the cable
equalizer with the next higher value. If the channel 2 level is less than the high end level,
the next lower value equalizer must be installed.
Flatness can be measured if a sweep signal is present on the cable. The test cable described
previously will be required. Flatness should meet applicable system requirements. A
qualified technician can make minor adjustments on the MDR board to improve flatness.
Automatic Level Control
The thermal control jumper must be in the
ACB
position for the following steps. It is again
assumed that the unit has been bench aligned and only minor adjustments are to be expected.
It is desirable, although not essential, to perform the steps outlined in the preceding section for
adjusting the manual thermal gain control. Checking the gain reserve in the manual control
positions, either
LO
or
HI
, assures that operation in this mode is available and preset, should it
become necessary to revert to manual thermal gain control. The “reserves” in the manual or
automatic control modes confirm that the amplifier is operating within its dynamic window of
gain control.
