18
Preinstallation Procedures
MB-750D-H and MB-750D-H/40 Installation Manual
Manual Thermal Gain Control
Manual thermal gain compensation is standard in the amplifier and is designed to compensate
for cable attenuation changes with temperature. Control is selectable and depends on the
position of the thermal control selector. This jumper is located on the circuit board and can be
positioned to
HI
,
LO
or
OFF
. The
HI
position is recommended for instances when 20 dB or more of
cable precedes the amplifier. The
LO
position is used for less than 20 dB of cable, and the
OFF
position is recommended for buried cable plant. Refer to Figure 2 in Section 2, Overview, or the
legend on the amplifier cover to locate the thermal control jumper. Position the jumper as
required by the installation.
Apply power to the unit and check the output of the power supply. The voltage at the power
supply test point should be + 24 Vdc
±
0.2 V.
Apply a sweep signal to the amplifier. Standard sweep testing procedures apply and should be
executed by a qualified technician. Turn the gain control to maximum (fully CW) and then
reduce the gain at 750 MHz as indicated by the required gain reserves in Table 2 for the current
ambient temperature.
Table 2
Gain reserves versus temperature
Required Gain Reserve
Degrees Celsius
Degrees Fahrenheit
7.0 dB
– 29
– 20
6.0 dB
– 12
10
5.0 dB
4
40
4.0 dB
21
70
3.0 dB
35
95
2.0 dB
49
120
Flatness
The output of the amplifier at this point should be flat, i.e., the low end signal level (channel 2) is
equal to the high end frequency signal level. Peak to valley response variations can be corrected
by adjustment of the MDR board and trimmer capacitors C80, C81, and C82 on the main board.
If the response exhibits tilt, an improper equalizer has been installed and should be replaced.
Less gain at the low end frequencies indicates that the next lower value equalizer may be
required, while loss of gain at the high frequencies indicates that the next higher value of
equalizer needs to be installed.
Figure 11 shows the location of the controls, while Figure 12 shows the approximate effects on
the frequency response of the amplifier. The changes in response, as they are shown, were
obtained by comparison against a normalized trace to emphasize the effect of the various
controls.
n
Trimmer capacitors C4, C6 and C80, together with potentiometers R6 and R7, control the
response at the low end frequency. These controls are used primarily to compensate for
diplex filter roll. Capacitors C80, C81 and C82 are located on the main PC board (Figure 5).
n
C81 and C82 control the high end bandpass.
n
Trimmer capacitor C3 and potentiometer R4 are used to obtain overall flatness.
