888-2406-002 4-13
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
Maximum RF output is 425 watts. Typical output level is 300 to 340 watts. The
power module is designed for single amplifier field replacement. In the event an RF
power MOSFET fails, field replacement is not practical due to the cost of
components, time and test equipment involved in repair and alignment of phase,
gain and response. This is a highly critical area, and if not done correctly, improper
module and transmitter operation will occur.
As an additional consideration, please observe the following warning from the
MOSFET manufacturer regarding beryllium oxide.
!
WARNING:
PRODUCT AND ENVIRONMENTAL SAFETY-TOXIC MATERIALS. THIS PRODUCT
CONTAINS BERYLLIUM OXIDE. THE PRODUCT IS ENTIRELY SAFE PROVIDED
THAT THE BEO DISC IS NOT DAMAGED. ALL PERSONS WHO HANDLE, USE OR
DISPOSE OF THIS PRODUCT SHOULD BE AWARE OF ITS NATURE AND OF
THE NECESSARY SAFETY PRECAUTIONS. AFTER USE, DISPOSE OF AS
CHEMICAL OR SPECIAL WASTE ACCORDING TO THE REGULATIONS APPLY-
ING AT THE LOCATION OF THE USER. IT MUST NEVER BE THROWN OUT WITH
THE GENERAL OR DOMESTIC WASTE.
Failed PA boards should be sent in for repair or exchanged. This module was
designed for very easy single amplifier replacement in the field. Refer to the
Maintenance section of this manual for the RF amplifier replacement procedure.
4.3.4.4.1Isolation Boards
The Z Plane has an associated Isolation Board which contains the isolation resistors
and disconnect switches for the two 4-way wilkinson combiners and the isolation
resistor for the 2-way Wilkinson combiner. The isolation resistors for the four way
combiners, R1-R8, are each 50 ohm. When all modules are working properly, these
resistors dissipate little or no power. However, when a PA fails approximately half
of the power of one module will be across the isolation resistor. The resistors will
handle the imbalance of a failed module for about 20 seconds. Therefore when a
module has failed, the isolation resistor will start to heat up. This rise in temperature
is detected by a thermistor, RT1-RT8, which is physically attached to the resistor.
The thermistor is monitored by the associated PA Controller Board. The PA
Controller then shuts off the module RF output via the PA gate voltage (Vg = -18V)
and activates the associated RF switch on the isolation board. The switch
disconnects the isolation resistor from the combiner and grounds output of the PA
(E1-E8). The entire Z Plane will be operating with a slight impedance mismatch (it
is now a 3-way combiner instead of a 4-way) but is well within the ratings of the
modules. The effects are minimized by the 2 way combiner and its isolation resistor,
which will tend to absorb imbalances.
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