888-2408-002 4-11
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
When the amplifier is used as the IPA, VG is used to linearly vary the IPA output
which then controls the transmitter power output. For an IPA, VG comes from the
Life Support Board and is controlled by the APC loop on the Master Controller
Board. Pressing the RAISE and LOWER controls on the front of the transmitter will
vary the APC voltage which in turn will vary VG to the IPAs. This will vary the IPA
RF output, which will raise or lower transmitter power.
The MOSFET outputs are combined in T2, a 1:4 transformer. C19 and C20 fine-
match the 50 ohm load impedance at the PA output to the output impedance of T2.
The +52Vdc or VD (Drain Voltage) from the main power supply enters at location
C-1 and is decoupled and filtered by C11, C12, C13, C14, C16, C24, L1 and L2.
The DC voltage reaches the drain of MOSFETs Q1 and Q2 through the center tap of
T2’s primary winding. The advantage of this method is to allow direct DC
connection to the MOSFETs and have an AC RF output through the coax without
the need for a blocking capacitor.
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 mainly due to the
minimal cost involved. Refer to the Maintenance section of this manual for the RF
amplifier replacement procedure.
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