4-26 888-2408-002
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
4.5.2.2 Rectifier Board Circuit Description
Refer to the Rectifier Board schematic for the following. The Rectifier Boards
perform three primary functions:
• Dynamic secondary tap switching using SCRs, to maintain a relatively constant
supply voltage and thus maintain constant transmitter power output despite fluc-
tuations in the AC line voltage. It also allows the controller to optimize transmit-
ter efficiency by selecting the best power supply voltage within its limited range
of about 6 volts.
• PA Power Supply Soft-Start.
• PA Power supply Discharge.
4.5.2.2.1 Tap Switching Circuit Operation
The control signals for the SCRs are active high and come directly from the PS
Controller Board. They enter the Rectifier Board at:
• J4-1 - DRIVER 1 (activates SCRs for 48V tap)
• J4-3 - DRIVER 2 (activates SCRs for 50V tap)
• J4-5 - DRIVER 3 (activates SCRs for 52V tap)
• J4-7 - DRIVER 4 (activates SCRs for 54V tap)
NOTE:
The circuit for the 48V tap will be explained here as the other three are identi-
cal, but are attached to different transformer taps.
The DRIVE_1 signal is used to activate the 48Vdc tap on T1 and T2 secondaries.
When active (HIGH) it causes the output of U2-17 to go low. This turns on Q1
which applies 20VB-SS, now called DRIVE_1, to R16.
NOTE:
The 20VB-SS (SS = Soft Start) will only be present if the DISCHARGE sig-
nal at J4-6 is inactive. See PA Power Supply Discharge later in this section for
more information.
The 20VB-SS forward biases diodes CR3, and CR4. This turns on the associated
SCRs, CR4 and CR8 on each power supply. Each of the SCR anodes are tied to
ground with the cathode connected through a 100 amp fuse to a tap on one of the
transformer secondaries. Note that both power supplies have selected the same
relative (lowest voltage) taps on each of the transformer secondaries.
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