888-2406-002 4-23
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
4.5.1.2 Rectifier Board Circuit Description
Refer to the Rectifier Board schematic and Figure 4-13 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 3 phase AC line voltage. It also allows the controller to optimize
transmitter efficiency by selecting the best power supply voltage within its lim-
ited range of about 6 volts.
• PA Power Supply Soft-Start.
• PA Power supply Discharge.
4.5.1.2.1Tap Switching Circuit Operation
The control signals for the SCRs are active high and come directly from the PS
Controller Board. They enter the “B” 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 both secondaries. When
active (HIGH) it causes the output of U2-17 to go low. This turns on Q28 which
applies 10VB-SS, now called DRIVE_1, to R12 on the “B” side of the 48V section.
Interconnect cable J3 then connects the same signal to R25 on the “A” side Rectifier
Board 48V section.
NOTE:
The 10VB-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 10VB-SS forward biases diodes CR4, CR8 and CR12 on the “B” side and
CR13, CR17 and CR21 on the “A” side. This turns on the associated SCRs, Q4, Q8
and Q12 for the “B” side and Q13, Q17 and Q21 on the “A” side. Each of the SCR
anodes are tied to ground with the cathode connected through a 30 amp fuse to a tap
on one of the transformer secondaries. The “B” side is connected to L5, M5 and N5
with the “A” side connected to L4, M4 and N4 (see the Overall System Block
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