4-22 888-2408-002
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
4.5.1.1 Rectifier Board
The Rectifier Board contains the SCRs which are used for the tap switching as well
as rectification of the 3 phase AC. Each secondary winding has four taps, see Figure
4-11. Each of these taps have one SCR to ground. The Power Supply Controller can
turn on any one of the four SCRs at a given time. If the voltage is not high enough at
the DC output, the PS Controller can change the transformer turns ratio by turning
on a different SCR and therefore utilizing a different secondary tap. The taps closest
to the center will provide the least amount of secondary turns in circuit and will
therefore give the lowest DC output voltage. This means that the taps which are
furthest from the center will give the highest DC output voltage due to more active
turns in the secondary.
There are two rectifier boards mounted on the top of each power transformer. The
two boards were originally made as one board which is broken apart for assembly.
Therefore, there is an “A” side and a “B” side, both of which are shown on the same
schematic. The left side of the schematic is the “B” side and the right side is the “A”
side (except on sheet 3 which is “B” side only). There is also only one part number
for both boards. The “A” Rectifier Board is for the transformer secondary “A” with
the “B” Rectifier Board used with transformer secondary “B”. The Rectifier Boards
are operated in parallel and are tied together by a ribbon cable, J3, which is shown
in the middle of the schematic connecting the “A” and “B” sides.
4.5.1.2 Rectifier Board Circuit Description
Refer to the Rectifier Board schematic and Figure 4-11 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.
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