4-16 888-2408-002
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
Each transformer has a 3 wire extended DELTA or extended WYE primary
(depending on your AC input configuration) and two secondaries labeled A and B
which are always WYE wound, with the center tap being the output. The outputs of
the 2 secondaries are combined through chokes and filtered by a single 120,000uF
capacitor. The combination of the extended primary windings and the dual
secondaries (each at a different phase angle) results in an output ripple frequency of
600/720Hz for 50/60Hz input. Each of the secondary windings has 4 output taps,
each of which are connected through an SCR to ground. The SCRs act as both tap
switches, activating the tap which gives the appropriate DC voltage output, and as
rectifiers. This gives a total of 12 SCRs per secondary, but only 3 SCRs will be
activated at any one time providing the ability to regulate the output DC voltage by
changing the transformer turns ratio. The 4 secondary taps basically change the
primary to secondary turns ratio allowing allow DC outputs of 48, 50, 52 and
54Vdc. The tap switching is controlled by the Power Supply Controller board. The
two transformer assemblies are designated PS1 and PS2. PS1 provides power for 8
of the 16 PAs. Four of these are on Z Plane A and four on Z Plane B. Therefore PS 2
provides power for the remaining 8 PAs. There is only one PS Controller for both
supplies, however each supply still operates independently. Also each Power Supply
is connected to the same PAs as its corresponding PA Controller.
PS1 and PAC1 connect to A1, A2, A5, A6, B3, B4, B7 and B8.
PS2 and PAC2 connect to A3, A4, A7, A8, B1, B2, B5 and B6.
This information is also shown in Table 4-1 on page 4-60.
4.4.1.1.1 Soft Start
During transmitter turn on, a fifth set of SCRs is activated. See Figure 4-8. These
are connected to the lowest voltage output tap of transformer secondary “B” (the
one with the fewest secondary turns). However, the Soft Start SCRs are connected
through a common surge limiting resistor to ground, instead of the direct ground
connection provided with the rest of the SCRs. This allows the “Soft start” by
limiting the maximum surge current while charging the power supplies. The soft
start lasts for about two seconds during turn on. After the two second soft start the
resistor is bypassed by activating the normal run-mode SCRs which are also
attached to the lowest voltage output tap but connect directly to ground. This allows
the supplies to come up to full charge. If the voltage is not high enough, the Power
Supply Controller will activate the next highest tap to increase the voltage. It will
continue this process until the DC output voltage is approximately 52Vdc or it
reaches the highest tap.
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