4-24 888-2406-002
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
Diagram for these connections). Note that both the “A” and “B” side SCRs have
selected the same relative (lowest voltage) taps on each of the two transformer
secondaries.
4.5.1.2.2PA Power Supply Soft-Start
Soft-Start is used to limit the surge current during transmitter turn on. The Soft Start
and PA Power Supply Discharge circuits are able to use the same resistor, R48 since
only one or the other will be active at any one time. A simplified diagram of the Soft
Start and PA Power Supply Discharge circuits in shown in Figure 4-9. The Soft-
Start control signal is active high and comes directly from the PS Controller Board,
via J4-8. A high SOFT START signal at J4-8 causes U2-12 to go low. This turns on
FET switch Q32 which then applies the 10VB-SS through R65 to CR28, CR29 and
CR30. This in turn activates SCRs Q34, Q35 and Q36. The three SCR outputs are
labeled SS1-B, SS2-B and SS3-B and are connected to the 48V taps on the
transformer “B” secondary. This allows the power supply to start charging through
R48, a 5 ohm 100W current limiting resistor. After about 2 seconds, the DRIVER 1
signal will activate the normal 48V tap SCRs which will tie the taps directly to
ground, bypassing R48.
4.5.1.2.3PA Power Supply Discharge
The PA Power Supply Discharge circuit uses R48 to bleed down the supply
whenever the transmitter is turned off. The DISCHARGE signal is active low and
comes from the PS Controller Board. When low, it causes the output of U2-14 to go
high. This does two things:
a. First, it reverse biases CR31 which allows the gate of Q33 to pull high
through R54, thus shutting Q33 off. This removes the 10VB-SS used to acti-
vate the transformer taps. This prevents the PA Supply (and Soft Start) from
being activated while the DISCHARGE signal is active.
b. Second, it allows the base of Q37 to pull high, shutting it off. R62 will then
pull the gates of Q29 and Q31 low turning them on. Q29 and Q31 are
switches which tie the 52Vdc to R48, and the supply is discharged.
4.5.1.2.4Voltage Samples
Two AC voltage samples are taken from the 48V taps using 301K ohm resistors.
One from the “A” secondary at J4-14 and one from the “B” secondary at J4-13. The
samples are pulsating DC with a frequency of 360Hz and an amplitude whose
average is equal to the DC power supply output. They are sent to the PS Controller
and used to control the tap switching based on the amplitude of the pulses. The
signals are also bandpass filtered at 60Hz to detect blown SCR fuses and 120Hz to
detect the loss of one AC input phase.
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