supplies the Buffer Amplifier. The winding also supplies 24
Volts AC for the interlock circuit relays.
The other secondary winding is tapped to provide two different
output voltages for bridge rectifiers, CR13 and CR14. The
center tap is grounded, so that each rectifier provides a positive
and negative output voltage. Rectifier CR13 pr8 VDC
and -8 VDC and CR14 pr22 VDC and -22 VDC. Large
electrolytic filter capacitors are used for all Low Voltage sup-
plies. Each capacitor has a bleeder resistor connected across its
terminals to discharge the capacitor when the supply is turned
off. All transmitter circuits, except Power Amplifier and Driver
Modules, operate from the Low Voltage supply.
4.3.1
Power Distribution Board, A39
The Power Distribution Board distributes the +8 VDC and +22
VDC outputs to individual printed circuit boards, where +5
VDC and +15 VDC regulators and zener diodes provide re-
quired voltages for circuits on the boards. The -8 VDC and -22
VDC outputs are also distributed to individual boards where
they are regulated to -5 VDC and -15 VDC.
4.4
PA Power Supply: Circuit Descrip-
tion
The PA Power Supply contactors K1 and K2 in the Step Start
panel are driven by transmitter logic circuits to provide a step-
start function on turn-on. Auxiliary contacts on K1 and K2
operate the PA Supply Discharge Circuit (Crowbar) to discharge
the supply when it is turned off. The PA Power Supply trans-
former T1 su230 VDC, +115 VDC and +60 VDC for the
Power Amplifier and Driver stages. Refer to Sheet 1 of the
DX-25U Overall Schematic, 839-7855-151, in the Drawing
Package, for the following description.
4.4.1
Step Start Panel
Eight identical MOVs (metal oxide varistors) are mounted in the
Step Start panel to absorb transient voltages on the incoming AC
lines. Two are used on the single phase 1 KVA feed, and the
remaining 6 are paired up and placed across the 3-phase lines.
When the PA Power Supply is energized by turn-on/turn-off
logic on the Controller, the single phase AC input voltage
energizes step-start contactor K1 through K101. AC power is
supplied to transformer T1 through three low-resistance high
wattage resistors (R1, R2 and R3). The series resistance limits
surge current as power supply capacitors charge.
When K1 energizes, an auxiliary contact also closes and pro-
vides a +30 Volt “K1 has closed” logic signal back to the
turn-on/turn-off logic on the Controller.
After approximately one second, the turn-on/turn-off logic en-
ergizes contactor K2 and applies primary power directly to
transformer T1, completing the step-start sequence. An auxiliary
contact provides a +30 Volt “K2 has closed” logic signal to the
turn-on/turn-off logic. Approximately one-half second later,
step-start contactor K1 de-energizes.
4.4.2
PA Supply Discharge Circuit (Crowbar)
This circuit is comprised of four power MOSFETs operating in
parallel to discharge the PA Power Supply anytime K1 and K2
are deenergized.
When the PA Power Supply is energized, transistors Q101,
Q102, Q103, and Q104 function as open switches. In this
condition, K1 is deenergized and K2 is energized. The AUX
contact of K1 is closed and the AUX contact of K2 is open. When
K2 deenergizes, the AUX contact closes and applies a control
voltage from the +230 VDC supply to the gate of each MOSFET.
This voltage switches the MOSFETs ON and discharges the
supply through resistors R101, R102, R103, and R104. Each
MOSFET has a zener diode from gate to source to limit the gate
voltage to 10V.
Resistors R105 and R106 provide a secondary discharge path
with a faster time constant than the combination of the individ-
ual bleeder resistors mounted across each filter capacitor. The
+115 VDC supply is also discharged through the PA Supply
Discharge Circuit through CR24.
4.4.3
Power Supply Shorting Switches
Switches S9, S10 and S12 short the +230 VDC supply bus to
ground if any Power Amplifier or Driver Compartment inter-
locked door is open.
4.4.4
PA Power Supply Transformer T1
Transformer T1 is a three-phase, open frame power transformer
with multiple windings in the primary to allow an input voltage
range from 360 VAC to 505 VAC. Three sets of secondary windings
provide AC for the +230 VDC and +115 VDC rectifier assemblies.
4.4.5
Interphase Transformer T3
The +230 VDC, six-phase rectified outputs are combined
through interphase transformer T3. The use of an interphase
transformer reduces the losses in the supply transformer and
rectifiers.
4.4.6
+230 VDC Supply
The +230 VDC 12-phase output of T3 is fed to the distribution
bus on Fuse Boards A24 and A25. A 12-phase rectifier assembly
provides a DC output with a small ripple component at 12 times
the power line frequency and reduces the supply filtering re-
quired. A parallel supply reduces the peak current requirements
of the rectifiers. To further improve overall efficiency, the total
number of rectifiers in each assembly is doubled. Each rectifier
is fused separately to provide built in redundancy.
4.4.7
+115 VDC and +60 VDC Supplies
The third secondary of T1 is wired in a WYE configuration to
provide the +115 VDC supply and the neutral of this winding
su60 VDC. The third secondary of T1 feeds the rectifier
assembly CR16 thru CR21. The +115 VDC output of CR16,
CR18, and CR20 is sent to the A24 Fuse Board for the Driver
and Binary Modules and filtered by C19 and C7. The neutral
DX-25U
4-4
888-2297-002
Rev. AA: 8-9-00
WARNING: Disconnect primary power prior to servicing.
