Section D
Driver Combiner/Motherboard (A14)
D.1
Introduction
This section includes a description of the Driver Com-
biner/Motherboard.
The Buffer Amplifier and 15 RF amplifier modules plug into the
Driver Combiner/Motherboard. RF drive from the Oscillator
enters the board to drive the Buffer Amplifier. The Buffer Am-
plifier provides drive to the Predriver. The output of the Predriver
is fed to through a series resonant circuit to the 14 RF amplifier
modules that make up the Driver stage. The Driver outputs are
combined and sent to the RF Drive Splitter to drive the 64 RF
amplifier modules in the Power Amplifier. The Driver Com-
biner/Motherboard is located in the Driver Compartment. Re-
moval, interconnecting wiring, and component access is made
from the rear of the transmitter.
Refer to SECTION V, Maintenance, for pc board maintenance
procedures. There are no adjustments on this board.
D.2
Circuit Description
Refer to the Driver Combiner/Motherboard Schematic, 839-
7855-095, and Sheet three of the Overall Schematic, 839-7855-
151, in the Drawing Package.
Refer to the following sections of this Technical Manual for
descriptions of printed circuit boards which plug into the driver
combiner/motherboard:
a. SECTION B, Buffer Amplifier A16.
b. SECTION C, RF Amplifiers
D.2.1
Buffer Amplifier Connections
The signal from the Oscillator enters the Driver Combiner/Moth-
erboard at J28, and is routed directly to the Buffer Amplifier RF
input J1-39/40/41/42.
The Buffer amplifier supply voltage from the +30 VDC supply
goes through rheostat R2, enters the motherboard at J28-1, and
is then applied to the Buffer Amplifier connector J1-23/24.
The Buffer Amplifier RF output enters the board at J1-5/6, and
is routed to the Predriver connector J16. The RF output signal
also goes to a peak detector CR1, R6, C5, and R5. Zener diode
CR2 limits the detector’s maximum DC output voltage. The peak
detector provides a Buffer Amplifier “RF sense” signal to fault
and overload circuits on LED Board A32.
D.2.2
Predriver
The Predriver RF output leaves the Driver Combiner/Motherboard
at J27 and passes through series-tuned network L1, C2, and the
primary windings of IPA Splitter T8. The value of C1 is frequency
determined and T8 primary turns are selected at TB5 to provide
coarse adjustment for Predriver tuning. Coil L1 is tuned for maxi-
mum RF drive to the 14 RF amplifiers in the Driver stage. The RF
output from the splitter reenters the board at J17 through J20 to
drive the 14 Driver Modules.
The Predriver supply voltage from the +60 VDC supply goes
through rheostat R1 and enters the board at J28-2. The supply is
fed through metering circuits R9, R10, R11 and R12 and then to
J1 where it is fused on the Buffer Amplifier. The supply is then
sent back to the Predriver connector J16.
Another of the RF feeds from the RF Drive Splitter is used to
provide the Predriver “RF sense” signal. The RF sample enters
the board at J17-1 and is routed to RF detector circuit, CR3, C6,
R7, R8, and CR4. Zener diode CR4 limits the DC output voltage
from this peak detector. The peak detector DC output voltage is
sent to fault and overload signals on the LED Board.
D.2.3
Driver Stage
RF Amplifiers D1 through D7 and D9 through D14 operate from
the +115 VDC supply which enters the board at E1 and E2.
RF amplifier D8 operates on a regulated supply from the Driver
Supply Regulator. This 0-115 VDC supply enters the board at J29.
The Driver stage consists of fourteen RF amplifier modules, D1
through D14 which plug into J2-J15. The RF drive inputs from
the IPA Splitter T8 enter the board at J17, J18, J19 and J20. The
driver outputs are combined through T2 through T15 and effi-
ciency resonator coils L2 through L15.
D.2.4
Control Signals
Logic LOW turn-on signals are brought in to the board at J21, J22,
J23 and J24 from the Driver Encoder/Temp Sense Board. These
signals will turn the Driver Modules ON when the PA Supply is
energized. The Driver Modules are interlocked by series connec-
tions between each module. The interlock circuitry is on the Driver
Encoder/Temp Sense Board. The removal of any module will result
in a Cable Interlock fault to the LED Board.
D.2.5
RF Drive Combiner
The output combiner consists of 14 ferrite toroids with primary
windings connected to the RF amplifier module outputs. A
copper rod passes through the toroids for the secondary winding.
The copper rod is attached to the RF Drive Splitter.
The combiner adds RF voltages from the RF amplifier modules
as the secondary passes from one transformer to the next. The
RF voltage is low or zero at the ground point of the secondary
rod and increases until it is fed into the RF Drive Splitter. Because
of the low impedance of the RF Drive Splitter (about 2 Ohms),
the total RF voltage is less than 100 Vrms.
D.2.6
Driver Tuning
Capacitor assembly C3, connecting strap L5, and Buck/Boost
transformer T10 provides a series resonant circuit for driver
tuning. Capacitor assembly C3 and connecting strap L5 are
located above the Driver Combiner/Motherboard and are ac-
cessed through a panel on top of the transmitter. Buck/Boost
12-12-94
888-2297-002
D-1
WARNING: Disconnect primary power prior to servicing.
