Harris Platinum Z2 CD, Manual

Page: 103 / 196

888-2406-002 4-33
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
The APC loop starts at the forward power directional coupler. The forward sample
is connected to the Life Support Board where it is calibrated with R127 for 8.9V@
100% TPO at TP9. This voltage sample is then sent to the Master Controller
(schematic sheet 7) where it is squared by multiplier chip U1 (to make the voltage
from the coupler proportional to power) and divided by 10. This is then sent to U5-
6 where it is compared to APC_REF voltage (APC Reference).
APC_REF is the DC output of a D/A Converter or DAC whose input is called
DAC_APC_REF and is controlled by the RAISE and LOWER buttons on the front
of the controller (or by the remote control inputs) or can be varied automatically by
the Master Controller’s foldback routines. Pressing RAISE increases the APC_REF
voltage and pressing LOWER decreases it. The DAC_APC_REF is responsible for
all transmitter power control functions including power level, foldbacks and mute.
This makes it a very important number to know and understand for troubleshooting.
NOTE:
As calibrated from the factory, the digital DAC_APC_REF number should be
approximately 660 (this number should be on the factory test data sheet). This
number can be checked by pressing [HOME, MORE, CONFIGURATION
C,B]. The DAC_APC_REF output voltage APC_REF can be metered at TP8
or in the Diagnostics Display by pressing [METERING, B,B].
The output at U5-7 is the difference between the forward power sample and the
APC_REF voltage. This difference can be measured at TP3 on the Master
Controller and is called the APC voltage. R13 and R102 divide this voltage down
for display on the Transmitter LCD Display on the right side of the controller. The
APC voltage is not the same as the APC_REF voltage. The dipswitch, S1-10, is
present to allow the APC loop to be opened for troubleshooting. This APC voltage
is now sent back to the Life Support Board (Schematic, sheet [3,B8]) and is called
MSTR_IPA_CTL (Master IPA Control).
If the Master Controller is operational, then the MSTR_IPA_CTL signal will pass
through to U5-4 which will in turn pass the signal if there are no System Mutes
present. Due to the low impedance of the circuit, the Manual PWR REF pot R25 has
little or no effect on the circuit. CR4 and R30 shift the positive APC voltage to a
proportional negative voltage which is passed on to IPA_AB1 or AB2 depending on
the position of K2. Remember that the gate bias on the RF amplifiers is -18V for
OFF and -0.6V for fully ON.
Since the exciter power is fixed, changing the IPA bias will increase or decrease the
IPA output which will then proportionately increase or decrease the output of the
PAs.