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WARNING: Disconnect primary power prior to servicing.
Overall System Theory
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, press [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.
EXAMPLE:
The following is an example of how the entire loop would work. For our example
the power output of the transmitter goes down due to a failed PA. This would
decrease the output of the forward power directional coupler and the voltage at the
input to the difference amp at U5-6 on the Master Controller Board. Since U5 has
almost infinite gain, the loop will try to force the forward power sample input to U5
to be the same as the APC_REF input. This will increase the APC voltage at TP3
and on the front panel APC reading. The increased APC voltage will pass through
the U5 analog switches (on the Life Support Board) bringing the input to U15-3 less
negative. This less negative voltage is sent to the IPA, increasing its output. More
drive to the PA will increase the transmitter RF output to compensate for the failed
PA. If the RF power output were to increase, the APC loop voltage would be
reduced, lowering the IPA and PA output.
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