888-2408-002 4-55
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
The IPA Voltage sample is taken from the voltage divider made up of R45 and R48.
This is sent to the Master Controller for monitoring and protection.
4.7.6.1.8.1 U18 Supply Voltage
The supply voltage for U18, VDP1 and VDN1 comes from U22 and diodes CR16,
CR17 and CR6. The samples coming into U18 at pins 2 and 3 are about 52 VDC.
The supply voltage for U18 is floating at VDP which is the PA voltage plus 4 volts
and VDN which is the PA voltage minus 4 volts. Basically U22 is a small switching
supply providing a floating +/- supply for the op-amps U18 and U21.
4.7.6.1.9 Forward Power Sample
The forward power sample from the directional coupler enters the Life Support
Board at J2. The sample is detected by CR8 and buffered by U1-14. The DC voltage
at TP8 is proportional to the forward power output of the transmitter. This is sent to
the Master Controller where it is used for monitoring, display, automatic power
control or APC and calibration.
4.7.6.1.9.1 CPLR_DISC_FAULT
This is an interlock fault which will mute the transmitter if the cable to the forward
directional coupler is disconnected. The fault line is pulled up through R1 to +5V. If
the coupler is connected, the line is pulled low and no fault occurs. When
disconnected, the line will pull high and the Master Controller will mute the
transmitter.
4.7.6.1.10Reflected Power
The reflected power sample enters the board at J3. It is detected by CR10 and
buffered by U1-1. The voltage at TP7 is directly proportional to the amount of
reflected power coming back to the transmitter from the antenna or transmission
line. This DC sample voltage is sent to the Master Controller, via J1-A19, for
VSWR protection and monitoring when the Master Controller is operational. The
VSWR fault comparator output at U3-2 will only be able to effect transmitter
operation in Life Support Mode.
4.7.6.2 Life Support Mode
The following contains all pertinent circuit information for operating in the Life
Support Mode. This is when the Master Controller has failed and the Life Support
Board has taken over control of the transmitter.
4.7.6.2.1 Master Controller Failure
The Master Controller has what is referred to as a “Watchdog Timer”. As long as the
micro-controller on the Master Controller board is working and executing its
program normally, the Watchdog Timer will be sending out a pulse every 1.0msec.
These pulses are detected by 1/2 of U4 on the Life Support Board (U4-4), a re-
triggerable one-shot with a 2msec time-out, which will stay triggered (SET) as long
as the Watchdog Timer pulses continue. If the Watchdog Timer pulses are
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