Specifications

L.4.2.12.2 External Air Flow Monitor
Refer to the schematic diagram for the Air Flow Monitor (839-
7930- 022) for the following discussion.
Operation of this monitor is exactly the same as the onboard
monitor, except for the added cable interlock.
The 10 conductor ribbon cable from J1 on the Air Flow Monitor
is connected to J9 pins 1 through 10. J9 is a 20 pin jack. This
supplies the monitor with +5VDC at J9-3 and B+ voltage at J9-1.
TheAir FlowIn voltage from the monitor at J9-5 canbemeasured
at TP29.
When the cable is connected, the monitor presents J9-2 Interlock
with a ground. If the cable is disconnected, R205-1 is not
connected to ground and the transmitter can not be turned ON
due to an Air Flow Fault- TX OFF.
L.4.2.12.3 Summing Amplifier
The voltage from the Onboard Air Flow Monitor at U38-1 is
summed together by resistors with the voltage from the External
Air Flow Monitor at R208. This summed voltage is connected to
U38-13.
Summing amplifier U38-14 has an adjustable gain control R213
Air Flow Calibrate that is used to set the air flow trip point.
The voltage at U38-14 is fed to the Sensor Select Jumper J7-5.
A wire jumper on the plug J7 will connect J7-5 to J7-8 so that
the Air Flow Sense circuit is connected to the Sensor Select fault
comparators.
L.4.2.13 Temperature Monitor - Mod Encoder A29 Only
The Temperature Sense circuitry on the A29 Mod Encoder
consists of an External Temperature Sensor U1 that is mounted
on the heatsink of Big Step RF Amplifier RF1.
Poor cooling, high ambient temperature, or RF Amp overdissi-
pation due to mistuning or VSWR fault conditions will cause an
Air Flow Reduced - TX Foldback. The Controller will Foldback
the power in selected steps until a safe operating level is reached.
Further increases in temperature will cause an Air Flow Fault -
TX OFF.
The 3 conductor cable from U1 is connected to J6. This supplies
the sensor with +5VDC at J6-1 and ground at J6-4.
The Temperature Sensor In voltage from the sensor can be
measured at J6-2.
The voltage from the sensor is buffered by U38-7. As the tem-
perature of the sensor starts to increase, the voltage at TP6
Temperature will also start to increase.
The voltage at U38-7 is fed to the Sensor Select Jumper J7-6. A
wire jumper on the plug J7 will connect J7-6 to J7-8 so that the
Temperature Sense circuit is connected to the Sensor Select fault
comparators.
If the cable is disconnected at J6, U38-5 will be pulled low to
+6VDC and the transmitter can not be turned ON.
NOTE - No indication of this condition will be visible.
L.4.2.14 Sensor Select Fault Comparators
Operation of the Sensor Select Fault Comparators is the same if
it is connected to the Air Flow Sense or the Temperature Sense
circuits.
The voltage at J7-8 is buffered and inverted by U38-8 and can be
measured at TP33.
U37-1 and U37-14 form a two level fault comparator circuit. A
resistive voltage divider establishes voltages at the - inputs of the
comparators, +3.0VDC at U37-6 and +3.5VDC at U37-8.
The + inputs U37-7 and U37-9 are connected together.
If the voltage at U37-7 and U37-9 continues to increase, it will
become greater than the reference and U37-1 will go high.
When the output goes high, it will saturate the transistor inside
U35-1 and an Air Reduced/Temperature Fault-L TX Foldback at
J8-39 is sent to the Controller.
Inverter U49-6 will illuminate DS7 Air/Temp Fault RED, as a
visual indication of the warning.
If the voltage at U37-9 continues to rise, the U37-14 will go high
and saturate the transistor inside U35-7.
An Air/Temp Fault-L TX OFF will be sent to the Controller via
J8- 37.
L.4.2.15 B+ Voltage
The B+ Supply provides a nominal +7.2VDC to the Encoder
Output Drivers. See Factory Test Data Sheets for actual voltage.
This voltage is fixed and does not vary. It is used in conjunction
with the Modulated B- voltage to control the RF Amp turn-
on/turn-off time.
L.4.2.15.1 B+ Regulator
The input Unregulated B+ voltage at J5-7,8 can be measured at
TP10 Unreg B+. This input is fused by F3, while diode CR49
and capacitors C62 and C64 bypass transients and high fre-
quency noise on the unregulated input.
The B+ Regulator U42 is a variable voltage regulator in a TO-3
case. It is mounted to the heatsink underneath the board. Pin 1 is
the unregulated input, pin 2 the reference voltage, and pin 3 is
the regulated output.
The output voltage is determined by the reference voltage at pin
2 that is established by the voltage sample divider R124 and
R125.
Diodes CR42 and CR43 protect the regulator against incoming
transient voltages.
When the B+ Regulator is enabled, the output B+ voltage is a
nominal +7.2VDC and can be measured at TP12 B+. Indicator
DS8 will be illuminated GREEN.
When P13 is in the Normal position, J13 pins 2 and 1 connects
B+ to Encoder Output Drivers U7 through U14, U23 & U24,
and U27 through U32. Should the B+ voltage become too
positive, zener diode CR55 will clamp the line at +10VDC.
L-8 888-2339-002
WARNING: Disconnect primary power prior to servicing.