Specifications

Sense comparators will generate an Encoder Supply Fault-L that
will turn the transmitter OFF.
L.4.2.18.1 B+ Comparator
U37-2 is the B+ fault comparator. A resistive voltage divider
formed by R128 and R20-8 supplies one-half the B+ voltage to
the + input U37-5. A similar voltage divider establishes the
2.8VDC threshold on the - input U27-4.
Should the B+ voltage become lower than the threshold, the
output U37-2 will go low.
Inverter U49-2 will go high and saturate Q5, causing a Supply
Fault-L to be sent to the Controller at J8-33. Inverter U49-4 will
also go high and illuminate DS9 Supply Fault RED.
L.4.2.18.2 B- Comparator and B- Fault Tacking
U37-13 is the B- fault comparator. A resistive voltage divider
formed by R131 and R20-5 supplies a sample of the B- voltage
the - input U37-10.
The B- Fault Tracking amplifier U47-7 takes a sample of the
modulated B- drive prior to the B- regulator. A similar voltage
divider establishes a reference signal of the modulated B- drive
on the + input U27-4.
Should the B- sample become greater than the reference signal,
the output U37-13 will go low.
This produces the same response as a B+ fault.
L.4.2.19 Normal/Bypass Jumper
Refer to the Emergency Operating Procedures for more informa-
tion on Mod encoder bypassing.
L.4.2.19.1 Normal Position
P13 is usually inserted into J13, the Normal position.
In this location, J13 1-2 connects regulated B+ to the B+ buss
(for Encoder Output Drivers) and the B+ Fault Sense. J13 4-3
connects modulated B- to the B- buss (for Encoder Output
Drivers) and the B- Fault Sense.
L.4.2.19.2 Bypass Position
When P13 is inserted into J12, the Mod Encoder is in the Bypass
mode.
In this location, J12 1-2 and 3-4 connects an unregulated B+
voltage supplied by resistors R132, R133, R211, and fused by
F2 to both the B+ buss and B- buss. This will turn OFF all RF
Amps controlled by the Mod Encoder by supplying a positive
voltage to the RF Amp turn-on/turn-off control line.
The B- Fault Comparator is bypassed because the + input of
U37-11 is pulled high by CR47 and R146. When this positive
unregulated B+ voltage is on the B- buss, indicator DS10 Bypass
will be illuminated RED.
L.4.2.20 Power Supplies
L.4.2.20.1 +22VDC
The unregulated +22VDC supply is used for the Module/Cable
Interlock and Fuse Fault functions. It enters the board at J5-1 and
is fused by F1. Indicator DS12 +22 will be illuminated GREEN
when the supply is present and this voltage can be measured at
TP9.
L.4.2.20.2 +5VDC Regulators
The Mod Encoder uses two +5VDC supplies, +5VA and +5VB.
The +5VA supply is used to power all the digital audio circuits.
The +5VB supply is used to power the fault and control circuits.
Both supplies are identical circuits and only the +5VA regulator
will be discussed.
The Unregulated B+ input from J5-7, 8 is fused by F5 and
connected to In terminal of U46-1. Bypass capacitors are used
on the input and output terminals along with a protection diode
CR58.
The output +5VDC at U46-3 can be measured at TP8 and DS13
will be illuminated GREEN when the supply is present.
L.4.2.20.3 -5VDC Regulator
U39 is a -5VDC regulator that is connected to a -12VDC supply
formed by dropping resistor R171. The output -5VDC is used by
the B+/B- Enable switch U44-7.
L.4.3
Troubleshooting Big Step Modulation Encoders
This board can be temporarily bypassed in an emergency. Refer
to Section 6A for Emergency Bypass Procedure.
Since all five modulation encoders contain the same circuitry,use
this procedure on any of the Mod Encoders A25-A29.
Faults with an encoder board can be classed into
a. Interlock/Fuse Fault
b. Air Flow Sensing
c. Supply Regulators, and
d. RF AMP turn on encoding.
Refer to the proper section for the area being addressed.
L.4.3.1 Interlock/Fuse Fault
If the Control Front Panel Displays a Module/Cable Interlock
Fault upon Step Start, and the Transmitter troubleshooting sec-
tion has referred to this section, begin troubleshooting by observ-
ing which Indicator DS1, DS2 through DS5, and DS6, is
illuminated on the Mod Encoders. This can be done with only
theLowVoltageon.
L.4.3.2 Cable Interlock DS2, DS3, DS4, or DS5 illuminated
NOTE
The circuitry for each indicator is identical therefore only one
example will be given. Use the example for DS2 listed and refer
to the schematic for the appropriate component locators for trou-
bleshooting DS3, DS4 and DS5 indicator illuminated.
If DS2 is illuminated RED on the Modulation Encoder, one of
the RF amplifiers or the encoder cable is not connected or open,
or the Interlock circuitry has failed.
L-10 888-2339-002
WARNING: Disconnect primary power prior to servicing.