Specifications
The cable interlock function is provided by two series connec-
tions. When all the RF Amps and Mod encoder cables are
properly connected, a closed circuit is established from J1-9 to
J1-10 (by four RF Amps and one cable) and from J1-20 to J1-19
(by four RF Amps and one cable).
Due to the voltage divider action of R89, R91, R53(7,8),
R52(6,1), and R53(5,6) and R52(4,1), the voltage at the + input
of comparator U34-11 will be greater than the voltage at the
-input U34-10.
The voltage established at J1-9 and J1-19 is approximately
15VDC.
Normally, when the transmitter is ON the fuses short the
+230VDC around the 56k resistors and LEDs.
This reverse biases diodes CR11 & CR12 which does not allow
any current to flow from the +22 Volt Supply through the 30k
resistor on the RF Amp.
Typically what causes a supply voltage fuse to open is one or
more shorted MOSFETs on one side of an RF Amp.
For example, shorted MOSFETs on the “A” side of the RF Amp
apply a ground to the cathodes of the diode CR11 and LED DS1.
The supply fuse will open and current flows through the 56K
resistor from the supply voltage illuminating the indicator RED
on the RF Amp with the open fuse.
Current also flows from the +22V Supply on the Mod Encoder
Board through the 30K resistor and diode on the RF Amp.
Therefore the voltage levelat U34-11 drops below that at U34-10
forcing the output at U34-13 to go low.
When the output of U34-11 goes low, diode CR25 will conduct
and establish approximately 0.6VDC at U48-9, Fuse Fault-L.
Whenever U48-9 is pulled low by CR24, CR26, CR28 and
CR30:
a. Inverter U48-8 goes high and saturates the transistor inside
U35-14. A Fuse Fault-L at J9-29 is sent to the Controller.
b. Inverter U48-6 goes low illuminating the Fuse Fault indi-
cator DS6 RED.
The Cable Interlock line U34-14 will not go low because when
U34-11 voltage decreases the reference at U34-8 also decreases,
and U34-14 will stay high.
L.4.2.11.2 RF Drive Open Fuse Detection
The RF Drive Fuse detection circuits operate on the same daisy
chain as the supply fuses, and in basically the same manner.
Normally, with good fuses, the RF Drive is rectified by CR19,
allowing C11 to charge up to approximately +20V.
This reverse biases CR17 and the detector chain is unaffected.
If F3 were to open, the voltage on C11 would discharge through
the 20k Ohm resistor R19.
This causes CR17 to be forward biased, which pulls the voltage
on the Interlock chain to ground through CR17 and R19.
This pulls U34-11 lower than U34-10, which causes U34-13 to
go low, causing the same transmitter action as explained in the
previous paragraphs.
L.4.2.11.3 Fuse Fault Enable
When the transmitter is ON, the B+/B- Enable switch U44
supplies +5VDC at pin 12. This voltage is connected to the anode
of DS6 Fuse Fault via R96. The Fuse Fault indicator is enabled
at this time.
When the transmitter is OFF, the +230VDC supply and the RF
DrivetoeachRFAmplifierisalsooff.
However, the Fuse Fault indicator DS6 will not illuminate RED
because the supply from the B+/B- Enable switch U44-12 will
be 0VDC.
L.4.2.12 Air Flow Monitoring - Mod Encoders A26 and A28
Only
The Air Flow Sense circuitry on the A26 and A28 Mod Encoders
consists of an Onboard Air Flow Monitor and an External Air
Flow Monitor that are summed together.
Mod Encoder A26 uses Air Flow Monitor A48 and monitors the
Center compartment.
Mod Encoder A28 uses Air Flow Monitor A49 and monitors the
Left Compartment.
Loss of a blower or restriction of air flow due to dirty filters or a
loose fan blade, will cause an Air Flow Reduced - TX Foldback.
The Controller will Foldback the power -6dB (1/4 the selected
power).
Further losses will cause an Air Flow Fault - TX OFF.
L.4.2.12.1 Onboard Air Flow Monitor
The Onboard Air Flow Monitor consists of an ambient tempera-
ture sensor, a “heated” temperature sensor, and a differential
amplifier.
Theambient temperature sensor U40 is mounted under theboard.
Its output is connected to the differential amplifier - input U38-
2.
The heated temperature sensor U41 is thermally connected to a
30 Ohm 20W resistor R115 by an angle bracket. Both are
mounted underneath the board, in front of the heatsink that is
cooled by the air channel in the compartment. Its output is
connected to the differential amplifier + input U38-3.
The differential amplifier U38-1 will only amplify the difference
between these two inputs.
Whenever the transmitter is ON, B+ voltage is supplied to heat
the resistor.
The output voltage of the heated sensor will increase as the
resistor heats up and then will eventually stabilize because air
flow is passing over both components.
Normal air flow in the transmitter will cause the voltage at U38-1
to stabilize. If air flow in the transmitter is decreased, the output
voltage will rise.
888-2339-002 L-7
WARNING: Disconnect primary power prior to servicing.