Handbook

PBL UHF1/1A 1 & 2 Air Interface
Maintenance Handbook
H/book Number:-60-055900HBKM
Issue No:-2
Date:-20/10/2003
Page:-
51 of 85
5.1.6 3 Stage Amplifier Alarm Boards (12-002201)
5.1.6.1 Description
Amplifier Alarm Boards are fitted to monitor the bias conditions of AFL Class A
amplifiers which remain constant in normal operation. Any departure from normal bias
conditions is a result of device failure, excess temperature, over-driving or oscillation
(excessive power).
In normal operation, the Class A bias circuit of the amplifier develops a constant voltage
of 1.20V across the collector current setting resistor. The Amplifier Alarm Board is a
window comparator device, which is adjusted to sense a departure from this condition.
Several different alarm outputs are provided to simplify interfacing, (Relay Contact, Open
Collector, and TTL Logic Levels)
The basic version of the Alarm Board (12-002801) monitors a single amplifier stage. A
three-stage version (12-002201) is used on complex amplifiers where three separate
comparators have their outputs logically combined to a common output stage. Failure of
any one stage will activate the alarms.
Note that the alarm board has a green Light Emitting Diode located near to the centre of
the printed circuit board, which is illuminated on ‘Good’, and extinguished on ‘Alarm’. It
is therefore a simple matter to identify an active module failure, by searching for an
Alarm Board which has its green LED extinguished. A simple test of the alarm board is
possible by shorting across the monitor inputs, pins 1 and 2, 3 and 4 or across pins 5 and
6. This last monitor input is inactive if the board has been converted to a two way alarm
board. (Refer to relevant amplifier alarm wiring diagram.)
1) Volt-free change over relay contacts.
2) Open collector NPN transistor pulls low on alarm.
3) TTL driver.
The use of precision voltage sources and resistors has eliminated the need for initial
adjustment or calibration, and the board will function correctly with a wide variation in
power supply voltage (8 to 30 volts, nominal supply is 12 or 24Volts).
There are two selectable link options on the three-way board:
LINK1 - Removed to convert to two-way alarm board.
LINK2 - Removed to isolate 0V from chassis earth.
The one way alarm board only has the 0V isolation link (LINK2) fitted.