User's Manual

Weehawken Tunnel VHF Repeater System
User/Maintenance Handbook
Handbook N.-Weehawken_VHF Issue No:-A Date:-05/08/05
Page:-
55 of 75
7.7 3 Stage Amplifier Alarm Board (12-002201)
7.7.1 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.