500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 5-10
15 January 2005
(l) Standby 2 code variation, which is a 0.125
second blip that occurs 0.875 seconds before the
first ident character, shall be detected in the
ident code.
(m) Disconnect shorting jumper from TB3-8 (TB3-
12).
5.3.15 FAULT THRESHOLDS/ CHANGE-
OVER CHECK:
Check that the carrier level and
modulation depth fault thresholds are set to the
desired levels that will result in a main to standby
transmitter changeover occurring when RF output
falls below a predetermined minimum carrier level;
when the modulation envelope falls below a
predetermined percentage or keying is not present, for
a predetermined period of time; as follows:
5.3.15.1 Preliminary Changeover Procedures:
Verify the previous functional test/adjustment
prerequisites have been completed and then check that
the associated potentiometers and switches have been
set to the settings required to start the changeover
procedures as follows:
(a) Verify carrier level checks of paragraph 5.3.8
have been completed and are being met.
(b) Verify modulation depth requirements (tone
only) of paragraph 5.3.9 have been completed
and are being met.
(c) Connect an oscilloscope to
RF MONITOR
connector.
(d) Set switches as tabulated for
Test Setting
in
table 4-1.
(e) Set
TEST-Power/Mod
switch to
FWD PWR
.
(f) Set
RF
switch to
ON
.
(g) Verify the transmitter's RF output is the
intended carrier level, as indicated by
TEST-
Power/Mod
meter's forward power indication.
(h) If
necessary,
adjust
O/P POWER
potentiometer
A4A2R48 (A5A2R48) to set transmitter's RF
output to the intended carrier level.
(i) Set
MOD
switch A4S2 (A5S2) to
ON
.
(j) Oscilloscope
waveform's modulation envelope
shall be the intended modulation depth.
(k) If necessary, adjust
TONE LEVEL
potentiometer
A4A2R19 (A5A2R19) until the modulation
envelope on the oscilloscope waveform is the
intended modulation depth.
(l) Set
KEYING
switch A4S1 (A5S1) to
ON
.
(m)
SET THRSH
lamp A10DS1 shall be flashing on
and off, provided the keying frame contains
identification code.
NOTE
Refer to figure MD-34 to locate monitor PWB A10
and
SET THRSH
lamp A10DS1
.
5.3.15.2 Carrier Level Fault Threshold Check:
Verify the carrier level fault threshold is set to initiate
a changeover from the selected side (main transmitter)
to the other side (standby transmitter) when the carrier
level has decreased by 3.0 dB from the intended
carrier level or to a user determined minimum level,
as follows:
NOTE
International Civil Aviation Organization (ICAO)
standards dictate the transmitter to be turned off or a
warning alarm be generated when the RF output has
decreased by 3.0 dB or more from the intended
carrier level. The following procedures use -3.0 dB
as the carrier level fault threshold. If the user decides
to select a different threshold level, the user selected
threshold level shall be inserted where -3.0 dB is
referenced. The desired carrier fault threshold level
shall be recorded in
'Established References
section
of table 5-2
.
(a) Verify requirements of paragraph 5.3.15.1 have
been completed and are being met.
(b) Verify
TEST-Power/Mod
switch is set to
FWD
PWR
.
(c) Simultaneously monitor forward power
indication on
TEST-Power/Mod
meter and of
SET
THRSH
lamp A10DS1, while decreasing RF
carrier level by adjusting
O/P POWER
potentiometer A4A2R48 (A5A2R48) counter
clock-wise until
SET THRSH
lamp just turns full
on and remains on or forward power indication
has decreased by more than -3.0 dB.
