Daniels VT-3/140-SWA2, Instruction Manual

Page: 53 / 262

Transmitter Main Board Instruction Manual 3-7
4 Turn the transmitter on.
5 Adjust the balance compression level (R38) for compression at ±60% maximum
deviation.
6 Set the audio frequency to 2.4 kHz, then adjust the deviation control (R29) for
maximum deviation.
7 Reset the modulating frequency to 1 kHz and re-adjust R38 for ±60% maximum
deviation.
8 Repeat steps 6 and 7 until both conditions are met.
9 Vary the audio signal from 1 kHz to 3 kHz and measure the positive deviation and
then the negative deviation. Adjust the symmetry control (R14) until the ± deviation is
symmetrical. The variation between ± deviation levels should not exceed 300 Hz over
the 1 kHz to 3 kHz range.
10 Repeat steps 6 and 7 and re-adjust if necessary.
11 Adjust the balanced input level control (R31) until the deviation produced by a 1 kHz
tone at -8 dBm falls below ±60% maximum deviation, then adjust R31 so that the
deviation increases until compression is observed. The deviation should be ±1.5 kHz
or ±3 kHz for narrowband and wideband channels respectively.
12 A 1 kHz tone at -8 dBm input level should produce ±60% maximum deviation. If not,
go back to step 4 and make sure the pot is set for maximum gain and repeat the
procedure. If so, increasing the input level by +20 dBm should not increase the
deviation. This confirms that the AGC action of preamplifier U2 is working.
13 A 2.4 kHz tone at the desired audio input level should produce the maximum
deviation. Increasing the input level by +20 dBm should not increase the deviation.
This confirms that the limiting action of U4a and U5a is working.
14 Set the audio frequency back to 1 kHz at -8 dBm output. Confirm and record audio
distortion with the appropriate filter on the communications test set.
15 Confirm the audio frequency response by referencing all output deviation
measurements to a 1 kHz input tone at ±20% maximum deviation (±500 Hz for
narrowband or ±1 kHz for wideband).
16 Remove the signal to the balanced input (pins B18, Z18).