Vectronics VEC-1290K - Manual (Page 18)

VEC-1290K Instruction Manual AM Radio Transmitter
Using afield strength indicator: If you have a general purpose volt-ohmmeter,
the schematic below shows a simple and easily constructed RF field strength
meter. Most of the parts can be found at Radio Shack. Assembly is not critical
due to the very low operating frequencies.
The diode must be a germanium type. The 3-foot wire "antennas" should be
run near the VEC-1290K transmitter antenna. Adjust C11 for a maximum DC
voltage reading on the voltmeter. Keep the field strength meter pickup antenna
wires as far from the transmitting as possible to prevent interaction.
Use the
lowest DC voltage setting on the voltmeter for maximum sensitivity. Some
voltmeter test lead jacks may allow the diode leads to be slipped between the
test lead plugs and meter jacks-if so, the meter leads may be spread apart and
used as the pickup antenna.
Audio Gain Setting:
The VEC-1290K will accommodate a wide range of audio levels, from "line level"
cassette or CD deck feeds, or even the very low output from a local mike.
Removing capacitor C2 will reduce the amplifier gain; this is the preferred
configuration for usually high line-level audio sources such as when taking an
audio feed directly from a speaker output.
Setting the gain control. The gain control sets the modulation percentage.
Setting the gain control too high results in over modulation and considerable
audio distortion The proper setting for the gain control RI can be found by
starting at it lowest setting, and with an audio source connected, slowly
increasing the gain the control until the audio starts to sound raspy or clipped.
Once clipping is heard, the control setting should be reduced until the
distortion,goes away. This is most noticeable on music sources. Experienced
technicians may use an oscilloscope to observe the RF modulation envelope for
clipping, but those procedures are beyond the scope of this manual.
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