Elenco AM Radio Kit - Manual (Page 33)

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With the power turned OFF, connect the equipment to the
circuit as shown in Figure 30. DO NOT connect the clip
lead from TP2 to Q1. Set the RF generator at 540kHz,
400Hz 80% modulation, and a low level of output. Turn
the tuning capacitor fully counter-clockwise. Turn the
power ON and a 400Hz tone should be heard coming
from the speaker. Tune the oscillator coil (L2) for a peak
on the oscilloscope. Adjust the RF generator output
during this process to maintain a peak at 0.5Vpp or less.
After peaking L2, set the RF generator frequency to
1600kHz and turn the tuning capacitor (C1) fully
clockwise. A 400Hz tone should be heard coming from
the speaker. Tune the oscillator trimmer capacitor on the
back of C1 for a peak on the oscilloscope (see Figure 31).
SETTING OSCILLATOR RANGE
With the power turned OFF, connect test equipment to your
circuit as shown in Figure 32. Set the RF generator at
600kHz, 400Hz 80% modulation, moderate signal
strength. Set the oscilloscope to read 0.5Vpp and turn the
power ON. Turn C1 fully counter-clockwise, then slowly
turn C1 clockwise until a 400Hz tone can be heard coming
from the speaker. Slowly slide the antenna coil back and
forth on the ferrite rod to obtain a peak on the oscilloscope.
For maximum signal, your location of the antenna coil may
have to be on the end of the ferrite rod (as shown in Figure
33). Change the frequency of the RF generator to 1400kHz and adjust C1 until a 400Hz tone can be heard coming
from the speaker. Carefully peak the reading on the
oscilloscope by adjusting the frequency of the RF
generator. Now tune the antenna coil to this frequency by
adjusting the antenna trimmer on the back of C1 (see
Figure 31). This process should be repeated until both
settings of the antenna track the oscillator tuning. Once the
antenna is properly aligned, carefully apply candle wax or
glue to the antenna coil and ferrite rod (as shown in Figure
33).
ANTENNA ALIGNMENT
After peaking the oscillator trimmer capacitor, return the
RF generator to 540kHz, and capacitor C1 to the fully
counter-clockwise position and readjust L2. Repeat the
last few steps until both settings of the oscillator are
correct. This process sets the oscillator range at 995kHz
to 2055kHz. If a frequency counter is available, you may verify this alignment by measuring the frequency at the
emitter of Q1 for both ends of the tuning capacitor (C1).
Be careful not to mistune the oscillator during this
measurement. A coupling capacitor of 82 picofarads or
less to the frequency counter is recommended.
Figure 32
GND
TP10
Oscilloscope
Probe
Generator
GND
TP10
Output Adjust
Close to antenna
Antenna trimmer
3 Leads
Oscillator trimmer
Antenna trimmer Oscillator trimmer
4 Leads
Figure 31
Wax
Wax
Coil
Holders
Figure 33