Elenco AM Radio Kit - Manual (Page 20)

-19-
Figure 18
STATIC MEASUREMENTS
With the power OFF, connect the negative lead of your
VOM to any ground and the positive lead to the emitter
of Q3 as shown in Figure 18. Set the VOM to read 9
volts DC and turn ON the power. The voltage at the emitter of Q3 should be approximately 1 volt. If your
reading is different by more than 0.5 volts, turn off the
power and check your battery of power supply voltage.
Also check components R7, R9, R10 and Q3.
Q3 BIAS
If you do not have an RF generator or oscilloscope, skip the following test and go to Section 4.
DYNAMIC MEASUREMENTS
With the power turned OFF, connect the oscilloscope
and the RF generator to the circuit as shown in Figure
19. Set the RF generator at a frequency of 455kHz, no
modulation and minimum amplitude output. Set the
oscilloscope vertical sensitivity at 1 volt/division. The
scope probe must have an input capacitance of less
than 50pF or it will detune transformer T3. Turn the
power ON and slowly increase the amplitude of the RF
signal until you have 4 volts peak to peak on the
oscilloscope. Tune transformer T3 for a maximum output while readjusting the RF generator amplitude to
keep 4Vpp at the oscilloscope. After T3 is aligned,
move the scope probe tip to the base of Q3 and record
the peak to peak amplitude of the signal here:
Vb=__________Vpp. Turn the power OFF. The AC
gain of the second IF amplifier at 455kHz is equal to
4/Vb, and should be greater than 100. If your gain is
less than 100, check components C7, C8, R7, R9 and
R10. Also, make sure that transistor Q3 is properly
installed.
AC GAIN
Figure 19
Oscilloscope
Generator
Probe
.02
μF
Output
Adjust
V
COM V
TP8
TP8
TP8