Static measurements, Dynamic measurements – Elenco AM Radio Kit User Manual
Page 28
-27-
Figure 27
STATIC MEASUREMENTS
With the power turned OFF, reconnect your VOM to test
point 3 (TP3) as shown in Figure 20. Set the VOM to
read 2 volts DC accurately and turn the power ON. The
voltage should be approximately 1.5 volts. If your circuit
fails this test, turn the power OFF and check Q2 and R6.
Q2 BASE BIAS
With the power turned OFF, connect the positive lead of
the VOM to the emitter of Q2. Connect the negative lead
of the VOM to TP10 and turn the power ON. The voltage
should be approximately 0.8 volts. Since the current in
Q2 is equal to the current in R6, I(Q2)=0.8/R6 or
approximately 0.8 milliamps.
Q2 CURRENT
If you do not have an RF generator or oscilloscope, skip the following test and go to Section 5.
DYNAMIC MEASUREMENTS
With the power turned OFF, connect the RF generator
and the oscilloscope to your circuit as shown in Figure 27.
Using a clip lead, short TP5 to R8 as shown in Figure 27.
This short prevents the AGC from lowering the gain of the
first IF ampifier. Set the RF generator to 455kHz, no
modulation, and minimum amplitude output. Set the
oscilloscope for a vertical sensitivity of 1 volt/division and
turn the power ON. Increase the amplitude output from
the RF generator until approximately 4Vpp registers on
the oscilloscope. Tune the IF transformer (T2) to maximize
the 455kHz at TP4. After tuning T2, adjust the RF
generator amplitude in order to keep 4Vpp at TP4. Now
move the oscilloscope probe to the base of Q2 and record
the peak to peak level of the 455kHz signal here:
Vb=____________Vpp.
The AC gain of the first IF amplifier is equal to 4/Vb. The
AC gain of this amplifier should be greater than 100. DO
NOT TURN THE POWER OFF. GO TO THE NEXT
TEST.
AC GAIN
Move the oscilloscope probe back to TP4 and adjust the
RF generator for 4Vpp if necessary. Remove the clip
lead shorting TP5 to R8. The AGC should reduce the
signal level at TP4 to approximately 0.3 volts.
AGC ACTION
GND
TP10
Output Adjust
Generator
GND
TP10
Oscilloscope
.02
μF
Probe
Wire lead
or clip lead