Static measurements, Dynamic measurements – Elenco AM Radio Kit User Manual
Page 22
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Figure 21
STATIC MEASUREMENTS
With the power turned OFF, reconnect your VOM to test
point 3 (TP3) as shown in Figure 14. 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 any DC ground 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 21. Using a clip lead, short TP5 to
R8 as shown in Figure 21. This short prevents the
AGC from lowering the gain of the first IF amplifier.
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 0.8 volts.
AGC ACTION
.02
μF
Probe
Output
Adjust
Oscilloscope
Generator
TP8
TP8
Clip Lead