-22-
Figure 18
Output
Adjust
Oscilloscope
.02
m
F
Clip Lead
GND
R6
GND
R6
Generator
STATIC MEASUREMENTS
With the power turned OFF, reconnect your VOM to test
point 3 (TP3) as shown in Figure 12. 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 .8 volts.
Since the current in Q2 is equal to the current in R6,
I(Q2)=.8/R6 or approximately .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
18. Using a clip lead, short TP5 to R8 as shown in
Figure 18. 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 .8 volts.
AGC ACTION
Summary of Contents for SUPERHET AM-550TK
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