Q3 Bias - Elenco Electronics AM/FM-108TK Assembly And Instruction Manual

STATIC MEASUREMENTS

Q3 BIAS

Amps COM V/Ω
TP15
With the power turned OFF, connect your VOM to the
circuit as shown in Figure 39. Set your VOM to read 9
volts DC and turn the power ON. The DC voltage at the
base of Q3 should be approximately 1.8 volts. If your
answer varies by more than 2 volts, turn the power OFF
and check components R7, R8, R11 and Q3.
If you don't have an RF generator and oscilloscope,
skip to the FM Oscillator Assembly Procedure.
AC GAIN
The AC gain of the mixer is set by the impedance of the
primary side of T1 and by the current flowing in Q3. The
current in Q3 is set by the resistors R7, R8 and R11.
Connect your test equipment to the circuit as shown in
Figure 40. Your scope probe must have an input
capacitance of 12pF or less, otherwise the probe will
detune T1 resulting in an incorrect measurement. Set
your scope to read 10mV per division. Set your RF
generator at 10.7MHz no modulation minimum voltage
output. Turn the power ON and slowly increase the
amplitude of the generator until 4 divisions or 40mVpp
V
Figure 39
are seen on the scope. With an alignment tool or a
screwdriver, adjust T1 for peak. Reduce the generator
amplitude to maintain 4 divisions on the scope. Move
the scope probe to the base of Q3 and record the input
voltage here:
Vb = __________mVpp.
Turn the power OFF. The gain can be calculated as
follows:
AC Gain = 40mV / Vb.
Your calculated answer should be about 3.
Record your calculation:
AC Gain = __________
Because the signal from the oscillator is injected at the
emitter of Q3, the emitter resistor is not bypassed to
ground. This is why the gain of the mixer is low
compared to the other IF stages.
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