First Am If Amplifier; Q8 Base Bias; Q8 Current - Elenco Electronics AM/FM-108TK Assembly And Instruction Manual

FIRST AM IF AMPLIFIER

The operation of the first IF amplifier is the same as the
second IF amplifier with one important difference. The
gain of the first IF amplifier decreases after the AGC
threshold is passed to keep the audio output constant at
the detector and prevent overload of the second IF
amplifier. This is accomplished by making the voltage
on the base of transistor Q8 lower as the signal strength
increases. Since the voltage from base to emitter is
fairly constant, the drop in voltage at the base produces
a similar drop in voltage at the emitter of Q8. This drop
ASSEMBLY INSTRUCTIONS
R34 - 1MΩ Resistor
(brown-black-green-gold)
TP6 - Test Point Pin
(see Figure A)
CAUTION: Test point must
not touch can of IF Coil.
STATIC MEASUREMENTS

Q8 BASE BIAS

Connect your VOM to the circuit as shown in Figure 13.
Set your VOM to read 2 volts DC and turn the power
ON. The voltage at TP5 should be approximately 1.5
volts. If your circuit fails this test, check Q8 and R37.
Turn the power OFF.
If you do not have an RF generator and oscilloscope, skip to Section 5.
SECTION 4
lowers the voltage across R37 and thus, reduces the DC
current through R37. Since all of the DC current from
the emitter of Q8 must go through R37, the DC current
in Q8 is therefore lowered. When the DC current in a
transistor is lowered, its effective emitter resistance
increases. The AC gain of transistor Q8 is equal to the
AC collector load of Q8 divided by its effective emitter
resistance. Raising the value of the effective emitter
resistance, thus, lowers the AC gain of Q8.

Q8 CURRENT

Connect the positive lead of your VOM to the emitter of
Q8 and connect the negative lead to ground point
TP15. Turn the power ON. The voltage should be
approximately .8 volts. Since the current in Q8 is equal
to the current in R37, I(Q2) = .8/R37 or approximately
.8 milliamps. Turn the power OFF.
-21-
Q8 - 2N3904 Transistor
(see Figure C)
C35 - .02µF (203)
or .022µF (223) Discap
R37 - 1kΩ Resistor
(brown-black-red-gold)