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It is advisable to use a digital meter because of the
small voltage changes in the following test. Connect
your VOM to the circuit as shown in Figure 5. Set your
VOM to read 1 volt DC and turn the power ON. Record
the base of Q10 here:
Vb1 = _____ volts.
Now set your VOM to read 9 volts and connect the
positive lead to test point TP1. Record the output bias
voltage here:
Vo = ____ volts.
Turn the power OFF. With a 1M ohm resistor (brown-
black-green-gold), R34, connect the power supply to the
circuit as shown in Figure 6.
Power Supply
Figure 6
If you do not have an audio generator, skip the following test and go directly to Section 2.
AC GAIN
The AC gain can be calculated in the same manner as
the DC gain except for two differences.
capacitor C42 bypasses the emitter resistor R46
leaving only the effective emitter resistance, and there
is a resistance seen at the output of Q13 and Q14. The
AC gain of Q10 can be calculated as R44 / Rj or 3300
/ 22.6 which equals 146. When the input signal is
positive, there will be a current flowing in Q11, which we
the Beta (β) of transistor Q13 or β x I(Q11). The total
will call I(Q11). This current will then be multiplied by
current at the output is equal to I(Q11) x (1 + β). The
is effectively divided by β, R48 / β.
resistance of R48 is also seen at the output. The
Assuming β of the output transistors are equal to 100
resistance
than the resistance seen at the output is equal to 1
ohm, 100 / 100. This means that there is a voltage
divider between the output and the 8 ohm speaker. The
signal is now divided down so that the output is equal
to the AC (gain of Q10) x (8 / (1+8)), or 146 x (8 / 9)
which equals 130. This is also true when the input
+
–
TP15
1MΩ
R34
For AC,
Turn the radio ON and turn the power supply ON.
Increase the supply voltage until the voltage at TP1 is
equal to Vo. Now increase the voltage of the supply
until the voltage at TP1 decreases by 1 volt. Move the
positive lead of your VOM to the base of Q10 and
record the voltage here:
It may be necessary to change scales of your VOM for
a more accurate reading. Turn the power OFF and
disconnect the power supply. Since the DC gain equals
the DC change at the output divided by the DC change
at the input, the DC gain of the audio can be calculated
as: 1 / (Vb2 - Vb1). Your answer should be near the
calculated DC gain of 47.4.
signal is negative. The only difference is that Q12 and
Q14 are now conducting. Connect the VOM and audio
generator to the circuit as shown in Figure 7.
Normally the AC gain is measured at a frequency of
1kHz.
Your VOM, however may not be able to
accurately read AC voltages at this frequency.
Therefore, it is recommended that this test be
performed at 400Hz. Set the audio generator at 400Hz
and minimum voltage output. With the power ON, set
your VOM to read an AC voltage of 1 volt at test point
TP1.
Increase the volume control about half way.
Slowly increase the amplitude of the audio generator
until your VOM reads 1 volt AC.
generator at this setting and move the positive lead of
your VOM to the base of Q10. Record the AC input
voltage to the amplifier here:
Vin = __________ volts.
-11-
Vb2 = ______.
COM
TP15
Leave the audio
V
V
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