Instruction manual

-10-
Frequency Response
If you do not have an audio generator and an
oscilloscope, skip this test. Make sure that the
power is OFF and connect the audio generator and
the oscilloscope as shown in Figure 11 (replace the
multimeter leads with scope leads). Set the
generator to 400Hz minimum output and set the
oscilloscope to read 1V/div. Put the volume control
in the fully clockwise position. Turn the power ON
and adjust the audio generator output until a
waveform similar to the one shown in Figure 13 is
obtained. You may have to adjust the oscilloscope
time base for correct comparison. Without changing
the generator output level, slowly lower the
frequency of the audio generator until the sine wave
at the speaker falls to 0.707 of the original setting
(3dB point or 1/2 power point). Record that
frequency here:
Speaker Frequency = __________________
Without changing the generator output level, slowly
raise the frequency of the audio generator above
the original 400Hz until the sine wave at the speaker
falls to 0.707 of the original setting and record here:
Speaker Frequency = __________________
During this second measurement, the voltage at the
speaker may go higher than the original value due
to speaker resonance. The frequency at which a
speaker resonates changes with speaker size,
mounting method, and speaker enclosure. The
bandwidth of your audio circuit is equal to the
frequency difference between these 2 half power
points and should be no less than 3,000Hz. If you
fail this test, check that capacitors C2, C3, and C15
are the proper values and are correctly installed.
Example
3dB Point Low Side = 100Hz
3dB Point High Side = 4,000Hz
Bandwidth = 4,000 - 100Hz
Bandwidth = 3,900Hz
Bandwidth = __________________
Figure 13
0.707 of
original peak to
peak voltage
Original
Voltage
Procedure 2
If you do not have any test equipment, you can test
the audio circuit as follows:
Make sure that the power switch is turned OFF.
Connect a fresh 9V battery to the battery snap. Put
the volume control in the mid position. While holding
one lead of a 0.1µF capacitor on the R8 resistor
lead, place the other lead on the speaker wire
marked “+” and turn the power ON (see Figure 14).
The audio should make a buzzing sound. The pitch
will change if you vary the volume control setting. If
this test fails, turn the power OFF and check for
solder shorts between the pins on U2 (LM-386
integrated circuit). Check that all of the parts are the
correct value and are installed properly. Check that
regulator U3 is installed properly.