Instruction manual

-18-
Squelch Control Check
The function of the squelch control is to eliminate
static when there is no signal present. This control
sets the level of passable signals. The squelch
control works by raising the voltage to the (
–) input
to the audio amplifier and thus disabling the
amplifier output.
The squelch control can be checked with the circuit
shown in Figure 21. Turn the squelch control fully
counterclockwise. Set the signal generator to
455kHz and the output amplitude to minimum.
Move the multimeter probe from pin 13 to pin 11 of
U1. The voltage should be near 0V. Turn the
squelch control clockwise until the voltage jumps to
about 2.6V, disabling the audio amplifier. If the
voltage does not jump, use a voltage divider to
reduce the signal generator output to 5mV peak to
peak and try again. If the voltage still does not jump,
check the wiring to the squelch control pot. Also, be
sure that the temporary jumper installed in block 1
was removed.
Procedure 4
If you do not have test equipment, it is possible to
check block 2 with the following procedure:
1. Make sure that the ON/OFF switch is in the
OFF position.
2. Reconnect the battery.
3. Turn the volume control fully clockwise.
4. Turn the squelch control fully counter-
clockwise.
5. Hold a 0.1µF capacitor on pins 5 and 13 as
shown in Figure 22.
6. Turn the power ON.
The audio should produce a hissing sound. Turning
the squelch control clockwise should quiet the
audio. If you do not get a hissing sound, check that
the integrated circuit U1 is properly installed. Check
that all of the leads installed in block 2 have been
soldered and no shorts exist. If the hissing is
present, but the squelch control does not remove it,
check the leads going to the control and compare
them to Figure 20.
Turn the ON/OFF switch to the OFF position and
remove the 9V battery. Before moving on to the next
block, try answering the questions in Quiz 2 to see
if you missed an important fact about the operation
of the second i-f, squelch control, or quadrature
detector.
Figure 22
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