Ramsey Electronics FX-440 Manual page 101

upuonot:
TEMPORARY VCO TUNING TEST CW
( RECEIVE ONLY ) +8 VOLTS
~
2K, SK, or 10K
TP1
Q
POTENTIOMETER
w
GROUND
ccw
Higher Voltage
=
Lower Capacitance
=
Higher Frequency
Lower Voltage
=
Higher Capacitance
=
Lower Frequency
VCO AND RECEIVER TEST PROCEDURE:
1. Make sure work area is tidied up and that the PC board has been
checked for stray scraps of wire, etc.
·2. Use your own hookup wire and potentiometer to connect to TP1 ,
+
av
and GND as illustrated.
3. Turn power switch OFF and squelch fully counter clockwise.
4. Connect 12V DC, speaker and antenna (2' of wire will do).
5. With power on and volume at a low level, s-1-o-w-l-y turn the
temporary tuning potentiometer.
See if you can hear a local repeater or nearby transmitter. As you tune
through the UHF business band, you should hear plenty of signals.
Because the control is sweeping through FORTY MHz of spectrum,
actually tuning in a station will be VERY touchy.
The demonstration can be more interesting and useful if a digital
voltmeter is added to monitor the voltage changes at TP1 , and a Ramsey
Frequency Counter is connected to TP2 or the top of R113. REMEMBER
that the indicated VCO frequency is 21.400 MHz LOWER than any signal
you happen to tune in. Observe how much frequency change there is per
.1 volt or less of voltage change. Stay on a given frequency for a minute
or so and notice how the indicated frequency drifts slightly either up or
down. This observation helps us appreciate the precision control that will
be offered by the PLL circuit. Finally, notice that the higher the voltage
applied, the lower the varactor diode's capacitance which means the
higher the VCO frequency.
When you are finished with this optional test, disconnect DC voltage.
Then, cut the wire to TP1, leaving 1/4" soldered in place to serve as a
permanent test point. If you do not wish to conduct this test, simply
double-check the accuracy of your work in this stage and proceed to the
Frequency Synthesizer section.
[ ] [ ] F20. Install U3, IC type UPB569. Do not use a s_ocket for U3, just
be sure to orient the notched end as pictured on the PC board.
FX-440 Assm. 41