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FX 146
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VCO AND RECEIVER TEST PROCEDURE:
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1. Make sure work area is tidied up and that the PC board has been
checked for stray scraps of wire, etc.
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2. Use your own hookup wire and potentiometer to connect to TP1, +8V
and GND as illustrated.
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3. Turn power switch OFF and squelch fully counter clockwise.
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4. Connect 12V DC, speaker and antenna (2' of wire will do).
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5. With power on and volume at a low level, s-l-o-w-l-y turn the
temporary tuning potentiometer. See if you can hear a local repeater,
nearby transmitter or business band transmissions. Perhaps the easiest
signal to hear is the local NOAA weather transmitter located around
162.5 MHz. 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.
If you do have a meter and counter connected, an initial setting of the slug in
L7 can be made now. Adjust the voltage for 4.0 volts and tune L7 to
approximately 138 MHz. (Note: final tuning of L7 will be even easier after the
PLL circuitry is completed). 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.
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F22. Install U3, IC type 12017. Do not use a socket for U3, just be sure
to orient the notched end as shown on the board.
FX-146
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49
FX-146 MASTER COMPONENT INDEX
The following pages were prepared to serve these purposes:
1.
A general cross-reference for circuit study and servicing;
2.
Additional help for parts sorting and identification;
3.
An accommodation for experienced builders who prefer to work out their
own assembly sequences. (If you have ANY questions or doubt about a
part, refer to the specified assembly steps and the schematic diagram.)
Key to Component Index format:
A. Part designator number (e.g., C1, Q12, Y2, etc.)
B. Experienced builders always double-check!
C. Part value or manufacturer identification
D. Assembly step number (which also identifies general circuit stage per the
schematic.)
E. Component function note (or space for your own note.)
Key to assembly step letter-number codes:
The letter code indicates the circuit stage(s) in the step-by-step assembly
instructions. The numeral designates the specific step. If you have any
question whatsoever about a particular part, pleas e consult its assembly step.
A: DC Power input
B: Receiver audio amplifier
CR: Receiver FM detector and squelch
DR: Antenna input, T-R switching, filters, RF preamplifiers
EF: VCO (Voltage Controlled Oscillator)
D11,D12
6.2 V Zener
RS276-561
VR1
7808
ECG964, SK3630 or RE
VR2
7805
RS276-1770
U1
MC13135
RE or Motorola
U2
LM380
ECG740A, SK3328 or RE
U3
MC12017
RE or Motorola
U4
LM324
RS276-1711
U5
LM358
ECG928, SK3691 or RE
U6
MC145152
RE or Motorola