FX 146
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FX- TROUBLESHOOTING GUIDE
The FX- transceiver is designed to W-O-R-K with a minimum of adjustment
or alignment. By imprinting the location and correct orientation of each
component on the PC-board itself in addition to publishing detailed assembly
steps, we have virtually guaranteed the successful operation of your
transceiver from the moment you turn on the switch.
A detailed guide to servicing a transceiver requiring so few internal
adjustments is virtually pointless to compile. (It would be like writing a
troubleshooting guide to a jigsaw puzzle: "Make sure all puzzle parts are in
the right place, and it will be a nice, complete puzzle. That's it!")
The best assurance of trouble-free operation is to build, understand and test
your transceiver in the stage-by -stage sequence presented in this book. If
you chose to install all parts first and then test, and then encountered
difficulty, we recommend that you compare your assembly work to the stage-
by-stage steps: USE the double-check spaces.
We simply MUST accept that any malfunction is caused by:
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incorrect part selection
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incorrect part orientation (diode or capacitor polarity, transistors,
IC's etc.)
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soldering error (missing connection, solder bridge)
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omitted part
?
part damaged during assembly
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part defective in manufacturing (extremely rare)
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part damaged by incorrect installation of other part(s)
?
part breakdown after extended operation
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defective external device or cable connected to the transceiver
circuit board.
The above possibilities may seem vague, but they cover what can go wrong
in any multi-stage electronic device. For this reason, it is important for you to
LEARN the FX circuit design in as much detail as possible, which is why
both the schematic and assembly instructions are presented stage by stage.
Troubleshooting is a process that considers EVERYTHING possible,
ESPECIALLY including every external device connected to the unit:
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power supply
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ALL cables and connectors
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microphone, speaker
?
antenna AND
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ALL cables and connectors!
If you are at all hazy about the binary programming of the "N" number,
FX-146
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Stage G: The FX- Transceiver PLL Synthesizer
A reassuring feature of the FX Transceiver is that if you build it correctly and
make just a few adjustments, it will work fine whether you understand how it
works or not. For most amateurs and beginners, the functions of the PLL
Frequency Synthesizer and VCO will present the greatest challenge to
understanding. In fact, these stages, which are at the heart of the
transceiver's design, are not easy to explain in everyday terms, because
some level of engineering training or serious amateur curiosity is definitely
required to catch on to the many concepts and principles involved. In short,
don't feel bad if the functions of U5 through U10 seem somewhat mysterious
to you at first.
The author's suggestion is that you study the FX transceiver's more complex
sections simply for the fun and satisfaction of it, catching on to a little bit here
or a new concept there as you go along. You'll get some insights from our
published circuit descriptions, and you'll pick up more from studying other
publications such as the ARRL HANDBOOK, discussing the circuit with
friends with engineering backgrounds, or doing whatever helps you best
catch on to new ideas and concepts.
The VCO, integral to the PLL, was discussed and tested in the preceding
assembly stage.
What we have so far is a master oscillator for the transceiver that can be
tuned by varying the voltage applied to the varactor diodes. All we really
need now is a super reliable way of controlling the voltage applied to those
varactor diodes so that we can put that oscillator on any VHF channel we
want with 5 KHz precision. And this control obviously must be extremely
stable, since a change of only a few picofarads can tune through many MHz
of VHF spectrum. How can we get such precision performance from small
tuning diodes, a very ordinary shielded coil, and a transistor with its
supporting capacitors and resistors?
We assume already that the answer must lie in the functions of the
Frequency Synthesizer IC. However, the key to grasping what this IC really
does lies in grasping TWO concepts: frequency synthesis and the "Phase
Locked Loop" or PLL. The more of a handle that you can get on these two
interconnected ideas, the less mysterious will be your FX transceiver as well
most other modern radio gear from ham transceivers to car stereos to
cellular phones. To begin, let's over simplify as much as we can.
UNDERSTANDING THE PLL:
In the following block diagram, the notion of "Loop" is seen in the simple fact
that the VCO output is routed right back into the VCO itself via the other
circuit sections. Try to imagine the unending and rapid circle of activity in
which the VCO output is repeatedly affecting its own oscillating frequency,
self-adjusting many times per second. Hence, an un-ending circle or loop.
