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How to covert an ordinary table model AM radio, or clock radio, or any old
transistor radio into a shortwave receiver.
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Well, you may ask, why be limited to converting a *car radio* into a
shortwave receiver? A car radio does have a few non-trivial disadvantages.
It comes in an unattractive, even clunky metal case, not really suitable
for display. You need to build a separate power supply for it, and though
details on doing this without much effort are in the enclosed file
POWERSUP.TXT, it does involve extra work and expense, not to mention the
extra space that the power supply case and external speaker hog.
Say you salvage this nice old clock radio out of your attic. The clock still
keeps time fine and the radio pulls in local AM stations nicely without too
much static. Hey, wouldn't it be nice to be able to tune shortwave stations
on it?
Yes, it is possible to convert many AM radios to shortwave receivers using
the converter described in the enclosed CONVERT.TXT file. The disadvantages
relative to a car radio are that the converted AM radio will be somewhat
more subject to interference from strong AM broadcasters plus noise and
static, and it may also be necessary to take extra precautions with the
"transformerless" power supplies of some older table model and clock radios
(see below). The main advantages are reduced parts count and lower cost. It
is quite possible to convert an AM radio for a total cost of $5 or less,
depending on how many of the needed parts are on hand.
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The procedure:
[Ignore all references in the following section to AC or AC power supply if
you are working with a battery-powered portable radio.]
Make certain the AM radio you will be working on is unplugged from the power
line. Open the case of the radio by removing the screws that hold it
together. Save the screws. You may have to disable or outwit spring clips or
other clever devices in order to get to the circuit board. Disregard the
warning that there are no "user serviceable parts inside". [Note that if you
are working on an AC-powered radio and do not know what you are doing, you
can cause damage to the radio and your test equipment, not to mention
hurting yourself. Get experienced help if you are new to this, or stay with
battery-powered radios, where the amount of damage you can do is limited.]
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First, you MUST determine whether the AC-powered radio has an unsafe
"transformerless" power supply. If the radio has an AC power cord and the
power cord does not have a "polarized" plug (three-prong or two-prong with
one prong wider so that it can be plugged in only way), and if the radio
has no step-down or isolation power transformer, then it has an UNSAFE
power supply and extra precautions MUST be taken. If you cannot fully
understand what has just been said, then STOP and do NOT proceed further
without supervision by a technician or experienced electronics hobbyist. AC
power can be dangerous!
If the radio has an unsafe power supply and the power cord does not have a
polarized plug, this condition must be remedied before proceeding further.
Replace the unpolarized plug with a polarized one oriented so that the AC
power "neutral" always connects to the chassis and/or circuit board ground,
and NEVER the AC "hot". Alternately, you may add an isolation transformer
that completely isolates the chassis and circuit board from the AC line.
Doing so requires measuring AC line voltages with a voltmeter, not a
procedure for beginners, and therefore details of this are deliberately
omitted. If you do not know how to do this, STOP and get help from a
technician or experienced electronics hobbyist. Or better yet, work with a
battery-powered portable radio, and you can avoid this problem completely.
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You will need to locate three points on the circuit board or nearby: the
circuit ground, the filtered DC voltage, and the point where you will inject
the output from the SW converter. This requires a DC voltmeter or multi-
tester and your powers of observation.
Finding the circuit ground is the easiest. The ground is usually identif-
iable as a large "run" around the etched side of the PC board. Another
sure bet is the negative terminal of the highest value miniature filter
capacitor on the circuit board, usually 100 - 500 �F. As always, stay away
from the AC power supply.
The +DC voltage you need is at or near the positive terminal of the above
mentioned filter capacitor. Now you will need to power up the radio,
carefully. WARNING: KEEP FINGERS AND TOOLS AWAY FROM THE AC POWER SECTION
OR YOU WILL BE EXTREMELY SORRY! Put the voltmeter on the 20V DC scale and
measure the voltage between what you think are the ground and +DC points.
Your reading should be in the range of about 6 to 15 volts DC. If you have
an oscilloscope handy, you might wish to confirm your diagnosis by verifying
that there is no AC ripple between the two points, to make sure you have a
filtered DC voltage available to power the SW converter. If you do not have
a scope, you can test this using an ordinary red LED with a series current
limiting resistor, about 470 �. The LED should burn bright and flicker free.
If you find AC ripple on the AM tuner PC board, this means the radio has an
UNSAFE power supply, and you MUST remedy this by the procedure given in the
previous section.
Unplug the radio. Now solder a black wire to the ground point and a red one
to the +DC power you just found. These two wires you now attach to the
ground and +V power points on the SW converter. You may do this using a
mini plug and jack combination that will hook to the case you install the
converter into. The SW converter will now be conveniently powered by the
radio and need no external power supply or battery.
The radio is still unplugged. Make certain that there is a wire leading
from the output of the SW converter. Solder one on if necessary. Connect
the black and red power leads from the radio PC board to the SW converter.
Carefully plug the radio into the outlet and turn on the power, observing
all precautions. Put the radio on the AM band if it is not so already. Turn
up the volume and twist the tuning knob. You should hear local AM stations
as usual. If this is not the case, shut off the power and check your
connections.
Now comes the tricky part. Your AM radio receives AM stations with the
converter powered up, but not yet injecting the down-converted shortwave
signals into the radio's RF stage. You will need to find, mostly by trial
and error, the point at which to inject the signals. Attach a "long-wire"
antenna (5' or longer) to the antenna input of the SW converter and insert a
tested crystal into the socket for it on the converter. Now locate the AM
radio's tuning capacitor, a small plastic square connected by a small shaft
to the tuning knob on the radio. The tuning capacitor will have 4 - 6 leads
attaching it to the PC tuner board. Touch the bare end of the wire from the
output of the SW converter to each of these leads in turn, hold it in
contact and twist the tuning knob of the radio. At one or more of these
leads you should hear the bleeps and static and voices speaking in unknown
languages. There's your culprit. The SW converter output connects to this
point. Solder a wire there. [Note: this part of the procedure should be done
at night, when short wave reception is best, and using a crystal for a known
SW band.]
Turn off the power to the radio. Now consider whether to install the SW
converter inside the AM radio or to house it in its own case externally.
External mounting has the advantage of making it easier to change the
crystal (and therefore the band of reception). Internal mounting makes
everything nice and neat, but you gotta unscrew and open up the radio case
to switch frequency bands. My personal preference is the external mount,
with every lead hooked up to plug 'n jack sets.
Reassemble the radio and turn on the power to see if it still works. If
not, check all leads and joints, and try again. If you have it working,
mount everything neatly by the method of your choice. Now you can enjoy the
pleasures of shortwave listening (SWL) on the cheap.
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Notes:
(1) For adequate reception you will need a "long wire" antenna at least
4' long. A whip antenna may be long enough, depending on your loca-
tion. Best reception on most SW bands is after nightfall.
(2) Converting an AM radio to short wave is a relatively simple project
as such things go. However, it is emphatically NOT recommended for
beginners, especially if working with an AC-powered radio. Do not
hesitate to get help from someone more experienced if you are beyond
your depth. Do not under any circumstances take risks with AC power!