93 lines
5.6 KiB
Plaintext
93 lines
5.6 KiB
Plaintext
OLD CAR RADIOS
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You have just purchased an old car radio for a dollar or two at a yard sale.
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So, how do you go about hooking up a 12-volt supply and speaker to it, much
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less a shortwave converter kit??? What you have in front of you is the bare
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metal case of the radio with the on/off switch and tuning knob in front and
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either a bunch of colored wires or just a funny-looking connector in back.
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So, what's next?
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Well, you need the proper tools. Get a couple of assorted screwdrivers, both
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regular and Phillips to remove the cover from the radio. Dig out a
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voltmeter or VOM, or anything that will measure ohms (if you don't have one,
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borrow one from a friend). Fetch your trusty soldering iron and solder and
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your needle nose pliers and wire stripper/clipper. These are the basic
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tools. If you don't already have them and can't somehow get a hold of them
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then you have no business building electronics projects anyhow. A scope
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would also help speed up things, but many hobbyists can't afford one. Oh,
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yes, you will also need a small 4 - 40 ohm speaker that you salvage from a
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broken transistor radio, or some such. Most hobbyists have a half dozen of
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these lying around.
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You will use the ohmmeter, your power of observation, and a bit of reasoning
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and common sense to figure out what the wires or tabs coming out of the car
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radio chassis mean. You need to identify the following terminals: the power
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- +12 volts and ground, and two speaker terminals (if the radio has wiring
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for more than one speaker, which is probable, then you need to find only the
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two terminals for any ONE of the speakers).
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Finding the ground terminal is easiest. If there are colored wires coming
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out of the radio, the BLACK wire is the likeliest suspect. Ok, power up your
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ohm meter and clip one lead to the metal chassis of the radio. In turn test
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each wire or tab for zero ohms (or just a fraction of an ohm) resistance
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from the lead to the chassis. The only one with the zero or near-zero
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reading is the culprit - the ground lead. Label it with a small piece of
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masking tape.
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Locating the +12 volt terminal takes a bit more work. If there is one and
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only one red wire coming out of the radio, that is probably it, but do not
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take it for granted. Now, open up the chassis by unscrewing one or more of
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the sheet metal plates enclosing the works of the radio. You need to get
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access to the back of the volume control, which also happens to have an
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on/off switch mounted on it. Find the two terminals on the back of the
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on/off switch. Twist the volume control so the on/off switch clicks ON. Now,
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measure the resistance from either terminal on the back of this switch to
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each wire or terminal coming out of the chassis of the radio (paying
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particular attention to the RED wire, if there is one). You will read a zero
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or near-zero resistance from only one wire or terminal to the on/off switch.
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This is +12 volts. Label this wire or tab. Reinstall the metal plates on
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the chassis to close it up.
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The final step is to find two leads for any one speaker. You will test the
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remaining, unlabeled leads or tabs. You will now for the first time power up
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the radio. Get your 12 volt regulated power supply (the enclosed file
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POWERSUP.TXT gives details on building one if you need to do so), and attach
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the +V and GND leads to the terminals on the radio that you have labeled in
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the steps above. Carefully apply power. Turn on the on/off switch of the
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radio. If the fuse on the power supply has not blown, then you are probably
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all right.
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Now with your VOM on volts function, read the voltage between the chassis
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(ground) and each of the unlabeled terminals. If you read +12 v on any of
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them, this is the lead supplying juice to a power antenna accessory, and you
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will label it so and leave it alone from now on. Most or all of the leads
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should give you a low or no voltage reading to chassis ground. Fine so far.
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If you have a scope, the rest is easy. Just connect scope in turn to each
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set of two unlabeled terminals. Set the scope time scale to 200 microseconds
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per division and the volts/division to about 5. With the power on to the
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radio, look for a scope display that looks like an audio signal (scrambled
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sine waves of various amplitudes). Sets of two terminals giving this display
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are likely suspects of being speaker terminals. Continue with tests below.
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If you can't get a scope or do not know how to use one, that is o.k. You
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will now use that old minispeaker mentioned above. With the power on to the
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radio, clip to the speaker leads (using alligator clip terminated test leads
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if you have them, if not, just plain old wires with the ends bared and
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crimped as necessary with needle nose pliers) each set of two radio terminals
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you want to test. For each test turn the radio volume up and tune the dial a
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bit. You will very soon find a set of terminals that works. If you had
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happened to hook up one terminal for each of two different speakers, you will
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get weird broken up sounds or other strange behavior. Test all the unlabeled
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terminals and you will find the pairs that sound best. These sets are
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intended to go to the same speaker.
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Now, assuming that you have not blown out the speaker (unlikely if you
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carefully! followed the instructions above), you can permanently connect the
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speaker to the terminals you found, or you can hook up a better speaker that
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you have been saving for the purpose, if that is the case.
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That all for this stage. Now on to building and installing the shortwave
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converter.
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