                                OLD CAR RADIOS


You have just purchased an old car radio for a dollar or two at a yard sale.
So,  how do you go about hooking up a 12-volt supply and speaker to it, much
less a shortwave converter kit???  What you have in front of you is the bare
metal case of the radio with the  volume control/on-off  switch  and  tuning
knob in front and either a  bunch of colored  wires  or just a funny-looking
connector in back. So, what's next?

Well, you need  the proper  tools.   Get a couple of  assorted screwdrivers,
both regular and Phillips to remove the  cover from  the   radio.  Dig out a
voltmeter or VOM, or anything   that will  measure  ohms (if you  don't have
one, borrow one from a friend). Fetch your trusty  soldering iron and solder
and your needle nose pliers  and  wire stripper/clipper. These are the basic
tools.  If you don't already have them and  can't somehow get a hold of them
then you have no    business building electronics  projects  anyhow. A scope
would also help speed up   things, but many hobbyists can't afford one.  Oh,
yes, you will also need a small 4  - 40 ohm speaker  (8  ohms is best)  that
you salvage from  a  broken transistor radio, or some  such. Most  hobbyists
have half dozen of these lying around.

You will use the ohmmeter, your power of observation, and a bit of reasoning
and common sense to figure out what the wires or  tabs coming out of the car
radio chassis mean.  You need to identify the following terminals: the power
(+12 volts and ground), and two  speaker terminals (if the radio has  wiring
for more than one speaker, which is probable, then you need to find only the
two terminals for any ONE speaker).

Finding the ground terminal is easiest.   If there are  colored wires coming
out of the radio,  the  BLACK   wire is the likeliest suspect. Ok, power  up
your ohm meter and clip one lead to the metal chassis of  the radio. In turn
test each wire or  tab for zero ohms (or just a fraction of  an ohm) resist-
ance from the lead to the chassis. The only  one with  the zero or near-zero
reading is the  culprit,  the  ground  lead. Label it with  a small piece of
masking tape.

Locating the +12 volt terminal  takes a bit  more work. If there  is one and
only one red wire  coming out of the  radio, that is probably it, but do not
take it for granted. Now, open  up the chassis by unscrewing  one or more of
the sheet metal plates enclosing the works  of the radio. You need access to
the back of the volume control,  which also happens to have an on/off switch
mounted  on it.   Find the two terminals  on the back  of the on/off switch.
Twist the volume control so the  on/off switch  clicks ON. Now, measure  the
resistance from either  terminal on the back of this switch  to each wire or
terminal coming out of  the chassis  of the radio (paying  particular atten-
tion to the RED wire, if there  is one). You  will read a zero  or near-zero
resistance from only one wire or terminal  to  the  on/off switch.   This is
+12 volts. Label this wire or tab. Reinstall the metal plates on the chassis
to close it.

The final step is to find two leads for any one speaker.   You will test the
remaining, unlabeled leads or tabs.   Now finally power up the radio.    Get
your 12 volt  regulated power supply   (the enclosed file POWERSUP.TXT gives
details on building  one if  necessary), and attach the +V and GND leads  to
the terminals on the  radio  labeled  in the  steps  above.  Carefully apply
power. Turn on  the  on/off switch of the radio.   If the fuse on the  power
supply has not blown, then proceed to the next step.

Now with the  VOM on volts  function,   read the voltage between the chassis
(ground) and each of  the unlabeled terminals. If you  read +12 v  on any of
them, this is the lead supplying juice to a  power antenna accessory.  Label
it so and leave it alone from now on. Most or  all of  the leads should give
you a low or no voltage reading to chassis ground. Fine so far.

Disconnect the power. Attach  one or  two feet of hookup wire to the radio's
antenna jack...  solder or crimp it on if  you do not have an antenna  plug.
This is so the radio  will now receive  broadcast   signals. If  you have  a
scope, the rest is easy.   Just connect  scope  in turn  to  each set of two
unlabeled terminals. Set  the  scope  time  scale   to 200 microseconds  per
division and the  volts/division to about 5. With the power on to the radio,
look for a scope  display that  looks like  an audio signal  (scrambled sine
waves of various amplitudes). Sets of two terminals giving this display  are
likely suspects of being speaker terminals. Continue with tests below.

If you can't get a scope or do not know how to use one,  that is o.k.    You
will now use that old  minispeaker mentioned above. With the power on to the
radio and a wire attached to  the antenna jack as above, clip to the speaker
leads (using alligator clip terminated test leads if  you have them, if not,
just plain old wires with the ends   bared  and crimped   as necessary  with
needle nose pliers) in  turn each set of two  radio terminals you will test.
For each test  turn the radio volume up  and tune the dial a bit. You should
soon find a set of terminals that works. If you had happened to  hook up one
terminal of each   of two  different speaker  outputs, you  will   get weird
broken up sounds  or  other strange   behavior.   Test  all   the  unlabeled
terminals and find  the pairs that sound  best.  These sets are intended for
the same speaker.

Now, assuming that you have not blown  out the speaker (unlikely if you have
carefully! followed the  instructions   above),    permanently  connect  the
speaker to the terminals you found,  or hook up   a better speaker  that you
have been saving for the purpose, as your prefer.

That's all for this stage.   Now on to building and installing the shortwave
converter.


Important note:
Make ALL ohms readings with the power  to the radio OFF, otherwise  you will
likely blow out your VOM.   When changing wired  connections, between taking
readings, it is safest to turn  the power off. While there  is little or  no
shock hazard while working on  low  voltage transistorized  radio circuitry,
there is always  the danger  that you  will damage  the  radio  itself, your
instruments, or your tools  if you do not know what you  are  doing. Have an
experienced electronics hobbyist or technician assist you if  you are new to
this.

Note on purchasing an old car radio:
Choose a cheap radio with  a  tuning dial, NOT with digital   tuning. Try to
get a radio with a bit of empty space inside the case, so you  can mount the
converter internally. Do not  buy  the radio if   it is too dirty or greasy,
has torn wires,  or loose parts clanking around inside. Do not pay more than
about $5 maximum  for  it (you should  be  able  to get one cheaper). Before
even buying a car radio, read  the enclosed  file   AMRADIO.TXT to see if an
ordinary table radio  could   fit your needs [it  would save both  time  and
money].
