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Modifying an existing radio.

Discussion in 'Electronic Basics' started by Noen, Nov 25, 2003.

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  1. Noen

    Noen Guest

    Hi.

    I have an 88-108 fm radio, and want to adjust it to 160 - 180.
    I know I probably need to replace the oscillating capacitor, but I dont
    know what to change it to. (And where I could locate it.)

    Hope anyone can help.
     
  2. The mods you describe would be far more complex than simply
    replacing a capacitor. They would involve a complete redesign and
    rebuild of the RF section.

    Also, remember that a typical FM broadcast receiver's IF section
    has a bandwidth of at least 75kHz. This makes them utterly useless for
    anything other than -- you guessed it -- FM broadcast reception.

    What is it you're really trying to do? Who or what are you trying
    to monitor?


    --
    Dr. Anton Squeegee, Director, Dutch Surrealist Plumbing Institute
    (Known to some as Bruce Lane, KC7GR)
    kyrrin a/t bluefeathertech d-o=t c&o&m
    Motorola Radio Programming & Service Available -
    http://www.bluefeathertech.com/rf.html
    "Quando Omni Flunkus Moritati" (Red Green)
     
  3. Noen

    Noen Guest

    Im trying to learn how to monitor radio communication... after Ive
    figured out how make my fm radio to receive on other frequencies, im
    thinking of plugging the output into my mic, and start coding programs
    for storing communication, filtering noise, squelch etc... Would be fun
    for an amatour radio operator (to catch messages while away)...
     
  4. That should be ± 75 KHz, for a total of 150 KHz bandwidth. That is
    why FM broadcast channels are 200 KHz apart.

    --
    I say, the boy is so stupid that he tried to make a back up copy of his
    hard drive on the Xerox machine!

    Michael A. Terrell
    Central Florida
     
  5. Jerry G.

    Jerry G. Guest

    It is a lot more than changing a cap value. If your set uses a variable cap,
    you will also have to change the coils in the RF and oscillator front end.
    There will be some re-engineering to get the proper matching.

    If you want to listen to UHF communications these bands, it will sound like
    a mess full of imaging, and noise. The IF bandwidth in your FM radio is +-
    75 kHz, or a total of 150 kHz for FM broadcast listening. You will have to
    change all the IF coils to ones with a higher "Q" and re-align the signal
    path. You will want to have a 15 kHz total bandwidth for communications
    listening.

    If your set is a PLL synthesized type, you will have to re-design the PLL to
    pre-scale in to the new band that you want. This will also involve
    redesigning the firmware that it is working under. The same will go for the
    IF amp design, as mentioned above.

    You would be best off going out and getting yourself a descent scanner that
    was designed to do the job. The reception will be much better than
    something you can do at home yourself.

    In the older days with the tube type FM radios, with some work done by
    someone that really knew what he was doing, it was possible to modify the
    radio to receive the aircraft bands that were in use in those days. The
    detector had to be changed to an AM type, and the IF amp stages had to be
    modified to have the required 15 kHz bandwidth. In most cases it still
    worked out cheaper and better to buy a hobby type aircraft receiver, which
    was engineered from its base design to do this.

    --

    Greetings,

    Jerry Greenberg GLG Technologies GLG
    =========================================
    WebPage http://www.zoom-one.com
    Electronics http://www.zoom-one.com/electron.htm
    =========================================


    Hi.

    I have an 88-108 fm radio, and want to adjust it to 160 - 180.
    I know I probably need to replace the oscillating capacitor, but I dont
    know what to change it to. (And where I could locate it.)

    Hope anyone can help.
     
  6. You could move it maybe 10 percent in that direction, but doubling the
    freq will probably not work because the transistors and other
    components would need to be replaced. You can remove a leaf from the
    variable capacitor, and turns from the coil, but once you do that, the
    alignment will suffer greatly, and you will have to realign it.

    And another major problem is that almost all of the stations you would
    pick up in those frequencies are narrow band FM. They are 1/15 the
    bandwidth of the normal FM broadcast station. This means that you
    will receive several stations at the same time, which is a real
    problem when you're in a city with a lot of activity in those bands.
    And adjusting the dial to a single station is nearly impossible
    because just moving it a slight amount will change it dozens of
    channels. I have an old Rat Shack Patrolman-5 radio that has similar
    bands and does something similar, and it is nearly useless when it is
    tuned to the VHF and UHF bands. I found out the hard way, I bought
    the radio thinking I could use it like a scanner. When I found out
    how bad it was, I went out and bought a real scanner.

    Good luck; you're going to need it.
    --
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    ###Got a Question about ELECTRONICS? Check HERE First:###
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    Subject: line with other stuff. alondra101 <at> hotmail.com
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  7. Well, not totally useless. But nearly so, especially in a crowded
    band with many active stations. If you were out in the sticks with
    very few stations, you might be able to hear just one. But nowadays,
    this is very rare. And you're right, I do _not_ recommend that anyone
    try to mod a radio like this, it's a waste of time.


    --
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  8. Mark Zenier

    Mark Zenier Guest

    I did that to a 1970 vintage stereo hi-fi tuner. That type and era
    radio is easy to work on because they're big, have air variable capacitor
    tuning and leaded discrete components, but modern enough to use ceramic
    IF filters and MOSFET front ends. I was trying to get the weather
    satellites at 137 MHz and managed to get the top end above 148 MHz,
    (the two meter ham band), by rewinding the oscillator coil with one less
    turn and tweeking the RF trimmer caps and coil adjustments. It worked
    fine for monitoring the local 2 meter repeater.

    But if you don't know what to change, you'll need to get the service
    data for the radio, or (if it's newer than a couple of decades old) the
    datasheet for the integrated circuit that the radio uses. The two most
    common radio chip manufacturers that I see are Philips and Telefunken ->
    Temic -> Atmel. Alas, (last I checked) Atmel seems to have taken the
    older Telefunken chips off their german web site.

    Mark Zenier Washington State resident
     
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