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Changing Shure wireless microphone frequency

Discussion in 'Electronic Repair' started by Guest, May 5, 2006.

  1. Guest

    Guest Guest

    Shure wireless handheld microphone. Spec can be found here if it can be of any help
    http://www.shure.com/pdf/discontinued/L-Series.pdf

    Microphone transmit at 180.4MHZ. It was made in 1990. It uses a 20.05MHZ crystal by MTRON
    as far as I can tell like the one uses in Radio Control car, but about half the size. Can the
    frequency be changed to something like 218.5MHZ by just changing the crystal?
    If so, what frequency should I use?

    Thanks for any info you can give.
     
  2. Arfa Daily

    Arfa Daily Guest

    180.4 divided by 20.05, is near as dammit 9, so the crystal oscillator stage
    is probably running through two tripler stages ( these are just 'dirty'
    amplifiers with appropriate tuned circuits in their collectors or drains, to
    pick off the third and then ninth harmonics. ) First stage may well be the
    oscillator itself.

    In theory, all you need to do is sub the crystal for one at 1/9th the
    frequency of the desired output, so for 218.5, you would need a crystal at
    24.277 MHz. The oscillator will probably be ok with this comparitively small
    change of frequency, but the tuned circuits in the multiplier stages will
    need to be adjusted. The first tripler stage will probably be in range, but
    the second will be nearly 40 megs off tune, so you may have difficulty
    getting it there without altering values in the tuned circuit. Also, if
    there is a following RF PA stage, this may well require retuning also,
    depending on whether any tuned circuits are just broadband tanks, or
    harmonic filters.

    Finally, crystals can have many different specifications in their cut
    scheme, and load capacitances, depending on whether they are designed for
    series or parallel resonance, or overtone use. Crystal cutters understand
    what is required if you can let them have a copy of the oscillator
    schematic. At this sort of frequency, it will probably be a series crystal
    with 30pf loading.

    Also be aware that the band that you are proposing moving the mic to, is not
    in general, a license-free band for this sort of use, so depending on where
    you are in the world, you would probably be contravening telecoms
    regulations, and could render yourself liable to prosecution, if you cause
    interference to other legitimate band users.

    Arfa
     
  3. Bob Urz

    Bob Urz Guest

    And unless you have a communications test set, i would advise against
    it. If your determined to do it, send it back to shure to have it done
    at the factory. Of course, its probably not worth it. Better to
    sell it on Ebay or such and find a used one with frequencies that are
    better suited to your uses. VHF wireless are on there way out. WHo knows
    what frequencies will be clear in the future. Basically in the USA, VHF
    is from around 169 to 215 MHz or so. You need to know what TV channels
    are in use in your area now.

    Bob
     
  4. Bob

    Bob Guest

    One thing to be accutely aware of is the fact that the "L" series is for use
    by Broadcast Stations only. The microphone (transmitter) HAS to be licensed
    under part 74 (Broadcast), and only TV/Radio station are eligible. If you
    are playing with this in your house, it's probably not a real risk. If it's
    at a public location, you need to use something that either doesn't require
    a license, or something that you can (and did) license. It's not as if the
    FCC is going to hunt you down. They now offer "snitch rewards", which gives
    motivation to individuals to look for unlicensed users. I know of a couple
    of underpaid local broadcast engineers that have played the game and won.


    COPY FEDERAL COMMUNICATIONS
    COMMISSION
    WASHINGTON, D.C. 20554

    GRANT OF EQUIPMENT
    AUTHORIZATION COPY
    Type Acceptance

    Shure Incorporated
    5800 W. Touhy Ave
    Niles, IL 60714-4608
    United States Date of Grant: 07/24/1989

    Application Dated: 02/13/1989

    Attention: Dillard Gilmore , Senior Engineer, Global Compliance

    NOT TRANSFERABLE
    EQUIPMENT AUTHORIZATION is hereby issued to the named GRANTEE, and is
    VALID ONLY for the equipment identified hereon for use under the
    Commission's Rules and Regulations listed below.


    FCC IDENTIFIER: DD4L2

    Name of Grantee: Shure Incorporated

    Equipment Class: Licensed Broadcast Station Transmitter
    Notes:

    Grant Notes FCC Rule Parts Frequency
    Range (MHZ) Output
    Watts Frequency
    Tolerance Emission
    Designator
    M4 74.861 174.0 - 216.0 0.05 0.005 % 60K0F3E


    M4: Operation of this unit is limited to use at stations licensed for
    use under Part 74 of FCC Rules.

    Mail To:
    None Specified,
    MET Electrical Testing Co.
    916 W Patapsco Avenue
    Baltimore, MD 21230




    8904278315022250
     
  5. Bob Urz

    Bob Urz Guest

    Really? there are thousands of those units still out there in churches,
    schools and such across the country. Even though they are suppose to
    file license applications, my guess is there is less than 5% compliance
    across the USA. The FCC could bust most of the churches in the country
    if they were stupid enough to do so.

    Bob

    The microphone (transmitter) HAS to be licensed
     
  6. Quoc Anh

    Quoc Anh Guest

    Thank you all for the info and suggestions.

    One more question. If I change the receiver's frequency to match the mic
    instead, would it require any retuning? The receiver's spec list a frequency
    range of 180MHZ to 250MHZ.

    Thanks.
     
  7. Arfa Daily

    Arfa Daily Guest

    If the receiver tunes across that range, then it just does. No tuning other
    than setting it to the frequency that your radio mic is outputting on, will
    be required.

    Arfa
     
  8. Mike Berger

    Mike Berger Guest

    That would be an awfully wideband VHF receiver. The frequency spec
    just means that the circuit is designed for that range. It
    doesn't necessarily mean that you won't have to change coils
    and capacitors, or retune.
     
  9. Arfa Daily

    Arfa Daily Guest

    It depends on how you read it I suppose. I was assuming that he had a
    tuneable receiver - some kind of VHF scanner or programmable handheld or
    something, which is why I said that if it tunes that range, then it just
    does. How it does this is of no consequence to the user. I have an Icom 706.
    It tunes from topband to 2 metres with a couple of button pushes - I don't
    have to go inside to retune coils, or change caps. My TV set tunes from
    470MHz to almost 900MHz virtually continuously ( small break between bands
    IV and V ). My satellite receiver tunes from 920MHz to 2050MHz, so I don't
    think that 70MHz is an " awfully wideband " receiver.

    Of course, if it is a fixed single frequency or channelised and rock-bound
    receiver that's merely *capable* of operating over that frequency range,
    then yes, you are probably right, unless it's synthesised, in that you would
    have to replace the L.O. crystal. I would not necessarily expect to have to
    retune the front end or change coils and caps though, if it is specced for
    that frequency range. It may actually have a real barn door antenna circuit.
    However, I agree that if it was mine, and the front end was tunable, I would
    probably peak it up for best performance on the specific frequency.

    Arfa
     
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