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crystal filter replacement. Has anyone tried this?

Discussion in 'CB Radio Modifications' started by loosecannon, Dec 2, 2018.

  1. loosecannon

    loosecannon Sr. Member

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    Hi all,

    I was thinking about receiver selectivity last night while falling asleep (i can't be the only one who does this) and an idea came to me that made me say 'huh'.



    I suppose i will end up trying this regardless of whether or not anyone else already has, but it would be nice to know if this has been done before.

    The idea is for the Uniden 8719 chassis, and so far, will be tried in a dual conversion type (148/2000/grant/madison).

    I would like to try replacing FT2 (7.8mhz IF crystal filter) with a crystal lattice filter like FT3, or a 7.8 crystal filter from an old 858 chassis or something similar.

    to my mind, this would give a much sharper 'skirt' and increase selectivity by a noticeable amount.

    what say all of you?
    LC
     
    Shadetree Mechanic likes this.

  2. Shockwave

    Shockwave Sr. Member

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    The single crystal filter is to remove mixer byproducts in the AM mode. It is wide enough to pass AM and SSB where the standard 7.8 crystal lattice filter is only wide enough to pass SSB. That is why the AM signal path feeds the 455 stage in front of the more selective lattice filter that gets bypassed in AM.

    Your idea of cascading two lattice filters will increase SSB selectivity but make AM sound "muddy" on receive just like an FT-101 without an AM filter. You can prevent that by switching the original, single crystal back inline for AM or getting your second lattice filter from a 142GTL. The single conversion 142GTL uses a filter that is just wide enough to let AM through too. It will also chop off some high end treble on the AM receive fidelity since it is narrower.
     
    Shadetree Mechanic likes this.
  3. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Remember when Low-Boy worked on his HR2510?

    He found this...
    XtalCF1.jpg

    The above is for 10.4MHz - but you may want to think about the caps used to Bypass the Xtals to pare down signals as they pass thru this multi-pole filter. - those caps apply a "hit" or an attenuation to the signal strength that is propagating thru this filter.
    XtalCF1resistor.jpg

    They also use resistors to "pad" and affect the bandpass as well as the signal strength used to "select" the signal out of the sheer noise of spectrum that filter needs to sort out.

    Cobra 1st IF and FT2 / FT3 mess...
    Cobra1481STIFFT3.jpg

    Note the locations - you will be changing parts in specific areas to attenuate the signal as it works thru the strip.

    Cobra 148 FT3 "build" breakdown...
    Cobra148GTLFT3LAYOUT.jpg

    So the next step is for you - read thru.

    Essentially, if I read you right, you want to change or somehow adjust the FT2 "response" and you want to use something like FT3 in it's place - not a good idea. Maybe you should temporarily try attenuating resistors to see if you can clear up the AM signal...

    They do that anyways in the AM section...
    Cobra1481stIFFT1455kHz.jpg

    So the choice is yours.

    I'll let you digest the above and then make your move...
     
    OldTech03 and Shadetree Mechanic like this.

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