Help To Identify Electronic Part

Discussion in 'Automotive Electronics' started by Charliebates, Sep 27, 2018.

  1. Charliebates

    Thread Starter New Member

    Sep 27, 2018
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    Hello, My first post here,
    I have a 1982 Phillips Berlina Coupe SE with the original AM FM Cassette radio and the Motorola male end pulled off when I was disconnecting from the radio and broke off the tiny inner wire inside the cable.
    Inside has an electronic part with "75 J" written on it. I was successful to simply solder the break back together with continuity.
    But I soldered the Motorola Male connector housing to the 90 degree metal cable cover so it couldn't pull apart again and I believe I over heated the "75 J" electronic in the process.
    I believe the "75 J" electronic is a filter to control noise from the power antenna when it is raised and lowered.
    Can anyone help me to identify the "75 J" part and source a replacement? I can replace the entire cable but is a huge job because it is routed under the glued down carpet.
    Thx in advance, John

    https://scontent.fyxd1-1.fna.fbcdn....=94b329ac3d0c97f71f7dbdda1b3df493&oe=5C2B7764
     
  2. Charliebates

    Thread Starter New Member

    Sep 27, 2018
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    I forgot to mention the radio is a the same as a 1982 Chevy Corvette and standard GM antenna cable for Power Antenna.
    Thx again, John <SNIP>

    Moderators note : remove private detail
     
    Last edited by a moderator: Sep 27, 2018
  3. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Polystyrene capacitor, the 75 is probably pF (a non preferred value). Try to get the right type - polystyrene dielectric has a specific temperature coefficient that is often matched to counteract that of other components in circuit.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    An other name could be styroflex.
    It could be 75 Pf and the J could give a tollerance of ± 5 %.

    Bertus
     
  5. LesJones

    Well-Known Member

    Jan 8, 2017
    2,052
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    It looks like a polystyrene capacitor. I would guess at it's value being 75 pF. The nearest standard values would be 68 pF or 82 pF. NOTE this is only a guess.

    Les.
     
  6. ian field

    AAC Fanatic!

    Oct 27, 2012
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    They probably wouldn't have bothered sourcing a non standard value without a reason.

    Polystyrene has a specific tempco that's often selected to counter other components - another reason to suspect the value is important.

    The old silvered mica type might do the job - but good luck finding any!

    I'd try a lower preferred value polystyrene - you can pad it up to 75pF by adding in parallel. Ideally more polystyrene caps - but you might get away with using other types for that. ceramic needs care - some types are very lossy at RF.
     
  7. ebp

    Well-Known Member

    Feb 8, 2018
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    Polystyrene capacitors have not been made for several years.

    Probably the best replacement, especially for noise filtering, is a C0G ceramic type. They generally have properties reasonably similar to polystyrene, though the temperature coefficient of capacitance is somewhat different. Polystyrene was popular for LC resonant circuits because the tempco of the cap was nearly equal and opposite that of the inductors often used, which maintained the resonant frequency over temperature.

    Silvered mica caps are reasonably readily available. DigiKey has stock of 75 pF 5%. They are rather expensive.
     
  8. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Specifically pot core inductors.
     
  9. Charliebates

    Thread Starter New Member

    Sep 27, 2018
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    Thx for the help Ian, should I be sourcing the "polystyrene capacitor -slivered mica caps 75 pf5%" for a replacement? Have I got this or am I way off?
     
  10. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Silvered mica were what they used before - might be similar, but good luck finding them.
     
  11. Charliebates

    Thread Starter New Member

    Sep 27, 2018
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    Well just about time to give it up.
    I see other suppliers of generic radio shielded antenna cables and come with the capacitor.
    If I just knew what they are using.
    Well I thank you for your input.
     
  12. ebp

    Well-Known Member

    Feb 8, 2018
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    Use a 75 pF 100 V or 200 V "C0G" ceramic capacitor. They are common and cheap and will outperform the original at radio frequency.
     
    Charliebates likes this.
  13. ian field

    AAC Fanatic!

    Oct 27, 2012
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    If that's all it is - it may not be very fussy...………...
     
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