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McIntosh 1900 fm tuner - no lower stations

Discussion in 'McIntosh Audio' started by pfejr, Jul 4, 2018.

  1. pfejr

    pfejr New Member

    Messages:
    27
    Location:
    belfast, me
    My 1900 fm tuner works great above 93 (or so) . It will tune stations below 93 for about a minute after being powered up then that part of the dial goes to static. What might I look for/do to fix?
     

     

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  2. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    Messages:
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    Location:
    Cleveland, Ohio
  3. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
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    Location:
    New Jersey
    Draudio knows better than me.
    That said, there are some tantalum caps that are known suspects (to go bad) usually replaced with films during rebuild.
    I suspect if one crapped I out you wouldn't get any stations
     
  4. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
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    Location:
    Angel Station, Alabama
  5. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    Messages:
    18,738
    Location:
    Cleveland, Ohio
    I have never, in over 40 years of servicing audio equipment, seen any component in a local oscillator fail that causes it to not work at a certain frequency. They either oscillator or they don't. Not saying it can't happen, but the likelihood of it happening is somewhere between "none" and "Get out of my office."
     
  6. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    26,482
    Location:
    Angel Station, Alabama
    I've seen marginal LO tubes do it, also flakey capacitors and cold solder joints. Rare in solid state, improper cleaning with sprays by the inexperienced invite capricious behavior. Sub par voltage supply can also be troublesome.
    I've an SAE Series Two tuner with a problem like this that I have yet to address, a lightning strike may have blown the voltage regulator and be putting excess voltage into the front end. It sort of works when first turned on, and gradually gets wonky and fades, LEDs stay lit.
     
    Last edited: Jul 4, 2018

     

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  7. c_dk

    c_dk Addicted Member

    Messages:
    6,240
    Location:
    West Michigan
    The time sequence certainly seems to point to a thermal issue with a semiconductor.

    Certainly a interesting RF challenge that certainly does not show up very often if ever. The use of a dual gate MOSFET between the two front end RF coils could lead to speculation if it is the cause.

    One of those challenges you do not know if you really want to own it if it lands on your bench.......
     
  8. TSmith8605

    TSmith8605 Senile Member Subscriber

    Messages:
    786
    Location:
    Sunnyvale, CA
    Based on having zero experience with this stuff, I vote for both of these issues being the problem.

    Anyway, I copied the text from dr*audio's linked post and filed it in my MAC1900 file for future reference. Thanks.
     
  9. pfejr

    pfejr New Member

    Messages:
    27
    Location:
    belfast, me
    Pio1980 and 4-2-7 like this.
  10. c_dk

    c_dk Addicted Member

    Messages:
    6,240
    Location:
    West Michigan
    Excellent. So no time variable but purely a physical contact issue within the tuning capacitor.

    RF is truly voodoo........
     
    Pio1980 likes this.

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