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MCS 3253 FM Section Deaf

Discussion in 'Tuners' started by MCS Guy, Oct 25, 2018.

  1. MCS Guy

    MCS Guy The MCS hoarder Subscriber

    Messages:
    538
    Location:
    FL
    I recently acquired a MCS 3253 and the AM section works great, but the FM is deaf (well, not totally).

    When in the FM section, there is noise and static like normal. But with a short piece of wire attached to the antenna input (300 or 75 ohm), only the strongest stations get through and then its barely audible.
    I removed the cover and inspected the circuit board and traces and I don't see anything that would be an issue. Attaching a short piece of wire to the second post of the FM tuning gang (second one from the pulley) brings the FM section alive and I'm able to tune stations and get stereo (though its not real sensitive).
    I seems I lost the first section of the RF?. Anyone have any pointers on what to look for? I'm posting the Tuner circuit from the 3275 (which is almost identical) so people know what the circuit is like. Its not a very good scan, but its the best I got.

    Thanks in advance
     

    Attached Files:

     

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  2. Punker X

    Punker X Moderator Moderator Subscriber

    Messages:
    3,986
    Location:
    Panic in Detroit
    Have you cleaned the front-end, following the excellent sticky at the top of the forum? Check that nothing it caught up in the finns. Blow out with compressed air. Tune to a station at around 106 and try the trim cap for the front section.
     
  3. Binkman

    Binkman AK Subscriber Subscriber

    Messages:
    6,512
    Location:
    Chapel Hill, NC
    You need an antenna. 75ohm Either rabbit ears or T dipole. I recommend mounting the dipole vertical. Also if fm muting optional turn it off and on with dipole and you'll note the difference. You can also look at making a loop antenna and experimenting with directional positioning.
    Simple DIY FM antennas: build an FM loop for about $20 | FM DXing
     
  4. MCS Guy

    MCS Guy The MCS hoarder Subscriber

    Messages:
    538
    Location:
    FL
    After spending some time with it, I believe the 2SK55 JFET is bad as its pulling down the voltage to ground (which is 10 volts on the schematic and its near 0 in circuit). Un-soldering the Drain from the circuit board brings the voltage feed back up and allows the rear antenna terminals to function again.
    Tried testing the JFET out of circuit and it shows nothing but dead shorts no matter how I test it.
     
  5. MCS Guy

    MCS Guy The MCS hoarder Subscriber

    Messages:
    538
    Location:
    FL
    *Update*

    After replacing the 2SK55 with a 2N5486, the tuner is working perfectly now. Sensitivity is very good and very low background noise.
     
    rcs16 and I LIKE MUSIC like this.

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