TFM-300 Suddenly Stopped Working

Discussion in 'Fisher' started by artsybob, Dec 7, 2018.

  1. artsybob

    artsybob Active Member

    Messages:
    100
    Location:
    Northern Virginia
    Last Spring I fixed up a TFM-300 tuner (replaced all electrolytic caps, had everything else tested) and for several months it worked perfectly with no incident. A couple of days ago while tuned into a station playing Christmas music the signal faded out and now I just get static. It doesn't pick up any stations. Did one of the three tubes in the front end go bad? Any help is appreciated.
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,906
    Location:
    Ball Ground, GA
    Definitely a possibility, or, one of the IF tubes could cause this as well........

    Dave
     
  3. fred soop

    fred soop Super Member

    Messages:
    2,115
    No IF tubes, but the front end is the tube unit as used in the 500-C. There is an RF amplifier that actually looks like a tube and 2 6CW4 Nuvistor tubes that are small cans slightly larger than a transistor. I suspect one of the Nuvistors. If it appeared to drift off frequency before failing, then probably the oscillator. If it faded without mistuning, then probably the mixer.
     
  4. audmod01

    audmod01 Super Member

    Messages:
    1,388
    Location:
    Tioga, TX
    One of the Nuvistors (actually a small vacuum tube) is used for the Local Oscillator and the other one is used for the Mixer function in the front end. If you still have intersation hiss or white noise with muting off, then the problem is probably the 6CW4 used for the Local Oscillator. You will need at least one 6CW4 replacement to trouble shoot the problem unless you can determine that the B+ (about +90VDC) is missing from the plate circuit of either one of the 6CW4 tubes. The RF Amp tube can be dead and the unit might still pick up local stations with strong signals.

    Joe
     
  5. fred soop

    fred soop Super Member

    Messages:
    2,115
    The oscillator gets its cathode current from pin 3W on the IF board. This is shown as being 12 V under "normal" conditions. However, 6CW4 Nuvistors can be swapped into the oscillator position and voltage measured on pin 3W. The lower the voltage, the weaker the tube.

    The oscillator and mixer use the same B+. But, when FM is not selected, the mixer is disconnected. The oscillator tube current can be checked here as well by measuring the B+ as audmod described. But, if FM is selected, then both tubes would have to be bad to get a much higher voltage.

    One of my major concerns with the design is that with the front end either missing tubes, or bad tubes, is that the same resistor chain supplies the 19 V to the IF circuits. The lack of current draw by the front end would significantly increase that 19 V supply. My redesign adds a regulator at the 19 V supply.
     
  6. fred soop

    fred soop Super Member

    Messages:
    2,115
    CORRECTION to post #5. I was thinking 600-T, not TFM-300. Obviously, the TFM-300 will always be in FM mode and the mixer does not get switched. The B+ can still be used to find a bad tube but needs to be checked with only one of the Nuvistors installed. It can be in either position.
     

     

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  7. audmod01

    audmod01 Super Member

    Messages:
    1,388
    Location:
    Tioga, TX
    A look at the schematic on page 9 of the service manual shows that the B+ is dropped down to provide both +19VDC and +22VDC for transistorized circuits. Check to make sure that you see those voltages in the power supply. The +90VDC voltage is supplied to both mixer and oscillator circuits in the front end. There is a separate +110VDC fed to the 6HA5 RF Amp tube. Make sure all of those key voltages are being supplied to their appropriate circuitry on the front end, IF and MPX boards.

    Joe
     

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