MF-300 & Remote Control Acquired

Discussion in 'Fisher' started by audmod01, Nov 2, 2017.

  1. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
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    Location:
    Tioga, TX
    Today I tried some other parts such as C28 the .05uF cap and that made no difference. I also increased the load resistance to the transducer circuit by adding a 10K ohm resistor as a load - all with no oscillation from the oscillator circuit. I removed the RCA SK3004 transistor and installed a 2N4403 silicon PNP transistor - still no oscillation. C25, C27 and C30 check good so I do not suspect any problem there. The switch contacts were cleaned early on in the investigation. I am about to suspect that the oscillator coil may have some shorted turns in it. It is a potted core ferrite construction. There is a tuning slug in the center, but I have not made any attempt to move it. I may reinstall the 2N632 transistor and try changing the tuning slug of the oscillator next.

    Matt is sending his units to me to restore, so I will get a chance to examine one more remote hand unit and another RK-20 for comparison. If the oscillator coil in my hand unit is bad I am hoping his remote hand unit has a good coil so I can check resistance readings to compare the two.

    For now I am frustrated and am taking a break from it. I find that usually helps. Going back to look at an item another day often helps by starting with a clear mind and progress is made.

    Joe
     

     

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  2. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    1,094
    Location:
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    I removed the oscillator coil from the board and checked resistances, recorded same and then put back in board with the OEM 2N632 oscillator transistor. I am waiting for Matt's remote pieces so I can compare resistance readings of the oscillator coil in circuit.

    I did receive the ultrasonic transmitter transducer I ordered from DigiKey yesterday. I had taken apart the OEM transducer to look at the metalized mylar disc. It had many damaged areas scattered about the center portion of the disc and around the outer edges much of the metalized surface was worn away where the circular metal backing plate touched it. So the OEM transducer has real problems. However, even with the transducer disconnected, the oscillator circuit will not oscillate, so there are still problems with the oscillator circuit. Things are pointing to the coil assembly being defective at this point.

    As a backup measure I am considering using 555 timer ICs to generate the desired frequencies that would be needed to talk to the RK-20 receiver circuits. 555 timers are very stable in operation and so would make a good choice for such service. They could be housed in a small plastic project case. I could still keep the original remote hand unit for looks. If it turns out the 555 timer circuits were small enough to house inside the Fisher remote hand unit case, I would do that instead.

    Joe
     
  3. rufleruf

    rufleruf AK Subscriber Subscriber

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    There is usually some slight logic to Fishers naming conventions. Anyone care to guess at "MF" as in MF300? I guess for that matter RK ought to be RC...
     
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    R for AM/FM Tuner
    FM for FM only Tuner
    MF for Multiple Frequency (Band) Tuner?
    RK for the Remote considering the "GERMAN" Engineer's. Remote Kontrol? Kathode=Cathode?!?!
     
  5. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
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    Location:
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    R does seem appropriate for AM/FM models based on a number of example Fisher chassis.
    FM seems self explanatory and there are numerous examples of Fisher's FM only tuners.
    MF - doesn't seem to apply in the case of the MF-300, which is an FM only tuner. The later R-200-B AM/SW/FM tuner is a multiband Fisher tuner. Is there a German word for automatic that begins with "M"? Maybe Magic FM?
    RK=Remote Kontrol does seem logical. Fisher used the same mnemonic for their wired remote controls so there was definitely a precedent there.

    Joe
     
  6. rufleruf

    rufleruf AK Subscriber Subscriber

    Messages:
    1,008
    I guess it could be arbitrary but scaling with position in the lineup like the numbers associated with the different receivers 400, 500, 800, or integrated amps 100, 101, 202, 1000, or tuners for that matter 50, 90, 100, 200, 300, 1000. All manufacturers seem to have an element of this bigger is better approach, but Marantz with their 70's silver face line has an element of the literal to it one finds them selves trying to reason out of the numbers
     
  7. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
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    Location:
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    I am pretty certain that the oscillator coil in the remote hand unit I have is bad. I have been investigating using 555 timer ICs to generate the required frequencies to create the two primary frequencies of 38.285kHz and 41.805kHz. I need to determine the modulating frequency that activates the reverse relay and use another 555 timer for that. I need to get some perf-board so I can build up the circuits. They may well be small enough to mount inside the original remote hand unit case and that way would look original. Lots of work lies ahead. I got a good buy on some 555 timer ICs and bought 100 of them. I will eventually have a use for them for other projects. The frequency stability of the 555 timers are quite good as long as good quality resistors and capacitors are used with them. Silver mica, polypropylene or polystyrene capacitors are some of the most stable.

    Joe
     
  8. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
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    The 555 timers arrived today in a long box from Mouser Electronics. I could not understand why they used such a long box until I opened it and saw the ICs were in a long anti-static plastic tube. They arrived in good shape though, so I am glad to have them.

    Joe
     
  9. rufleruf

    rufleruf AK Subscriber Subscriber

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    Well, half the naming mystery of the MF-300 is solved - it's called the "Motor Tuner" on a manual I saw.
     

     

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  10. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
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    So maybe "Motorized FM"? That makes sense.

    Joe
     

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