Fisher MF-300 tuner

Discussion in 'Fisher' started by vendo81, Aug 2, 2017.

  1. vendo81

    vendo81 AK Subscriber Subscriber

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    I decided to work on my new Fisher tuner tonight. I haven't seen a lot written up on the MF-300 so I thought I'd add some info.

    These are very interesting tuners and fun to use. Put the selector switch in automatic, push either of the bottom center buttons (tune o matic) and it will seek out the next station in that direction. Hold the button and it skips over stations until you let go. It automatically goes into stereo or mono depending on the signal received. Putting it in Mono disables this feature and in stereo locks the tuner in stereo. The 3 way muting switch not only eliminates the noise between stations but also provides 3 degrees of sensitivity for the automatic search circuit. There is a sensitivity pot on the rear that adjusts how sensitive the tuner's seeking is as well. AFC helps lock the station precisely in center of a station while scanning as well as keeping it from drifting. Fisher really talked it up in their brochures on how well the MF-300 finds the exact center of a station and it does seem to work exceptionally well. I notice much less drifting out of stereo mode on weak stations than when I tune by hand. The stereo filter switch does the same thing as a high filter. The MF-300 has lights to tell you what's going on. When seeking and it finds a station, the station indicator on the right side of the dial lights.Two lights on the left of the dial light up when a stereo station is being received. The dial pointer has a small light that moves along with it, showing you from a distance where you are on the FM dial. On the rear, besides the mentioned sensitivity adjustment pot are left and right volume pots. Left and right main output as well as left and right rec output jacks are provided.

    Using the RK-20 wireless remote, you can turn the tuner on, volume up or down, and turn it off. You can also seek the next station in either direction, all from your easy chair. Very advanced for 1964!

    Tonight I took a look under the hood. Nothing looked burnt or unusual and it looks to have never been worked on. I only replaced about 3/4 of the caps as I have a few on order. I will still need to finish the rest and do the filter caps, which for now are staying cool. This tuner had the typical bad power switch that I had to jump to test things out. The switch will need a good cleaning or more likely replacement. I brought it up on the variac watching the current draw and checking voltages. At 90 volts I heard relays click and it came to life!

    This is a very, very sensitive tuner and it catches every station out there! The Los Angeles-San Diego area has so many AM and FM stations it only travels about a 1/4" before it finds the next station. All this with just a single wire clipped to one FM terminal. This one sure doesn't need an alignment and frequency is spot on. Everything is working like it should and it sounds fantastic. It is nice to find something that actually works well and just needs basic servicing.



    MF300 working.jpg
     
    Last edited: Aug 2, 2017
    I LIKE MUSIC and radioactive like this.
  2. audmod01

    audmod01 AK Subscriber Subscriber

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    It is so nice to see this thread appear. I only ever saw one while I worked in Berkman's Hi-Fi in Austin during the early to mid 1960s. We were a warranty service station for Fisher and a number of other major brands. We had an addendum service manual that applied to the MF-300 model. I have not seen one of those addendum service manuals since. If you happen to have one, a scan of it would make a good contribution to the AudioKarma.org database in the Fisher folder. The unit I service at the time had only minor issues that were corrected by application of contact cleaner and little else. I don't think it even had a bad tube at the time. Of course this was back when the units were still pretty new.

    Congratulations for having one of these fine tuners!

    Joe
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Very, very cool Vendo! It would be interesting to know how the motor control circuit operates that moves the pointer up and down the dial. Buttons provide direction of rotation, while working with the muting circuits would be ideal for the system to know what stations to consider, and which ones to ignore. Still, having it stop precisely on the dead center of the station would be interesting. An extention of the Microtune/AFC circuits no doubt, but does it have the ability to correct itself if any drifting occurs? I would think that the motor control circuit would need to operate in some kind of servo (feedback) control circuit if so to keep itself centered on the carrier frequency.........

    Thanks for posting on such a rare Fisher model!

    Dave
     
  4. vendo81

    vendo81 AK Subscriber Subscriber

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    Dave.
    On my large Saba automatic radio if you move the tuning knob slightly it corrects itself and recenters the station. On this tuner, once automatically centered, a relay kicks in and the station indicator lights up. You can then manually move the tuning knob and it stays wherever you put it until one of the two tune-o-matic buttons are again pushed. It does say in the owners manual that the AFC circuits are fully temperature compensated and will not drift more than .02% under any circumstances. I'm sure that only applies with new components and tubes.
     
    Last edited: Aug 2, 2017
  5. vendo81

    vendo81 AK Subscriber Subscriber

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    Hi Joe.
    That's great that you actually worked on these classic Fisher items back in their early days :) This tuner had all original tubes and only 2 were bad. All of the 6AU6's, the 12AT7's and the 12AX7 tested as new so I don't think this was used much.
    Wish I had the addendum service manual! It would be nice look at.
     
  6. audmod01

    audmod01 AK Subscriber Subscriber

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    Maybe one day someone in the group will locate a service manual addendum and scan it for the group. I generally search Fisher Vintage Electronics on eBay and if I ever spot one I will buy it and do so. Many people are not even aware of this model.

    Joe
     
  7. vendo81

    vendo81 AK Subscriber Subscriber

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    Well I did find one problem last night while listening to the tuner. While searching in either direction, when the dial pointer gets to an end it automatically goes into reverse and starts scanning for a station the other direction. This was working in one direction but would sometimes "freeze" in the other. There is an adjustable contact at each end that make when the pointer comes into contact with them. My first check was the contact at the "frozen" end but this was adjusted and working fine. Each contact engages a solenoid that in turn does 2 functions. It releases the previously selected solenoid, opening it's contacts and in turn closes a set of contacts for the other direction that are mounted on the newly selected "other direction" solenoid. All of this is mounted on the drive motors gearbox and when the other solenoid is selected, motor direction reverses and it also moves a set of gears back into mesh with the main drive gear. When a station is found, both selenoid's drop out and the shaft that they are connected to now centers. When centered it's 2 outer gears no longer mesh with the main drive gear. This is necessary for precise locking of the pointer at the instant the search circuitry finds the exact center of the stations tuning. This shaft had gummed up and was having problems working quickly and consistantly.
    Having worked on my jukeboxes for 20 years I'm used to such contacts and mechanical systems and was able to lubricate the gear train and burnish and adjust all contacts for correct operation. It runs quietly and very smooth in selecting and reversing direction now.

    I bet this drove many techs crazy the first time they had one come in with the pointer stuck at one end :eek:
     
    Last edited: Aug 3, 2017
  8. audmod01

    audmod01 AK Subscriber Subscriber

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    That's great news! Sounds like it is now working in top shape again after some attention by the master!

    Joe
     

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