My Dynaco FM-5 drifts, any ideas?

Discussion in 'Tuners' started by N8Nagel, Jan 13, 2017.

  1. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    Subject says all

    worked fine when I tested it but after bringing it home and hooking it up for real, I found that it'll start drifting after a few minutes of operation. If you leave it on for a good hour or so and then lock in a station it'll be fine. I thought this was something that happened to tube tuners not SS, but there you have it. If I leave it on "Dynatune" before that first hour or so it'll actually drift so far that it'll mute, I'll have to get up and nudge the dial.

    Since there are no vents in the case I am assuming it's actually clean inside and that cleaning the tuning cap won't accomplish much.

    Is there anything I can try at home, or is this a "take it to a pro" kind of thing?

    thanks for any advice!
     
  2. Punker X

    Punker X AK Subscriber Subscriber

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    Clean it anyway. It not really the dust and dirt, but oxidation on the ground tabs that.
     
  3. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    If that is the case should I just run the tuning knob back and forth, end to end a few times?

    Actually I'm going to do that anyway now. thanks for the idea.
     
  4. Pete B

    Pete B AK Member Subscriber

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    I've owned an FM-5 since the early 1970s and noticed the drift in tuning - that is what the auto lock is for - No?
    I remember measuring mine, finding that the power supply voltage drifts, and noticed that it
    uses a very primitive voltage regulator that lacks thermal compensation. Expect it to drift as
    it warms up a modern 3 terminal voltage regulator should fix it.
     
  5. audmod01

    audmod01 AK Subscriber Subscriber

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    Due to the age of these units now I would recommend replacing the electrolytics in the power supply. You could first measure the DC voltage going to the front end and see if the voltage is slowly changing from cold start. If it is, it points directly to the power supply. The regulator mentioned above would be suspect too, but electrolytics are known problem causers in older solid state equipment.

    Joe
     
  6. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    Going to have to try that. I've just been "dealing with it" but soon as I get another unit done (current patient: Marantz 4270) I'll try clipping some grabbers on the PS and monitoring it as it warms up.
     
  7. gadget73

    gadget73 junk junkie Subscriber

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    if the front end has a varactor diode in there for AFC purposes, any voltage drift will make that change value. Old zener based supplies did tend to do that a bit. A fixed voltage regulator like mentioned earlier would probably sort that out.
     
  8. Pete B

    Pete B AK Member Subscriber

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    These tuners worked when new but drifted when the Dynatune was not engaged,
    if they drift more as they age there is a good chance that a cap in the voltage regulator
    or Dynatune circuit
    has become leaky with reduced breakdown voltage so that it takes a very long time to
    get up to full voltage. A recap will probably fix it. A 3-terminal regulator might make
    it stable without using Dynatune.
     
  9. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Many times a ceramic cap or the varactor itself becomes temperature sensitive, causing drift. If none of the above fixes it you have to go through the LO circuit hitting each component with a small amount of freeze spray to see what makes it detune the most.
     
  10. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    Well, before I had a chance to address this, my PAT-4 decided to develop a 60 cycle hum, so I'll be swapping something else into the rotation while that is addressed :(
     

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