Fisher 700T--First Solid State Restore

Discussion in 'Fisher' started by Dave451, Nov 21, 2018.

  1. fred soop

    fred soop Super Member

    Messages:
    2,065
    That may not get rid of the thump because you are still stuck with that damn output coupling capacitor.
     

     

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  2. Dave451

    Dave451 AK Subscriber Subscriber

    Messages:
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    Location:
    Philly area
    Ahhhh. I remember your earlier input on same. Perhaps this and the power supply/output coupler are an area for mods in the future? For now, things are 'thumping along' in more-or-less stock condition.
     
  3. fred soop

    fred soop Super Member

    Messages:
    2,065
    You can't make a modification to remove the output capacitor. The power transformer winding does not have a center tap. I studied this issue for a long time in an attempt to modify my Heathkit AR-15. Without being able to make that change, the unit is only worth repair to keep it working, not worth other upgrades or complete restoration. Another option, but there is not enough room, would be to convert the output stage to full bridge with both speaker terminals floating at half the supply voltage. That has other problems, such as DC offset of 2 amplifiers, bias tracking for 2 amplifiers, etc. Would be easier to find a different transformer.
     
  4. Dave451

    Dave451 AK Subscriber Subscriber

    Messages:
    1,263
    Location:
    Philly area
    Well, sounds like we're staying stock and putting up with the thump for now. Though there may be some imperfections in the design of the 700-T, it seems to be a pretty pleasing receiver as is and worthy of some work to get it going. I'll see how it measures and post here later.
     

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