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Cleaning up a Fisher 500C

Discussion in 'Fisher' started by Mikeorg, Sep 24, 2018.

  1. Mikeorg

    Mikeorg New Member

    Messages:
    42
    Location:
    OREGON
    OK thanks one and all I've ordered all Capacitors and a new 8 Amp bridge rectifier, will post when build is complete or need your help :dunno: again! I really do appreciate this site!...
     

     

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  2. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,240
    Location:
    Melbourne, FL
    Don't be a stranger. We'd like to hear how it all goes after you install the parts.
     
    Mikeorg likes this.
  3. RUmad

    RUmad AK Subscriber Subscriber

    Messages:
    936
    Location:
    New York
    Ayuh. What he said. And keep asking questions.
     
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  4. Catmanboo

    Catmanboo AK Subscriber Subscriber

    Messages:
    483
    Location:
    Atwater, ohio
    I have 2 500c's , one with o.e. bridge, still in spec. Other one got a 4a-200 bridge transplanted, no issues there, working fine-when I use it.:)
     
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  5. 604man

    604man Well-Known Member

    Messages:
    821
    The original bridge in my 500c only provided -12 bias , not the required -17.
    This could burn up expensve 7591A's.

    Best to replace while you have the covers off , A good insurance policy.
     
    Mikeorg likes this.
  6. audmod01

    audmod01 Super Member

    Messages:
    1,219
    Location:
    Tioga, TX
    The bridge selenium rectifier for the bias voltage in Fisher amps, tuners and receivers was very common for years. Fisher used selenium bridge rectifiers in the FM-100B and FM-200B tuners and some others for the HV B+ too. Most of us do replace them when doing a restoration just as insurance against failure later on. Loosing negative bias voltage on output tube can be quite costly - loss of the tubes themselves and worst case loosing an output transformer. I replaced the bias selenium in my 500-C with a 2A@200V from Mouser Electronics. It is not critical about current rating here since tube bias circuits use very little current themselves, but the main thing is to be sure that enough current is available if it is also feeding tube filaments in the preamp circuits of a given unit. In a Fisher 500-C or 800-C there are two 12AX7 tubes with their 12.6VAC filaments in series in each leg of the circuit. Each leg uses .15A so with two legs the bridge rectifier needs to supply .3A in order to handle the current needs of the preamp tubes. As mentioned above the current feed to the grid circuits of the output tubes is quite small. Any adjustable bias control circuit will consume current, but still far less than the filament circuits.

    Joe
     
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