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1965 Fisher 200-T one channel not working

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  • #46
    Not sure what you mean.
    Input is at 0 VDC (at least when a cartridge is connected) and the transistor base is at a positive voltage.
    So input side is the negative end - completely normal.

    Whenever in doubt just use your meter across the cap to determine the polarity of the voltage.
    - Own Opinions Only -

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    • #47
      Ok, I wasn't sure if it was a mistake on the schem or not. I looked up the 500/800T and it's the same there too so it's just something I needed to learn about solid state.

      Thanks
      --Jim


      He's like a new set of strings... he just needs to be stretched a bit.

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      • #48
        Electronics principles are always the same. There's no tube electronics vs. SS electronics. Just different circuits and components.
        Same rules, same basics.

        Where did you see an input coupling ecap with positive end connected to input? Hardly with tubes.
        Maybe with a circuit using pnp (Germanium?) transistors?
        Last edited by Helmholtz; 05-23-2022, 08:21 PM.
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        • #49
          Originally posted by gui_tarzan View Post
          Another question - I haven't seen a capacitor on an input with the negative side pointing to the incoming signal. I circled them in the attachment.
          Has to do with single-ended DC supply to circuit. Like most guitar pedals, the effective zero point within circuit is half of B+. Everything on the circuit side is positive with respect to the rest of the world hence cap polarity is selected as you see.
          This isn't the future I signed up for.

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          • #50
            I see no polarity indication at all, are they even electrolytics? A film cap has no polarity. The curved line on the symbol indicates outside foil.
            Education is what you're left with after you have forgotten what you have learned.

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            • #51
              Originally posted by Enzo View Post
              I see no polarity indication at all, are they even electrolytics? A film cap has no polarity. The curved line on the symbol indicates outside foil.
              Hardly a film cap at 1 (given the age of the amp).. See C105 for symbol comparison.
              Last edited by Helmholtz; 05-23-2022, 09:49 PM.
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              • #52
                Originally posted by Enzo View Post
                I see no polarity indication at all, are they even electrolytics? A film cap has no polarity. The curved line on the symbol indicates outside foil.
                Yeah, they didn't bother with the + sign on the drawings, either layout or schem. They just used the || and |( method.
                --Jim


                He's like a new set of strings... he just needs to be stretched a bit.

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                • #53
                  Originally posted by Helmholtz View Post
                  Electronics principles are always the same. There's no tube electronics vs. SS electronics. Just different circuits and components.
                  Same rules, same basics.

                  Where did you see an input coupling ecap with positive end connected to input? Hardly with tubes.
                  Maybe with a circuit using pnp (Germanium?) transistors?
                  I see what you're saying but I guess I should have said that I don't remember seeing tube amps with electros on the input side of tubes in the signal path, only film caps. When I look at the schematics between the two (SS vs tubes) there are many differences so I'm not following your statement completely. I know the principles, rules and basics are the same but from a component view they aren't the same. It's really as simple as I see something I haven't seen before and I asked why it would be so.

                  --Jim


                  He's like a new set of strings... he just needs to be stretched a bit.

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                  • #54
                    Originally posted by gui_tarzan View Post

                    I don't remember seeing tube amps with electros on the input side of tubes in the signal path, only film caps.
                    Exactly. The reason is that tube circuits are typically high impedance which don't require F coupling ecaps.
                    But that follows from basic electronics rules which are the same for tubes and SS.
                    Look up cut-off frequency of high pass filters.

                    - Own Opinions Only -

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                    • #55
                      Ok, I see what you mean now.
                      --Jim


                      He's like a new set of strings... he just needs to be stretched a bit.

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