Fisher 400-CX hum woes

Discussion in 'Fisher' started by gkargreen, Aug 21, 2017.

  1. gkargreen

    gkargreen Active Member

    Messages:
    341
    Location:
    Fairfax, VA
    I have a 400-CX I am working on that has an annoying hum in both channels, although louder in the rt. channel, this is coming from the AUX position. The strange thing is that with aux 1 shorted and the chosen input I get the hum, but if I chose aux 2, the hum goes away even though that channel input is not shorted. Also, if I choose the phono input, no hum there either. I can see the hum on pins 3 and 6 of both V7 & V8 (1 mv) and pins 3 & 6 of V6 & 5. It is also present at pin 6 of V4 & 3, but much reduce. The PS has been rebuilt by another respected tech, who basically rebuilt the whole thing. Any suggestions? Thanks!
     
  2. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Can you determine if the hum is 60Hz or 120Hz. I was originally thinking heater hum, but all the tubes are DC heated. But just for shits and giggles, check the heater wiring dress, and C32A & C32B for leakage. Do you have any hum on V1 and V2?
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Sounds suspiciously like a ground loop -- as in the ground system got disturbed when it was totally rebuilt. You did not state it, but may I assume that the hum is volume control dependent?

    Dave
     
  4. gkargreen

    gkargreen Active Member

    Messages:
    341
    Location:
    Fairfax, VA
    Thanks, guys, the hum is 120 hz and yes, the hum is volume control dependent. I'll check on the V1 & 2 to see if they have hum as well, although I believe they do not, thanks!
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Almost surely a ground loop then........

    Dave
     
  6. vendo81

    vendo81 AK Subscriber Subscriber

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    One time while visiting my good friend, who's also a great tech, I noticed a 400c on his bench he was working on. It came to him rebuilt but with a hum problem. He mentioned that this is a common problem with some versions of the 400c. I know one of the things he had done is change out some components being careful to where they were located and he checked and repaired some chassis grounds. He moved some grounds as well. He did a couple other things that I dont recall but it was really quiet when he was testing it.
    I don't know if this is your problem but he said you have to be careful rebuilding the 400c.
     
    Last edited: Aug 22, 2017
  7. gkargreen

    gkargreen Active Member

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    341
    Location:
    Fairfax, VA
    Thanks guys. One thing I forgot to note is that the waveform is basically a sine wave with the bottom flatten, so not a typical sine waveform, here is a pic of it below. Note sure if that helps, but I would think that it points to a particular issue.
     

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  8. gkargreen

    gkargreen Active Member

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    341
    Location:
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    Looks like half-wave rectified...
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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  10. gkargreen

    gkargreen Active Member

    Messages:
    341
    Location:
    Fairfax, VA
    yep, it was a bad diode that was the cause, that's why the half-wave rectified waveform look, it WAS only have rectified!
     

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