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Sudden treble volume bursts in one channel ???

Discussion in 'Fisher' started by thornev, Sep 21, 2018.

  1. thornev

    thornev Well-Known Member

    Messages:
    557
    Location:
    Mid Hudson Valley, NY
    All of a sudden my 500-C is doing this thing where it will suddenly in one channel get really loud and trebly. I think it's only in Phono Stereo mode. If I turn the volume knob really fast back and forth it goes back to normal. Does this sound like a case for Deoxit in the volume control? The only changes I've made to my system are to install a moving coil cart in my turntable (but I doubt that's the cause), and to change the cabling into AUX and Phono Stereo (I hooked up both AUX and PHONO cables at the same time so I can simply choose on the 500-C which input source I want to listen to using the SELECTOR knob). I've tried jiggling the cables which does nothing. It won't do it for days and then will maybe do it several times during the night.
     

     

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  2. RS Steve

    RS Steve Tube Junkie Subscriber

    Messages:
    7,539
    Location:
    Jupiter, Florida
    If it's in phono only, go back and check all the solder joints in that area where things were replaced. Then look into the tubes in that circuit, test or swap them to see if it helps.
     
  3. thornev

    thornev Well-Known Member

    Messages:
    557
    Location:
    Mid Hudson Valley, NY
    I don't know which solder joints to which you refer. If you have a schematic, I'd appreciate component indicators. Which tubes are for the Phono Stereo selection?
     
  4. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

    Messages:
    3,282
    Location:
    NC
    Try just a short shot of Faderlube, sound like a dirty pot.
     
  5. llwhtt

    llwhtt Super Member

    Messages:
    1,488
    Location:
    SoCal
    If it's only in the phono position I would think the RIAA circuit, PC! or PC2 would be at fault, even the switching could do it. The RIAA uses feedback to greatly attenuate the treble and boost the bass. If the feedback is intermittent the treble would get loud.

    Craig
     
  6. Rockyhill

    Rockyhill No marigolds in the promised land Subscriber

    Messages:
    3,200
    Location:
    Maine
    I had a similar symptom with my 500B. Turned out the Loudness switch was ever-so-slightly engaged. I must have bumped it inadvertently. Once I turned it off the symptom stopped.
     
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  7. thornev

    thornev Well-Known Member

    Messages:
    557
    Location:
    Mid Hudson Valley, NY
    On the 500-C it's not a switch; it's a knob. I'll try turning it back and forth a few times to ensure that's not the root cause. For now I've worked around the problem by disconnecting the AUX cables. Before, when I was having the problem, I had connected cables so that I have AUX (for playing CD's) and PHONO both permanently connected and hence could switch between them using the SELECTOR knob (which is the normal way of cabling). Disconnecting AUX cables seems to have fixed the problem, but of course I'm back to having to move cables between PHONO and AUX.
     

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