Hum coming through tube system, most likely culprit?

Discussion in 'Audio Hardware' started by seed_drill, Apr 12, 2019.

  1. jea48

    jea48 Forum Resident

    Location:
    Midwest, USA
    Bill,

    Though, if the problem is with the ground wiring in the tone arm wouldn't the sound of the hum change when the OP touches the arm?

    The thing, imo, that rules out that the hum is caused by an AC mains ground loop is the OP said when he turns off the preamp the hum stops.

    TT:
    DUAL CS 5000, DUAL 1219
    Cartridge:
    Ortophon OM 40, Grado mono

    Quote:
    The Dual CS 5000 is a three speed, semi-auto, belt-driven turntable system with integrated low mass tonearm and pitch control.
    Specifications

    Motor: EDS5000

    Drive: Microprocessor controlled quartz belt drive

    Platter: non-magnetic removable

    Tonearm: aluminium tubular tonearm, Kardan, OPS

    Signal to noise ratio: 80dB rumble weighted signal to noise ratio

    Cartridge: OMB 20E HiFi DIN 45500

    Stylus: DN 20 NE Biradial

    Frequency range: 10Hz-28kHz



    DUAL 1219
    Quote:
    Specifications
    Power supply: AC, 50 or 60Hz, changeable by changing motor pulley

    Power supply voltage: 110/117V or 220V switchable

    Drive: Synchronous continuous-pole motor with radial-elastic suspension

    Platter: non-magnetic, dynamically balanced, 3.1kg

    Speeds: 33, 45 and 78rpm

    Pitch control variation: 6%

    Rumble: -60db (weighted)

    Tonearm: extra long, torsionally rigid metal arm, 4-point gimbal suspension, skeletal head design

    Cartridge holder: removable, accepts all 1/2" cartridges from 1 to 12

    .

    I assume the AC power plug on the TT is a 2 wire, non polarized type.
    Just for the heck of it I would unplug the TT from the wall outlet and check for the hum. If the hum is gone I would then try reversing the plug 180 in the wall outlet and check if that changes the sound of the hum.



    .
     
    Last edited: Apr 14, 2019
    seed_drill likes this.
  2. SandAndGlass

    SandAndGlass Twilight Forum Resident

    I was going to post the same thing as @KT88. Do you still have hum without anything connected to the phono input?
     
  3. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    I turned the plug around (currently have the 5000 in this system) and it's quiet for the time being.

    I'll leave it on a while. There is a tendency for the hum to be gone when I first turn the system on and then for it to appear later. I tried to do a needle drop last night and it got about two thirds of the way through side one before it started up again.
     
  4. SandAndGlass

    SandAndGlass Twilight Forum Resident

    So, are you getting the hum out of one channel or both?
     
  5. KT88

    KT88 Forum Resident

    Now that sounds like an AC polarity chassis ground capacitor, aka a "death cap". I would bet by the symptom you are describing now that the chassis is seeing some live voltage potential, perhaps as high as 60v. It should see a doctor.
    -Bill
     
  6. jea48

    jea48 Forum Resident

    Location:
    Midwest, USA
    If it does start humming again unplug the TT from the wall outlet. If the hum stops then that points to an AC power issue with the TT, imo.

    If unplugging the TT from the wall outlet doesn't stop the hum then the problem is somewhere else.
    Had you just powered up the Scott preamp? Or had it been turned on for a while? If you had just turned it on there may be a problem in the phono section. Possibly a bad cap. Might be a tube. Could be a cold solder joint on a tube socket. It could be as simple as a corroded tube pin to tube contact connection. The problem doesn't manifest itself until the phono section warms, heats, up.
    (Someone may have already mentioned all this in the thread)
     
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  7. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    both
     
  8. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    The way things work around here is, if something isn't working, I take it to one of several different repair guys who keep it for close to a year. Half the time it can't be fixed. The old guys are retiring and the young guys. Oh, wait, there aren't any young guys.

    This pre amp needs some new pots, but no one around here has a source.
     
  9. SandAndGlass

    SandAndGlass Twilight Forum Resident

    Probably not a tube issue.

    Grounding, or ground loop more likely.
     
  10. Beattles

    Beattles Forum Resident

    Location:
    Florence, SC
    Also, does it hum with both TT/Cart combinations? I don't know about the Grado mono, but I have had a Grado Wood Body RS (?iirc) that I could not get to stop humming.
     
  11. Just Walking

    Just Walking Forum Resident

    Location:
    Abingdon UK
    I've just been peering at photos of the preamp layout http://www.avidaudio.com/images/scott130c.JPG and the schematic. Now I don't know this particular piece of gear, but I have restored lots of similar stuff (mainly Tektronix test gear). The 130 was apparently last manufactured in 1960. I'd make the following observations, in no particular order:

    1. Selenium plate rectifier. That is a potential source of trouble at that age. Feeds the heaters with filtered DC (which is really good), but correct operation should be checked.
    2. Alan Bradley carbon composition resistors. These tend to drift high, often by 50% or more. Critical most places, but particularly in the RIAA bit.
    3. Lots of paper capacitors (the black tubular things with a white band). These either go electrically leaky and screw with tube bias points, and/or have zero capacitance.
    4. Allen Bradley carbon track pots. Although no longer made, similar pots are available from https://www.blore-ed.com/series45potentiometers . Are your existing ones causing problems?
    5. Whatever the smoothing caps were, they are likely to be dried out and misbehaving. I *think* they are Sprague "twist-lock" type. These tend to become disconnected from the terminals - the connection foil inside the caps rots through.
    6. Two wire mains connection with no safety ground. Typical of anything up to the late 60's/early 70's, but not possible now of course.
    7. The original phono sockets are ugh.

    Unless it has already been restored, there are ample opportunities for the sort of problem you are hearing. But definitely worth sorting out - the schematic shows that it is a really rather before-its-time piece of classic gear, is drop dead gorgeous and should sound superb.

    PS did you spot that there is a hum balance pot on the rear panel?
     
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  12. Ralph Karsten

    Ralph Karsten Well-Known Member

    Location:
    St. Paul MN
    @Just Walking makes a good point about the hum balanced control. Before anything else try it out and see what you find.

    Now if the phono hum returns after a while of operation, shut the preamp off and give it a minute or so and turn it back on. If the hum is gone then there is a good likelihood that a filter capacitor that feeds DC power to the phono section is failing. And if that hum is gone, its thus also a good bet it will return soon.
     
  13. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    It was restored 12-15 years ago. Except for the ughs. I mean phono sockets. Those are still original.
     
  14. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    I've played about half a dozen albums since flipping the turntable plug, and so far the hum has not returned.
     
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  15. Just Walking

    Just Walking Forum Resident

    Location:
    Abingdon UK
    That is good news! And good to hear it was restored - looks like it was an iconic product with great looks.
     
  16. nm_west

    nm_west Forum Resident

    Location:
    Abq. NM. USA
    Great! You kind of dismissed polarity advice, but it looks like you're all set. :)

     
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  17. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    I always try the easiest thing first. :wave: Still, I wouldn't be surprised to have it return.

    I had an old PAM clock that had some issues and every time there was a power outage or surge the hands would start going around backwards! This reminds me of that.
     
  18. richbdd01

    richbdd01 Forum Resident

    Location:
    London
    One thing you might have overlooked and, perhaps, an easy mistake....the humming isn’t part of the music is it? :shh:
     
  19. Angry_Panda

    Angry_Panda Well-Known Member

    Mental note to self - do not use Glenn Gould as a test record. :p
     
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