Oscilloscope ground

Discussion in 'General Electronics Chat' started by ZymDap, Nov 30, 2014.

  1. ZymDap

    Thread Starter New Member

    Nov 30, 2014
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    Hello,
    I've got an old scope C1-64 (http://www.cqham.ru/hpa/osc_c1-64.htm). And try see signal from my PC (let's say 1kHz). The scope only shows signal when + wire (phase wire) is connected to the source of the signal (to the middle pin of the RCA plug). When I connect ground ofthis plug, the signal in the screen dissapears and shows only ground line. Is it related to the settings I use? Or maybe ground isn't nessesary at all?
    My settings are:
    http://i.imgur.com/1rLCLVe.jpg
    (Everything in Russian, but experienced specialist I hope understand)

    Thanks in advance :)
     
  2. ZymDap

    Thread Starter New Member

    Nov 30, 2014
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    0
    My scope has this wire:
    • [​IMG]

    And when I connect like this:
    • [​IMG]
    The signal is shown in the screen of scope.

    But if I add ground:
    • [​IMG]
    There's no more signal in the screen, just plane line.
    I had different very small scope and I used both: phase and ground wires, so don't really know what's the problem here in this huge scope.
     
  3. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
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    I think you have an earthing issue here?

    Is your PC and scope connected to mains earth?
     
  4. ZymDap

    Thread Starter New Member

    Nov 30, 2014
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    I'm begginer with scopes. I suppose probe is simple shielded wire, because I measured with tester and signal goes straight through. Oh it's very good idea that all problems are due to bad ground connections. Yes, my computer has earth wire in the wall socket, but scope isn't grounded. To tell the truth, there's no such connection in the back of the scope for ground. Several banana plugs in the front maybe. The other small scope I have, does contain that ground connection. But maybe this C1-64 scope doesn't need to use ground when measuring..
     
  5. ZymDap

    Thread Starter New Member

    Nov 30, 2014
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    Tried on different scope, and yes, if there's no main ground, it's enought only one probe wire. Is it good or bad to measure like this: using only one probe wire? I've seen videos, where measurements are made using both probe wires and there's no main ground.. By the way, found ground connection in the fron of the scope. I'll try to connect it.
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,429
    3,360
    Something does not quite add up. Most oscilloscopes and probes are grounded to your AC mains supply.

    If your PC is also grounded then you should see a signal with or without the probe ground.
    If your PC is not grounded then you should see AC mains pickup at AC mains frequency without the probe ground.

    I would recommend getting a DMM and check your ground situation at scope, the PC and at the audio jack.

    Make sure your trace on the scope is the 1kHz signal expected and not AC mains pickup.
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    I think you need to show what circuit you are probing and the exact power supply used.
    When you connect the ground, have you tried increasing the vertical gain?
     
  8. ZymDap

    Thread Starter New Member

    Nov 30, 2014
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    After some settings changed seems to be working :D thanks all :))
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    You did not say how the problem was resolved. It is usually the right thing to do to bring closure to a discussion on the forum. Someone else could benefit from the education.

    My guess was you were observing AC mains pickup and not the the proper signal.
     
  10. ZymDap

    Thread Starter New Member

    Nov 30, 2014
    6
    0
    I've made several mistakes by setting not very good settings of Vol/Div and synchronization. Now going to change amplifier and stabilization potentiometers, which look like haven't been changed in 33 years :D
     
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