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Connected my oscilloscope up backwards

Discussion in 'Troubleshooting and Repair' started by gwfami, Apr 17, 2014.

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  1. gwfami

    gwfami

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    Sep 13, 2012
    Ok,
    Made a noob mistake and hooked the + to the - on my oscilloscope (was using 8 1.5 AA cells in series).

    The scope still seems to work, but now the display is about 1/4 the size of the original.

    Looked under the hood and didn't see anything obviously blown.

    Any ideas?

    Thanks,

    GWFAMI.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Where did you misconnect the batteries? In the battery compartment or in the measurement circuit?
    Which oscilloscope do you use?
     
  3. gwfami

    gwfami

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    Sep 13, 2012
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I'm not sure how that could have caused a problem.

    Do you mean you connected the ground leads of the two probes to different parts of the circuit having different potentials (and essentially shorted them out via the scope)? Or do you mean something different.

    Unless they were NiCad batteries, or maybe D size alkaline batteries, I can't see this damaging the scope. In the worst case you may have damaged the probes (melted the insulation on them perhaps?)

    A voltage of 12V across the input of a scope shouldn't cause any damage.

    Get your multimeter and measure the resistance across the probes (ground to probe tip) both with the leads disconnected and again with them connected.

    Assuming you get a very high resistance (tell us if you don't) then turn the scope on, set the vertical range for 0.5 to 1V per division and then probe a 1.5V battery. The trace in the screen should jump by between roughly 1.5 and 3 divisions depending on the battery voltage and the vertical sensitivity you've set it for.
     
  5. gwfami

    gwfami

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    Sep 13, 2012
    They say a picture is worth a thousand words, with apologies as to the quality of the photo. Notice how the trace no longer goes to the edge of the screen? Nothing I do will expand the trace back to the sides as it was before. This is what happened when the leads were connected and shorted out via the scope.
     

    Attached Files:

  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah, we get that the scope is badly damaged, but it would be useful to try to determine what caused it. The damage seems not to be confined to the input circuitry (that wouldn't change the beam width)

    We really need to know *exactly* what you did. Pictures would be good.

    Did you take the measurements I asked you to take?

    Did you try the test I suggested?

    I need answers to the questions I ask or I'm simply not going to be able to help you.
     
  7. gwfami

    gwfami

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    Sep 13, 2012
    Get your multimeter and measure the resistance across the probes (ground to probe tip) both with the leads disconnected and again with them connected.
    --1 probe > 2000k, other probe around 1000k disconnected.
    --1 probe < 2000k, other probe around 1000k connected.


    Assuming you get a very high resistance (tell us if you don't) then turn the scope on, set the vertical range for 0.5 to 1V per division and then probe a 1.5V battery. The trace in the screen should jump by between roughly 1.5 and 3 divisions depending on the battery voltage and the vertical sensitivity you've set it for
    -- Trace does not jump vertically, but moves horizontally.
    -- intensity and y position knob does nothing.
    -- machines smells faintly of "electricity", not burning nor ozone, just "hot" smell.

    It's actually been a while since I blew up the machine. I was working on a circuit that runs 12V at 7 Amp or so (8 fresh AA in series), and hooked the ground to the positive on the circuit, then used the probe to check the circuit. When I hit a direct line to the negative terminal, there was a pop and things quit working. Sorry, that's all I remember.

    Sorry, no pictures as my phone is AWOL at the present.

    Anything else I can provide?
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That is all useful information. It seems you have both a vertical and horizontal problem.

    The fact that it smells "hot" suggests that there is something bad happening inside.

    *WARNING* the device probably generates 10,000 to 20,000 volts inside. It BITES HARD. It won't kill you (probably), but it will make you sit up and take very special notice.

    I would be looking initially at a power supply fault. It is possible that it just failed coincidentally when you were using it.

    Leave it off for several hours, then carefully open it up and look for things that might be as they shouldn't be in the power supply.

    Don't poke your fingers or anything else around inside until someone tells you.

    If you find something, you will need to photograph it for us.
     
    Arouse1973 likes this.
  9. gwfami

    gwfami

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    Sep 13, 2012
    Thanks for the warning. I have already been "bit" by a computer monitor flyback, kinda suprising to say the least. Should I check the voltage of the power supply? If so, how?
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    No, the first thing to do is to simply look, and maybe smell.

    And all with the power long off...
     
  11. gwfami

    gwfami

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    Sep 13, 2012
    Got the hood off. Nothing looks burned or melted. No indication of blown resistor/capacitor/etc. Both fuses test good. I do get a whiff of the smell, but can't pin it down to an exact location. Power has been off for 24 hours.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, take some pictures of the power supply first. Let's see if we can pick anything odd.
     
  13. gwfami

    gwfami

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    Sep 13, 2012
    Probably have to wait until tomorrow to get the pics. Should I remove the power supply and photo it and the board beneath or just photo it in the case?
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yes, that is a good idea (take it out -- carefully -- and photograph both sides).

    Don't use a flash!
     
  15. gwfami

    gwfami

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    Sep 13, 2012
    Sorry for the delay, here are the pics.
     

    Attached Files:

  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The thing you have carefully photographed from all angles is a transformer. The power supply is the board to which it connects, most probably the one nearest the power connector.

    You'll note that in many of your photos the nintendo box is in perfect focus. That's what the subject needs to look like.

    Nevertheless, there's no obvious signs of damage that I can see.

    Unless you can spot anything, it's going to be a tricky fix unless you have access to a service manual.
     
  17. gwfami

    gwfami

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    Sep 13, 2012
    Dang, don't know what I was thinking. Sorry about the focus, my phone is still awol and had to use another. I'll see if I can get pics of the power board. Any ideas where a service manual can be found?
    Thanks,
    Gwfami
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Hahaha, don't worry. You had enough in the background that I could see a lack of large obvious burn marks at least.

    Much to my surprise (really!) Google knew the answer.

    http://www.testequipmentdepot.com/ezdigital/pdf/os5020-5020c_manu.pdf

    It's pretty rare to find manuals so easily, especially for what appears to be a "no-name" brand.
     
  19. gwfami

    gwfami

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    Sep 13, 2012
    Unfortunately that is the users manual, not a service manual. Google is not my friend in this case.
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The title is "OS 5020/5020C Analog Oscilloscope Service Manual" so you'll understand why I was confused into thinking it was a service manual.

    Why did they put that on the first page then follow it with the user manual?

    On the positive side, you do have the actual manufacturer's name and address, so you could look it up and see if there's a direct email and ask them for a copy.

    Here is a web site. It has a phone number and a name of someone in South Korea. They also appear to have a presence on alibaba, and you can send messages to them via this route.

    A little extra googling reveals that "EZ Digital" were previously called "LG Precision" and that this scope dates from when they had the older name (and LG stands for Lucky Goldstar, which harks back to their days when they were known as just "Goldstar"). So armed with that, we are lead here. (Google really is your friend). A little more poking around reveals a second source (registration required).

    I downloaded it from the latter because it's free. And I've uploaded it here for you.
     
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