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Oscilloscope Problem

Discussion in 'Troubleshooting and Repair' started by rhaugen, Mar 9, 2014.

  1. rhaugen

    rhaugen

    21
    0
    Feb 21, 2014
    I have an old Leader LBO-514A Analog scope 15MHz that I bought a while back. I have the user manual for it and just went through the user calibration procedure and did some very minor adjustments and everything checks out.

    Recently I accidentally shorted a probe with Mains 120V 60Hz and it melted the ground lead on the probe. It may be a coincidence but since that time AC signals do not seem to read correctly on the old scope. Ie I have hooked it up to a stepdown transformer 120V to 18V. When I hook it up to the scope with a V/Div of 5 and probe set at 1X it reads full scale. Digital multimeter reads 17.82 VAC - scope at 10v/Div shows about 50p-p

    I have tried the same reading with a new probe and on each channel.

    I tried setting the probe to 10X which I believe should make the scope see 1.8V. It to is way out of wack. I am not sure if I understand the 1X - 10X properly.

    I have the schematics for the scope. I did open the scope today to look for any obvious components that had failed but could not find any.

    Anyone have any idea what could be going on here ? Did I "brick" the AC reading capability of my scope ?

    Any help would be appreciated.

    Regards all

    Ray
     
  2. rhaugen

    rhaugen

    21
    0
    Feb 21, 2014
    Ok... I have done some more investigating. I attached a 10VAC source. Measurement confirmed with two independent DMMs. With this known value running into the scope, both channels read the value as more than twice what it should be. I am able to adjust the value back to a correct reading by adjusting the variable dial for each channel. However they have to be turned off of the cal'd setting which is not ideal.

    I am hoping that there are some O'scope experts/ repair techs around that can help out.

    Ray
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,192
    2,694
    Jan 21, 2010
    10VAC (sine wave, RMS) will be 28V peak to peak.

    What are you seeing?
     
  4. rhaugen

    rhaugen

    21
    0
    Feb 21, 2014
    Well I am beginning to think that there is nothing wrong with the scope...but something wrong with me :) So Steve... I set the scope as follows:
    10V/Div - Probe set to 1X -Input signal 10VAC (RMS)
    Reading showed........ 28 V Peak to Peak.

    Apparently I have managed to get myself all mixed up on what reading it should show. (Of course my old scope does not display any numeric readings on the CRT just the wave form) My newbie question then would be ... if the input signal is 10V am I correct to say that the voltages should vary from +10V to -10V ? If that assumption is correct then with the scope set on 10V/Div should it not cover only 2 division ?
     
  5. rhaugen

    rhaugen

    21
    0
    Feb 21, 2014
    Steve.... I am such an idiot !!!!. University physics was a LONG time ago. I just re-read the relationship between peak-peak and RMS. Of course my reply above is WRONG. Of course 10VAC(RMS) equates to 28V peak to peak.

    So it wasn't the scope.... "It was the nut behind the wheel"

    I have only been on this site for a short period of time... but I want to thank everyone for all the help, advice and guidance that is offered.

    Ray
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,192
    2,694
    Jan 21, 2010
    Don't worry about it.

    We've all made those mistakes and more...
     
  7. elebish

    elebish

    177
    12
    Aug 16, 2013
    Remember, a X10 probe is actually a divide by 10 probe and offers isolation between the voltage measured and the scope vertical input of about 10 meg ohms. Ed.
     
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