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Old HC 5502 analog scope ( some display problems )

Discussion in 'Repairing Electronics' started by BGAmodz, Jan 21, 2014.

  1. BGAmodz

    BGAmodz Member

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    Hello everyone .

    I have received an old HUNG CHANG 5502 20 Mhz , its working but it got some issues .

    For example if i want to display a 6.5 volts sinusoidal waveform , the scope should display 6.5v * 1.42 , so if i choose 2v/div , i get 4.6 div , but in my case i get about 7 div witch is equivalent to an ac input of (7div*2v)/1.42 =9.85 v AC .
    The scope's v/div regulator is ranging from 5v/div to 5mv/div , and on the same regulator there is another small regulator for each voltage/div , i tried to adjust it but no change .

    The other issue on this one is that the wave form is displayed but with a lot of flickering .

    link of the video :
     
    Last edited: Jan 21, 2014
  2. JimB

    JimB Super Moderator Most Helpful Member

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    Not quite.
    Should be 9.86v

    JimB
     
  3. BGAmodz

    BGAmodz Member

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    Yes it is indeed 9.85 v witch is way bigger than my original input .
     
  4. dave

    Dave New Member

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  5. dr pepper

    dr pepper Well-Known Member

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    The old favorite with old 'scopes like this is dirty switches.
    A douse in switch cleaner might well sort out the issue.
     
  6. BGAmodz

    BGAmodz Member

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    I will do that and see , but could this this dirt be affecting also the voltage display accuracy ???
     
  7. rumpfy

    rumpfy Active Member

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    The 'flickering' looks to me like there is a problem with the intensity. The waveform you are displaying looks quite stable/well synchronised. When the display 'stops' it goes dark and then when the display starts again, there is an increased brightness. The CRO will have an intensity control for suppression of the display during retrace. Often this circuitry is tied up with the cro intensity grid and can be at a relatively high voltage. I'd be inclined to see if you can identify the cro tube used as a starting point.
    With the attenuator, these things are usually a 1 megohm/50 pf input impedance. You'll just have to trace it through. If your cro has a dual beam arrangement, you can apply the same input signal to both channels and use one input channel to fault find the other.
    hope this is of help.
     
  8. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I need a good pic of the controls. It almost looks like A intensified by B is set. It's a way of selecting the part of the waveform you want displayed using delayed sweep. So, the problem might be between the chair and the unit.

    The other problem is that the triggering is not stable. Again, probably between the chair and the unit.

    Basically, controls not set right!
     
  9. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    The flickering is due to the sweep rate not being in sync with the video's frame rate, not the intensity. You get the same effect when taking a video of a CRT TV screen.
     
  10. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Look closely. It looks like a portion of the waveform is intensified. i.e. setting up delayed sweep.
    And the trigger isn't set right.
     
  11. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    I see what you mean, I just assumed that was due to the sync (or lack thereof). Pretty hard to tell from the video though.
     
  12. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    That's why I'm asking to see the controls. You might also be able to argue that the intensity is too high as well.
     
    • Like Like x 1
  13. shimniok

    shimniok Member

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    Something amiss with triggering there. Either user setting or problem with triggering circuit I'm guessing? Seems like there's a sort of random holdoff/delay going on or something.

    Vertical issue--sure the knobs are installed correctly? :) Calibration knobs in default location? Maybe there is an issue with the vertical preamp or amp.
     
  14. BGAmodz

    BGAmodz Member

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    I hope so

    here is a picture of the controls

    hungchang-1361861724-56516.jpg
     
  15. BGAmodz

    BGAmodz Member

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    Yes the video is recorded with an old nokia phone , in real time it not totally like in the video , its like the signal is displayed very well but keeps turning on and off very fast .

    When i lower time base on dc , the line shows up perfectly , but still bad on ac signal
     
  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    For a line voltage setting on channel #1:

    Input to channel #1.
    AC coupled (AC DC GND) to AC
    Vertical mode Channel #1. (Have no idea why they are all pushed in)
    (Auto Normal Single) should be in Auto mode for now

    Trigger Source should be line at this point. (Ext Line Cha2 Cha1) Channel 2 is a guess for the label.

    Coupling should be in AC-LINE (DC TV AC-LINE); Guess label is AC-LINE for AC or LINE

    See if the trace is stable. This isn't the way to display all signals.

    Note that there is a CAL point on the the front panel. This is a test point for a square wave that is 1 V p-p. Settings for that will be different.

    FYI: The definition of auto trigger is that if there is no level/slope trigger signal in the allotted time, the scope triggers. So, you typically use Auto to get a signal and then switch to NORM and use the level/slope to get a stable signal. You can try that too after changing the trigger source to Cha 1
     
  17. BGAmodz

    BGAmodz Member

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    Sorry i didn't show the coupling and source selections .
    Coupling ( DC/TV/AC-LF/AC)
    Source ( EXT/LINE/CH2/CH1)

    i have adjusted the settings as you described but that flickering behavior is still present , the signal quality is good , also the flickring intensity changes when i vary time /div settings .

    here is another video :
     
    Last edited: Jan 22, 2014
  18. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    That's not too bad in my opinion. With one period displayed the trace was stable. With, say 10 periods, that's like if could be 0.2 sec per sweep if the line is 50 Hz. Phosphors won't persist that long. Remember, this is displaying in real time.

    Not sure if the intensity perceived in the video is real or not. If it is, try to focus and lower the intensity.

    Try the calibrator output. It's frequency is probably about a kilohertz.
     
  19. BGAmodz

    BGAmodz Member

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    Its not a big deal as long as the signals will be displayed correctly , but what am worried about is the voltage amplitude accuracy . like i mentioned earlier , i have an input of 6.5 vac but the oscilloscope displays 9.85 vac , am using 2v/div in this case , i get a 7 div amplitude
     
  20. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Let's look at the cal signal (1V p-p) before we go there.
    6.5 * 1.414 = 0-peak peak value
    *2 = the p-p value
    7 div * 2 is 14 V p-p whereas you should be reading 18.2

    Make sure the v/div knobs are in the CAL position. Usually the click at full counter clockwise.In your scope, it's full right.

    CHANNEL 2 is not in the CAL position.
    CHANNEL l #1 may be.
     
  21. BGAmodz

    BGAmodz Member

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    As far as i understand 1 square's Y axis has 5 divs , so if i got a 6.5 v input and choosing 2v/div , i should get 6.5*1.414 = 9.191 v max ( i guess oscilloscope shows only Umax ) , that means 9.191 /2 div = 4.595 div ; But in my case it displays approximately 7 divs .

    Please correct me if am wrong on something here , am not very familiar with scope measurements .
     

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