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Distorted oscilloscope trace

Discussion in 'Electronic Repair' started by vic, May 8, 2009.

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  1. vic

    vic Guest

    Hi,

    I recently bought an oscilloscope on eBay, and I don't know if the
    seller lied in the description or if it was damaged during the
    transport, but the trace is completely distorted and shows strange
    grid-like patterns. I made a few pictures and a little video :

    http://img222.imageshack.us/img222/1984/focused.jpg
    http://img212.imageshack.us/img212/6566/unfocused.jpg


    The shape of the trace changes when I change the y-position dial. This
    makes me think the problem is in the tube and not in the electronics.

    I'm wondering if this could be caused by a bent grid somewhere in the
    tube ? Could it have been damaged if the oscilloscope received a shock
    for example ?

    I think the tube is pretty much toast, as well as the scope since there
    is no way I could find a replacement tube (Tektronix 2225), but I
    wouldn't mind to be proven wrong :) Thanks.
     
  2. Guest

    Hi
    Check bypass electrolytics. It looks like the axis are cross talking
    through the supply lines.
    Dwight
     
  3. Archon

    Archon Guest

    Tubes busted internally, if it was signal distortion the shape would be
    the same top, bottom or middle of the screen.

    You will most likely here glass tinkling inside the tube if you tip the
    scope, also remove the blue implosion filter and check for tiny marks on
    the phosphor from debris.
    JC
     
  4. hr(bob)

    hr(bob) Guest

    You must get another scope and see if there are signals getting to the
    CRT deflection plates when the input is grounded. Bad electrolytics
    are the most likely problem!!! But, a scope is the only way to tell
    for sure.
     
  5. Archon

    Archon Guest

    Tubes busted internally, if it was signal distortion the shape would be
    the same top, bottom or middle of the screen.

    You will most likely here glass tinkling inside the tube if you tip the
    scope, also remove the blue implosion filter and check for tiny marks on
    the phosphor from debris.
    JC

    In fact unless your filter is dirty, I can see pock marks on the
    phosphor behind the filter, there should not be any marks at all.

    JC
     
  6. vic

    vic Guest

    Yes you're spot on, I disassembled the tube to check if any damage was
    visible, and there were little pieces of glass going around inside. One
    of the glass posts holding the deflectors and electron gun was broken.
    There are indeed several marks on the phosphor, as if it was scratched
    from the inside.

    Out of curiosity, do you know what element in particular causes the
    strange distorsion ? Since the shape changes whith the y-position, I
    figure this would have to be after the vertical deflection plates, but I
    don't see what it could be.

    v.
     
  7. vic

    vic Guest

    In this case shouldn't the distorsion be the same whatever the
    y-position is ?

    I have another scope (which I hoped to slightly upgrade), so I'll do as
    you suggest and check the signal on the vertical deflection plates, to
    be sure.

    v.
     
  8. Archon

    Archon Guest

    No, never really looked into it, buying scopes off Ebay if the seller
    doesn't pack the scope well (assuming it was ok before) then this is the
    result. I bin a dozen tubes a year, basically that's a dozen scrap
    scopes as tubes are unavailable.

    Did the auction listing show a trace when you bought it? If it was a
    straight line or undistorted waveform then damage occurred in shipping
    and you could file SNAD with Paypal.

    JC
     
  9. Jim Yanik

    Jim Yanik Guest

    Oh,yes,seen lots of them.
    I worked for TEK 21.5 yrs,Indy and Orlando service centers.
    don't be so sure;Ebay or Sphere in Canada may have parts.
    IMO,the CRT is toast;judging from the bent trace and glow on the left side
    of the graticule.But you should check the power supplies anyways.
    That's always step ONE.

    FYI,checking tube geometry is best done with a signal displayed.
    Then you can see the vertical distortions.

    If you do remove the CRT,tilt it and see if you hear little bits tinkling
    inside,that's from the electron gun glass rods that got cracked.Sometimes
    you can see them on the faceplate,or they make spots on the graticule.
     
  10. Jim Yanik

    Jim Yanik Guest

    It could be the final mesh screen,and/or any one of the defelection plates.
     
  11. PeterD

    PeterD Guest

    Regardless, it is a firm eBay rule that if the item is damaged in
    shipping, regardless of the circumstances, it is 100% the seller's
    responsibility. The seller cannot disclaim shipping damage, nor can a
    seller *require* shipping insurance as a seperate item conditional on
    the sale.

    Title doesn't pass to the buyer until the item arrives "in the buyer's
    hands in the condition described". So simply file an eBay/PayPal claim
    and be done with it.

    It is the seller's problem, not yours. Don't let him tell you
    otherwise.
     
  12. How would you prove, as a buyer, that you didn't drop it yourself
    after having received it?

    Regards,
    H.
     
  13. Archon

    Archon Guest

    OMFG WTF Here we go again

    JC
     
  14. vic

    vic Guest

    I never buy from this kind of shop since their business model seems
    dodgy to me. Basically they sell copyrighted works of others, and stuff
    that is sometimes freely available elsewhere.

    v.
     
  15. vic

    vic Guest

    The auction did not show the oscilloscope in operation, however the
    description said that it was in working condition. While technically it
    does operate, that's not what I consider working condition. I only paid
    40 euro though, so I think I won't bother with the procedure, especially
    if I have to send it back to the seller, with proper packaging this
    time. I'll probably save some more and buy a recent model one day.

    Cheers.

    v.
     
  16. vic

    vic Guest

    Just to give some feedback, I tested the deflection plate voltages and
    power supply and they look correct to me. So I'm a bit pissed off that
    an otherwise working device was destroyed by careless handling on the
    part of the sender. I guess my current very old, unreliable and
    completely decalibrated oscilloscope will have to do a little longer :)

    v.
     
  17. vic

    vic Guest

    The packaging was a repurposed LCD panel box, obviously too small for
    the oscilloscope, with the device tucked in a corner and various packing
    material to hold it in place.
    The transaction took place in France, I'm not sure federal law applies
    here ;P

    v.
     
  18. This is a very wise decision, imho. I think, that on-line purchase of
    fragile goods and delivery by mail always carries a considerable risk
    of receiving the article damaged - be it, because the sender broke it
    before or while packaging, because he packed it not well enough,
    because the delivery service threw it around, because a heavy other
    parcel was thrown onto it, or because the buyer dropped it himself
    while carrying it to his workbench, or because of something else.

    In the end, the problem imho is, to prove to the judge without doubt,
    what really did happen.

    The solution I prefer is, to buy such goods from sellers located near
    to my place, so I can drive over, eventually have it shown to work,
    and to bring it home myself. This way, I often meet nice people, too.

    Regards,
    H.
     
  19. If you think that everything else of the scope is ok, then why not sit
    at the bank of the river and wait for a replacement crt to swim by?

    Or even walk into the river and stir up one from the bench of some
    replacement guru? In such cases I find we are really blessed with
    the internet :).

    Regards,
    H.
     
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