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Malfunctioning LeCroy Oscilloscope

  1. Jul 14, 2010 #1
    I have a http://www.google.no/url?sa=t&sourc...DI_g4&usg=AFQjCNF_-nWNCIHWKCfcGHElPGaj-mJEXw" (100MHz, 200MS/s) where the CRT screen repeatedly turns itself off while the rest of the scope seems to keep going. It's been going on for a while (months), with some days better/worse than others. I think the scope has been lying around for some time (years probably) before I started using it. Up until now I've just turned the scope off and on again each time it has happened, but today has been on the worse side of things, thus this cry for help.

    I realize this might be a long shot with such scarce information, but I'm hoping some of the old-timers on PF have seen this before and can point me in the right direction (which hopefully is not towards the trash bin) before I bite the bullet and go look for a screwdriver.

    (Incidentally, I have another old analog scope where the CRT permanently died on me a while back, so all information on repairing scopes would probably be helpful.)
     
    Last edited by a moderator: Apr 25, 2017
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  3. Jul 14, 2010 #2

    berkeman

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    Staff: Mentor

    Have you checked the screen saver option? I'm not sure that 'scope has it, but it's worth checking anyway. It would be under the Display options button.

    When the display cuts off, though, does pushing any buttons restore it?
     
  4. Jul 14, 2010 #3
    Some scopes have an option that run a self-diagnostic routine. Perhaps this one has it? If so run it.

    If nothing else helps, then taking it apart would be the next step and dusting the inside and all the circuit boards.

    Check if all jumper and ribbon cables are connected to the boards, and check for any cold solders, although that could be a tough one.

    Then check all electrolytic capacitors for any leakage, or bulging - especially in the power supply module. Ideally, an ESR meter would come in handy to check the caps.
     
    Last edited: Jul 14, 2010
  5. Jul 14, 2010 #4
    It does have a screensaver option that kicks in after a while, but that just makes the screen dimmer--not completely dead as the problem seems to be. Pushing buttons etc does not restore it (as it does with the screensaver), unfortunately. I tried to turn OFF the screensaver, but the scope just died on me again just now.
     
  6. Jul 14, 2010 #5
    I can't find any self-diagnostic option anywhere. I'll probably just open it up when I have time, but the task seems a little daunting. With a little luck, maybe I'll find a blown electrolytic cap that can be easily replaced. I read somewhere that replacing all the electrolytic caps with fresh ones on old scopes was a good thing in any case.

    I'll keep your pointers in mind when I get around to it. Thanks.
     
  7. Jul 14, 2010 #6

    berkeman

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    The LeCroy 'scope shown in your link doesn't look all that old. I wouldn't be messing with the electrolytic caps -- that's no likely the cause of the problem.

    When the display shuts off, does the rest of the 'scope keep working? Like, do you still hear the re-calibration relays clicking every once in a while? If you change the time base while the display is off, when you power cycle the 'scope, does it come back up with the last visible time base (horizontal time per division), or does it come up with the new value that you changed it to while the display was off?

    Do you have the manual for that 'scope? It should have the self-test initiation sequence in it. It's something like holding down a couple buttons when you power the 'scope up. I'll see if I can find it for the LeCroy 'scopes that we have in our lab...
     
  8. Jul 14, 2010 #7
    Yes, I can change the time base while the screen is dead (i.e. it starts up with the new value as you suggested). No manual, unfortunately.
     
  9. Jul 14, 2010 #8
    Replacing all caps is probably reserved for restoration of vintage 50 year old equipment or server power supplies and mothersboards which ran under heavy load 24/7 for years.

    I used to be involved troubleshooting both commercial and consumer electronic products. The number one problem were failed capacitors at least 90% of the time. And in most cases it was just one or two caps that went bad, not all. To find that out, we used an ESR meter to check the condition of all capacitors in-circuit. And in most cases, the bad capacitors were located on switching power supply board.

    It's actually quite counter-intuitive, I had a case of replacing one 200 uF cap which had high ESR reading, and it made a difference between an LCD screen turning on or off in a commercial touch controller product.
     
  10. Jul 15, 2010 #9
    Thanks for sharing that. I'm not sure I have access to an ESR meter, but--after reading your comment--troubleshooting without it seems to be a futile exercise. Maybe I'll go ahead and build one according to the schematic http://ludens.cl/Electron/esr/esr.html" [Broken] (the link also contains relevant background info for any interested by-passers).
     
    Last edited by a moderator: May 4, 2017
  11. Jul 15, 2010 #10
    ESR meter has to be accurate within 1/10 and 1/100 ohms in order to make a good judgment call. Many schematics of ESR meters on the internet aren't that accurate. They could distinguish a toasted cap from a good one, but not one that looks suspect from a good one.

    The differences in ESR we are looking for are very subtle. To give you an example of what I'm talking about, an ESR of a 1000 uF cap should at most 0.1 ohms for good filtering. If you are reading 0.2 ohms then this cap is suspect, if it's 1.0 ohm, then it's really bad and should be replaced.

    That's why a professional ESR is meter is required for thorough troubleshooting. But they cost $300 -$500 bucks...

    A good alternative is a kit from Anatek, the blue esr meter, at a cost of about $80 bucks. It's very reliable and accurate.

    http://www.anatekcorp.com/blueesr.htm

    The worst case scenario would be if you were to go through all the trouble and build or purchase an ESR meter, but only to find out that all of the caps were good, and the problem with your scope was something else entirely.

    So my post focuses on caps as a likely culprit. One thing to keep in mind is that troubleshooting is really eliminating all the variables, and caps are just one variable.
     
    Last edited by a moderator: May 4, 2017
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