My oscilloscope just did something strange

Discussion in 'General Electronics Chat' started by adam555, Nov 20, 2013.

  1. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I bought a second hand oscilloscope a couple of months ago -an old analog Hitachi- and it's been working well so far; with the exception of the trace intensity being a bit low and a couple of buttons/pots not making good contact some times. Well, the case is that I was just using it, and suddenly the trace got quite bright; the intensity got way higher than ever before on its own.

    I don't know what to make of it; if either the intensity got fixed on it's own after 2 months, or if something was just damaged. I mean, if the oscilloscope arrived with the trace at this intensity I would have thought of it as normal; as I can bring it down to normal with its control. But since it has been low for 2 months, and suddenly went up on its own, I though that maybe some component has been damaged (or came already damaged).

    I would appreciate your thoughts and any advice on taking measures to prevent any damage or fixing the problem.

    Thanks in advance.
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    Sounds like the intensity adjustment was/is dirty and you just got a good connection between the element and wiper. Hope it stays that way, or try cleaning the pot.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Try using switch cleaner on the controls to get rid of dirt and debris.
     
  4. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    Thanks, I definitely need one of those cleaning sprays for this oscilloscope, as it has various buttons and pots that make bad contacts; especially the pots that also have pull-push switches.

    But it's odd that I didn't notice any bad contact with the intensity; not even now. I checked it a lot of times to see if I could get the intensity higher, but it seemed to be working well; it always went smoothly from beginning to end -as it also does now-, unlike the other pots with a bad contact. That's why I was wondering if it could be another component related to the intensity?
     
    Last edited: Nov 20, 2013
  5. #12

    Expert

    Nov 30, 2010
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    Of course it could be another component, or a bad solder joint. Anything intermittent can cause this kind of problem. Go poking around the intensity circuit (with a plastic stick) and see if anything is loose.
     
  6. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    My scope had a intensity problem and the problem was a resistor going bad... But now I have back feed which I have to work on now..
     
  7. #12

    Expert

    Nov 30, 2010
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    My scope had a vertical deflection problem. A 1.2 volt zener had fractured, and I didn't know you could even buy a 1.2 volt zener!
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Could be a flaky pot that controls the intensity: does the intensity control still adjust the brightness?

    If not, it means the high voltage regulator circuit has failed and it is running "wide open" at max brightness.
     
  9. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    If it's not the pot, could this damage the oscilloscope further? I mean, I'm a bit worried that the low intensity was the normal state (e.g. due to an old cathode ray tube) and that something is now feeding that circuit more current than it should; which I guess could end up burning something else in the long run.

    I realize it would be difficult to answer without having seen the oscilloscope, but if you had to guess: is this a possibility that I should worry about; or do you think there was a bad contact and now is in its normal state? I'm asking because as it is, it seems to be working better than ever; so I wouldn't want to mess with it without a good reason.
     
    Last edited: Nov 20, 2013
  10. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    What makes me doubt about the pot being the cause is that it always worked well; always regulated the intensity smoothly. The only difference is that now it reaches a much higher level of intensity.

    I don't know much about this, but I would guess that if it was the pot it would be failing along these 2 months; and it never did. As I said, always went pretty smooth.
     
    Last edited: Nov 20, 2013
  11. Brownout

    Well-Known Member

    Jan 10, 2012
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    Well now, that's something you haven't told us before.
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    What is the model number of the Hitachi scope?
    You might be able to find the schematic.
    From what you describe I would guess that the pot is ok.
    Something was flaky inside that just got fixed.
     
  13. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    running too much brightness will burn the phosphors on the screen.

    Are you saying you can no longer adjust it down to a normal level of intensity?
     
  14. BReeves

    Member

    Nov 24, 2012
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    I have always been impressed with the way you guys deal with some of the questions that crop up but please allow me a little rant.

    The OP explained (in his first post) very well that the pot is OK and that he can now adjust the intensity. Are you guys not reading what he is saying.

    Sorry, will now crawl back under my rock.
     
  15. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    In these 2 months I haven't felt anything wrong with the intensity pot; it runs smoothly from beginning to end, you don't see any jumps or sudden changed on the intensity, and it always regulated the same levels. Until yesterday, the intensity levels were rather low -sometimes at high frequencies it was even difficult to see the trace in daylight- but suddenly, and without even touching it, the intensity just went up... by a lot. But you can still regulate from 0 to bright. Actually, I would say it's regulating as it should for the first time.

    It's an Hitachi V-522. I got a PDF copy of the manual with the schematics. I'll attach the part where the pot is, but for now I couldn't find the rest where that connects to...

    Brownout is right; I didn't say it in the OP... and I guess no one reads the whole thread (I usually don't; just the OP and maybe the last posts).
     
  16. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I think this is where the previous schematic connects to...
     
  17. MrChips

    Moderator

    Oct 2, 2009
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    So with the intensity set to a normal viewing condition, measure the voltage at P1302 pin-1 wrt GND.

    Next time the scope goes dim, without adjusting the intensity control, measure the voltage at P1302 pin-1 again and compare.

    Edit: Also measure the voltages at all three terminals of the pot wrt GND.
     
  18. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I'll see if I have time to open it later on, but since yesterday night it never when back to the previous levels. I just tried by moving the pot and it's fixed on the new intensity levels.

    I'll also have a look later on to see if any of the components in that circuit are loose.
     
  19. MrChips

    Moderator

    Oct 2, 2009
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    My bet is there is nothing wrong with the pot and its immediate circuitry.

    The circuit shown on the second page has about 30 components each of which can affect the intensity. My first effort would be to look for a suspicious solder joint or loose connection.

    If it is working now, leave it alone. If the intensity goes dim again get out the can of cold spray.
     
  20. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I also thought it would be best to leave it as it -since now it seems to be working as it should-; but I was worried that if this wasn't the normal state, and it's just due to some some component that got damaged yesterday, it could make it worse in the long run.

    I think the guy who sold it to me said it wasn't used for a very long time; and I noticed that over these 2 months the pots and buttons that were giving problems at the beginning got better with use. I guess this could also be the case with the internal components related to the intensity.
     
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