My oscilloscope just did something strange

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

  1. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    YES, which is why I asked:

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

    Moderator

    Oct 2, 2009
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    This is what the op said on post #1.
     
  3. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    Something still wrong with it: the other day it just went back to the previous low level while using it, and after a minute it gradually increased the intensity on its own over a span of 3 seconds. You can also see some quick flicks some times, but always returns to the new high levels... until today: it dropped the intensity levels quite a lot; though not a low as when I bought it. So I decided to open it.

    As soon as I opened it, it completely dropped to the lowest levels of intensity; as low as at the very beginning. I took out the circuit where the intensity potentiomenter is connected to, and now it's back to high intensity levels.

    I pocked the components as #12 suggested but I can't seem to replicate the problem. All the soldering looks fine, I see no burns or loose pins, and nothing happens when I move the cables; so at this point I don't know what else to do.
     
  4. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    MrChips, I really need your help. :(

    I closed it, after testing it was working well. It continued to work well for a while, but sudently went back to the very low levels.

    I opened it again, but now there is no way to bring the bright levels back.
     
  5. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I left it rest for a while, and now it's back to the high intensity levels.

    I measured the voltages on the control panel diagram (the one in post #15), and they all seem the same when it fails as when it's fine.

    For example: at P1302 pin 1 I got -8V with the intensity pot at maximum and 0V at minimum; both when it's working well and when it's not.

    I also got the same results either way with pins 2 and 3; nothing at pin 2 (because it's disconnected) and -7.5V to -26.8V on pin 3.

    EDIT: corrected pin values
     
    Last edited: Dec 7, 2013
  6. Dr.killjoy

    Senior Member

    Apr 28, 2013
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    Do you know what the voltages are suppose to be????
     
  7. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I don't know where those -26.8V come from; it doesn't say anything about them on the diagram. But the rest are all according to what's stated; except for P1304 pin 6, which states -13V and I'm reading -15.9V. But again, I got the same reading both when working fine and when not.

    Link to the control panel diagram with the voltages

    Link to where the control panel connects to (P901 on bottom-left corner)
     
    Last edited: Dec 7, 2013
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Have you sprayed some electronic cleaner (100% evaporative type) in the brightness pot and worked the pot for a couple of minutes? It could be dust or corrosion in the pot.

    After that I would get a real strong magnifier and go right over the PCB looking for dry joints and circle fractures on joints, especially on any power components or anything that runs hot.
     
  9. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    Thanks,

    I haven't cleaned the pot yet, but I don't think that's the problem. The pot seems to be fine because it graduates the intensity really smoothly, no jumps, no flicks... just perfect. Also, when the intensity fails, I moved the pot... well, practically in every direction, pocked the pins and solder joints, and there was absolutely no change.

    Checking the PCB with a magnifier is going to be a real problem. I already did that for the small one where the pots are connected to, and I saw nothing wrong with it. but the big one with the main circuits is well hidden under a lot of cables, screws, and things, and I would need to dismount the whole thing before I could reach the bottom side to see the joints.

    In any case, I moved the components on that part of the circuit and also got absolutely nothing.
     
    Last edited: Dec 7, 2013
  10. MrChips

    Moderator

    Oct 2, 2009
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    I'll have a look at the schematics later.
     
  11. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    Thanks,

    So far it's been working well; though I haven't closed it yet. I'll leave it opened until tomorrow then.
     
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    OK, that makes it less likely the pot is mechanically faulty.

    I would still do some testing around there. Try measuring the voltage on the pot pins, as you adjust the brightness. Maybe one pin will have a DC voltage that moves when the pot is turned. If so, measure that voltage again when the low-brightness fault happens. That might give some more information. It might be something as simple as a bad connector where the pot wires plug into the PCB or something.
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    Take a look at the HV and Z-AMP circuit schematic you provided.

    You want to take as many voltage readings as possible at nodes that you can identify while the scope is working.

    Something is flaky. So the next time the scope goes dim you can compare the voltages with what it should be when working properly. Hopefully this will allow you to narrow down which component is acting up.

    There is a CRT BIAS pot shown as RV1011. Make a note of its setting. Try to measure the voltage at the center pin. Turn the pot and put it back to its original position.
     
    adam555 likes this.
  14. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I think you got it!

    It's the CRT BIAS pot. :)
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    Chalk one up for MrChips.:)
     
  16. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    If I remember correctly, this would be the second one for you. :)
     
  17. ian field

    AAC Fanatic!

    Oct 27, 2012
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    You have to be a bit careful with switch cleaner - some plastics parts of electronic components are ABS (like motorcycle helmets with loads of dire warnings not to clean with solvents). When I was in the TV repair trade, I remember squirting some switch cleaner on the mode switch in the bottom of a VCR - and watching in amazement as it turned into tiny granules before my eyes!

    Last CRO I had open, the intensity pots were all at high voltage! - not a lot of current, you're as likely to damage the PSU as yourself if you stick your finger in the wrong place.

    One old CRO I got from the tip and had repaired (bodged!) many times, was getting a bit weak on cathode emission - the diodes in the multiplier were actually long selenium stacks (many selenium pellets stacked in a long thin insulating tube) - the combined Vf of a single diode stick was enough to give you a shock. Yet another atrocious bodge - I replaced each stick with a series pair of TV line output stage flyback diodes to reduce total Vf and so boost the HT.
     
  18. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    Too late.. :(

    I just came back from buying the cleaner and I already used it on all pots. One of them gave me problems afterwards, but then it went back to normal.

    I read the warning after buying it, which says it should be tested before use on some plastics. I was a bit worried about the small pots (like the one I needed to clean) as they are completely covered in plastic; but I don't think they were damaged.
     
    Last edited: Dec 9, 2013
  19. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    Sorry, it doesn't warn about plastics, it warns about using it on rubbers and delicate plastics.
     
  20. Dr.killjoy

    Senior Member

    Apr 28, 2013
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    Trust me you will know if it destroy plastic ..
     
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