help with my scope

Discussion in 'General Electronics Chat' started by lokeycmos, May 17, 2012.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
    7
    i have a tecktronix scope model 564. last night when i turned it on i hear a soft pop sound and a flash on the side of the scope. but it still operated. the same thing happened this morning, it still worked. the wierd thing is that the power indicator was still on even though the switch was in the off position. the power switch feels solid with no play in it. now the scope wont power on at all. i took the side off, and found a fizzled capacitor. could this single cap be the cause of my problems? also, if you look at the pic of the side i attached, i noticed a few neon bulbs as part of the circuitry. what are these for? what do they do? thank you!
     
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  2. #12

    Expert

    Nov 30, 2010
    16,346
    6,833
    Oh yeah. Completely obvious that you have a dead electrolytic capacitor.
    Neons are sometimes used as over-voltage protection but I'm just guessing at that part.

    While you're in there, look for a fuse that the capacitor took out.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,368
    Neons are used as voltage requlators.
    First step would be to replace the bad capacitor.
    Note that there is a small spool of silver solder on that high voltage cover.
    Use that to solder the capacitor to the ceramic terminal strips.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Brings back memories of my early days in electronics. :) I think Tek was one of the few that mounted all the passive components on ceramic terminal strips for minimum high frequency dielectric losses and maximum insulation resistance for the mounting terminals. Also made it relatively resistant to humidity and moisture. Obviously was a rather expensive fabrication technique but they considered it was worth it to get the maximum circuit performance from their scopes.

    P.S. If you've ever tried to use one of those old analog storage scopes you'll realize how much of an improvement the modern digital scopes are in the ease of capturing, storing and analyzing high speed, non-repetitive waveforms.
     
  5. w2aew

    Member

    Jan 3, 2012
    219
    64
    Any of the electrolytic caps for an instrument of this vintage could be suspect, and ought to be checked/replaced. Depending on how the cap failed (and it sounded like it failed "shorted") due to the the fact that you had a "flash", it is likely that it may have wounded or killed other devices - most likely in the power supplies (diodes, resistors, etc.).

    And yes, use the silver-bearing solder that was included inside the scope (if it's still there). If not, you might find silver-bearing solder at Radio Shack - believe it or not, I found some there last year. If you don't use the silver bearing solder on the ceramic strips, you'll likely damage them by leaching off the metalization.
     
  6. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    One problem with the ceramic strips is that solder "whiskers" tend to grow on them, and can bridge between components, causing problems. Silver solder is less likely to do that but I'd inspect the terminal strips closely, anyway.

    W2aew is quite correct that all of the electrolytics are suspect, and all should probably just be replaced.
     
  7. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
    7
    the bad cap is in the power supply section. i circled the bad cap. is there any other possible parts it may have damaged down the line? there is definately something wrong with the switch. i get 120v up to it, but nothing out. at the green X's im measureing 1.3 ohms. doesnt that seem a little low for the primary? thnx
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    When they taught me, the mantra was:

    Most problems will be in the power supply, most will be bad caps....


    And whatever damage the caps did when they died.
     
  9. BSomer

    Member

    Dec 28, 2011
    433
    106
    The company I work for tells me to replace all electrolytic capacitors on any repair I do when there is at least one such cap that is obviously bad. Otherwise replace all the electrolytic caps if I suspect they may be old. Or something along those lines. The point being is that electrolytics dry up and age more quickly than the other types. Unless you have some high voltage or high capacitance capacitors they are generally pretty cheap to replace.
     
  10. #12

    Expert

    Nov 30, 2010
    16,346
    6,833
    Antique Electronic Supply has some HV caps. www.tubesandmore.com
    I wouldn't worry about the 1.3 ohms of DC resistance in a transformer winding.
    One of those X's is on a shield terminal. It would be connected to the case ground. Be mindful of your grounds and commons when measuring this beast.
     
  11. w2aew

    Member

    Jan 3, 2012
    219
    64
    If a cap in a power supply shorts - it usually causes damage "up" stream, not down-stream. Meaning, check for blown diodes, resistors, etc. that sit between the bad cap and the transformer.

    If a cap fails open (or high ESR), that can *sometimes* cause problems down-stream for circuits that can't deal with pulsating DC when it's expecting a steady rail.

    Note that a failed cap might test *open* after it failed *shorted*.
     
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