Heat failure

Discussion in 'General Electronics Chat' started by TexAvery, Oct 30, 2009.

  1. TexAvery

    Thread Starter Member

    Oct 7, 2009
    58
    0
    Hi ALL!
    I a building a scope clock and shorted the output to the CRT.
    Apparantly the HV NPN transistors shorted. Replaced all 4 , now here is the problem I am seeking help on. On powering up the clock the image is normal, but after 3 min or so the image distorts on the right side.
    My guess is a component (resistor?) heats up and partially fails.
    I could
    "A" go to college to learn electronics properly.
    "B" replace components until fixed
    "C" find the culprit by cooling each component with cold air from compress air can
    or "D" by a new scope clock (not much adventure in that)
    [​IMG]
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Probably a capacitor used to form the horizontal sweep ramp has lost capacity. Just a guess as to which one it might be, but caps are cheap.

    Choices:
    1. Mass replace capacitors.
    2. Have new glasses made to correct for CRT display distortion.
    3. Wait for better idea.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Your vertical (Y-axis) deflection is fine; it's the horizontal (X-axis) that's giving you problems.

    A can of freeze spray will be a big help in tracking down the offending component.

    Find the X-axis drive section, and start freezing parts.
    [eta]
    As Beenthere said, you could have a damaged cap - but it's most likely attached to just the X-deflection circuit.

    Having a schematic available would help a great deal.
     
    Last edited: Oct 30, 2009
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    You can also buy cold spray, and selectively spray parts to see which ones react. My bet is a chip or transistor, though capacitor is a definate possibility.
     
  5. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    1,758
    98
    I wonder how long it takes to burn in the CRT?
     
  6. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    I'm sure it might have something to do with CRT persistance; which is the length of time the image remains on the screen after the beam is turned off.

    A good way to burn in a CRT is to boost the anode voltage; from let's say 25,000V to 35,000V. :D Actually that would just make the image size decrease, but who knows, it might help. And then again, it might not.

    Austin
     
  7. TexAvery

    Thread Starter Member

    Oct 7, 2009
    58
    0
    Before I went off to buy some spray and caps, I swapped the X & Y to confirm X is at fault. Now the display is normal, I am still going to find the original fault for future ref.
    Thanks ALL!
     
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