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CRT Monitor Odd Behavior

Discussion in 'Electronic Repair' started by Michael Kennedy, Dec 16, 2007.

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  1. I've had this Viewsonic PF790 19" CRT monitor for quite a few years now and
    it has been used daily since it was new without any issues. Recently I have
    noticed that large white images will cause the horizontal size to shrink
    about 1cm overall. I keep my desktop set to black so if I minimize a window
    I see the picture expand or if I open a window I see the picture shrink. It
    is possible that it has always done this, but I don't believe that it has.
    Any thoughs on what might be going out?

    Mike
     
  2. I just set my monitor to 100hz, the setting I used to keep it at. With a
    mostly black picture the desktop streches beyond the edges of the screen and
    is dark and slightly purple. I guess I have some issue in the horizontal
    deflection circuitry. I'm a novice at PC monitors so any suggestions would
    be great.
     
  3. Guest

    Guest Guest

    No one needs a refresh rate that high. Set it to something
    reasonable (e.g. 75 or 85Hz) and see if that helps.
     
  4. Eeyore

    Eeyore Guest

    Large white images require more current.

    Clearly the supply voltage for that part of the circuitry is sagging under the
    additional current demand. A PSU problem.

    Graham
     
  5. If it used to work and is within the specs of the monitor scan rate, then
    that's not the issue.

    Something is failing in the deflection or power supply, and the regulation
    is suffering.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  6. AJ

    AJ Guest

    Effect is called "Blooming" and is directly related to the power supply
    circuits not able to supply the additional current required to display the
    brighter raster, you will also notice a slight de-focus of the information
    when the screen decreases in size.
     
  7. Jim Yanik

    Jim Yanik Guest

    my guess would be electrolytic caps,bad ESR.
    You MUST use low-ESR,105degF caps for replacements.

    Not a task I'd suggest for a neophyte,I suggest buying a new LCD monitor.
     
  8. Jerry G.

    Jerry G. Guest

    With the ageing of the monitor, the power supply regulation and scan
    circuit stability has decreased. The most common cause are electrolytic
    capacitors going high in ESR. If all the capacitors in the monitor were
    to be checked with an ESR meter, there is a possibility of finding many
    of them out of specs. This may not be a feasible repair for a monitor as
    old as the one that you have.

    --

    Jerry G.


    I've had this Viewsonic PF790 19" CRT monitor for quite a few years now
    and
    it has been used daily since it was new without any issues. Recently I
    have
    noticed that large white images will cause the horizontal size to shrink
    about 1cm overall. I keep my desktop set to black so if I minimize a
    window
    I see the picture expand or if I open a window I see the picture shrink.
    It
    is possible that it has always done this, but I don't believe that it
    has.
    Any thoughs on what might be going out?

    Mike
     
  9. Well it's not my first time working on a CRT, but I just don't have tons of
    expirence. I've fixed minor things in TV's before, cold solder joints, etc.
    I have an ESR meter so with a little time I should be able to find the
    suspect caps.
     
  10. Well the interesting thing is that it appears to be doing the opposite. It
    is clear when a large white image is on screen, but blooms when the image is
    dark.

    Mike
     
  11. As old? It is from 2001 I don't really think it is that old. I have
    monitors way older than that.. I have an IBM VGA monitor from 1990 and a
    Tandy CGA from about 1986 that works great still.

    It would be nice to get a smaller monitor on my desk, but money is an issue
    so I'll be re capping the monitor when I get time.

    Interesting thing is that picture color, quality, brightness is all ok.
    Guess it is probably in the deflection section.
     
  12. msg

    msg Guest

    <snip>

    Indeed, it 'floors' me each time I read this sort of time-perception in posts;
    soon we will all have concepts of longevity appropriate for fruit-flies.

    Anyway, one of our local universities recently put out hundreds of late-model
    monitors in hallways for giveaway since they had converted to lcd to save
    power campus-wide. Are you in an area with electronics recycling? Surely
    back alleys also are good sources for discarded monitors. I find good
    working ones nearly every day here in a smaller U.S. city, in all screen
    sizes ;-).

    Regards,

    Michael
     
  13. While I won't claim to have anywhere near the experience that you have,
    what I have seen suggests that in many cases, it may infact be a single
    bad cap with high ESR that is the problem, especially if the monitor or
    TV had no obvious symptoms before.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  14. James Sweet

    James Sweet Guest

    Well it depends on the part of the circuit the parts are in. Nothing in a
    monitor gets over 85C, but 105C caps will usually last longer. There's
    little reason not to buy them, but you can use what you have on hand and it
    will work in most cases.


    Agreed on the first part, but the LCD monitor depends on whether he likes
    them, I much prefer the image on a good CRT and know plenty of others like
    me.
     
  15. James Sweet

    James Sweet Guest

    Look around the horizontal deflection and HV areas, really with an ESR meter
    it's pretty easy to test almost all of the lytics in the monitor, hardest
    part is getting to them, but the Viewsonics are not too bad to work on.
     
  16. James Sweet

    James Sweet Guest

    I replaced a pile of caps in a 14 year old 17" monitor a couple weeks ago,
    the image looks better now than it has in years. It's old tech, but my mom
    is still happy with it so I figured why replace it?
     
  17. jakdedert

    jakdedert Guest

    Absolutely agreed. Craigslist and Freecycle (google either one) list
    free, working monitors every single day. Thrift stores will hardly
    accept them...although they usually have a few for $25 or less. I put
    out a working 17" on the curb the other day, just because I have so many
    that I'm tripping over them. I finally had to toss it....

    That said, I've got a 15" Sony composite sitting next to my desk as a
    'repeater' for the home theater setup (hooked to a 'Video Sender' rf
    receiver). It must be 20 years old, but works great.

    jak
     
  18. clifto

    clifto Guest

    Which reminds me that time flies like an arrow, but fruit flies like a banana.
     
  19. b

    b Guest

    start with the simplest: by changing the caps on the b+ line from the
    power supply to the horizontal/line stage.. typically rated 160 volts
    or more. don't bother measuring them, just replace. if no joy, then
    work back into the primary of the power supply.

    chances are this monitor has a few years of life left in it , so
    don't give up and buy one of those 'orrid LCD things.

    let us know how it goes.
    -B.
     
  20. Well I thought it was thermally sensitive since it worked fine until the
    monitor heated up... Now it appears to work fine for the time being. I'm
    sure that I will have to fix it at some point, but right now it is not
    showing any symptoms. I guess I'll have to wait for it to act up again.

    Mike
     
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