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Thread: Apple IIe colour sceen puzzel

  1. #1

    Default Apple IIe colour sceen puzzel

    While I had my Apples out today I noticed something odd with one of them. Here is a mystery for you Apple hardware hackers out there.

    One of my Apple IIes ( say machine A), when plugged up to my Apple Colour monitor shows horizontal instability. The text bobs about horizontally (but doesn't break up) and I can't stablise it no matter what control knobs I twiddle with. You could get seasick looking at it.

    However...my OTHER Apple IIe (Machine B - same motherboard type) shows quite steady text on the same monitor. There is a slight, slow, horizontal wave than starts at the top and moves down but it's not really noticable unless you really look for it. I can live with it.

    Now..here is the mystery. If I connect Machine A to a green screen monitor, the image is rock-steady?!

    So...is it the machine that's faulty, the monitor or both? Given that machine B works fine on the monitor it would suggest the monitor is ok and this issue is with Machine A? If so, why does machine A look so good using a green screen monitor. There is no horizontal movement at all. Unfortunately, I don't have another Apple colour monitor I can use to compare.

    The diagnostic routines of both machines check out, and it is not the PSU. I swapped them over and the symptom didn't change.

    Anyone come across this before, or have any ideas?

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  2. #2

    Default

    You say it's a IIe? I would check for bad electrolytics in and around the color-gen circuitry, or the color-killer circuitry. The IIe has that special upgraded color gen stuff, from what I can recall. So a monochrome monitor wouldn't do that, but a bad color burst signal on a color monitor certainly would.

  3. #3

    Default

    Quote Originally Posted by arfink View Post
    You say it's a IIe? I would check for bad electrolytics in and around the color-gen circuitry, or the color-killer circuitry. The IIe has that special upgraded color gen stuff, from what I can recall. So a monochrome monitor wouldn't do that, but a bad color burst signal on a color monitor certainly would.
    Sounds like a reasonable explanation. Thanks.

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  4. #4

    Default Confusing capacitors

    Ok, I've decided to replace the caps in the video circuitry to stop the horizontal sway as discussed in this thread. I've identified the caps which may be the culprits but I just want to check something. Here are the caps concerned...


    Now the numbers on the caps refer to 47pF and 100pF respectively. At least this is what the SAMS Facts manual I have for the IIe says. Here is the related circuit:



    Picofarads though? With such large thick capacitors? It seems unsual to me?

    I'm not sure what type of caps these are? They seem very thick and meaty for the specs. Would a ceramic replacement be ok? 100 picofarads = 0.0001 uf and this would often be written on a ceramic cap as 101, correct? And the 47pf would just be 47?

    I did swap out the 100pf one with 10000pf (0.01um) before I figured out the units correctly (opps!). This caused the horizontal video to be highly unstable as you would expect. Oh well, that was fun..lol

    Anyway, I'd be grateful if someone could assure me that standard old ceramic caps would do the job as a replacement for these ones in the photo? I'm assuming they will?

    Tez
    Last edited by tezza; February 8th, 2010 at 11:18 PM.
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  5. #5

    Default

    Hi Tez,

    My wife just took a look and the caps in the posted photo appear to be high-frequency mica caps from Cornell Dubilier (see link below). She advises that if this is the case, ceramics may not switch fast enough to do the job. I guess these are driving horizontal scan?

    http://www.cde.com/catalogs/CD17-CDV18.pdf

    If you look at the specs for general purpose ceramics, you can see what the problem is. The dissipation factor is a couple orders of magnitude higher:

    http://www.cde.com/catalogs/G.pdf

    More food for thought: The high precision mica's go for about $3 CAD whereas the ceramics go for about 30 cents apiece. This is a clue that the high precision caps were necessary as mfgrs will not pay more than needed.

  6. #6

    Default

    Ok, thanks. So I'll have to look for high frequency mica caps. Do you know if this type go under a different name? I can't seem to find that type in the listing of our local electronics store? There are lots of cap types listed, but I can't find those ones...or anything that even looks like them?

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  7. #7

    Default

    Micas are specialized, so it is unlikely you will find any at a general electronics store. Online stores and wholesalers will deal in these types of components though.

    Here are some results from the Farnell NZ catalog:

    http://nz.farnell.com/jsp/search/bro...lues%3D1000100

  8. #8

    Default

    Excellent! Thanks for the help!

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  9. #9

    Default

    Hmm..well I replaced all the capacitors which seem to play a role in the video circuity and the issue is still there.

    The image is rock solid on a mono monitor and also on a PAL colour monitor (which doesn't show colour with the Apple anyway). It's just unsteady on the Apple Colour monitor I have. The issue is not with the monitor though. The monitor is very steady with another Apple IIe I own.

    It's not the PSU. I've swapped several over and got the same issue.

    I find when I swap the monitor between the Apple IIes the vertical hold always needs re-adjusting. The two machines don't seem to be putting out the same signal.

    Any ideas?

    Tez






    Back to the drawing board...
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  10. #10

    Default

    If you have a television or VCR with a composite video input, I'd connect it to the Apple and see what kind of image you get.

    Or, going the other way, I'd connect the output of a VCR or DVD player to the Apple monitor, to see if it can display a steady image.

    (Of course, this is assuming Apple IIs in New Zealand put out PAL video... otherwise you may have some issues with PAL vs. NTSC inter-operability when trying this.)

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