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Thread: This distinctive symptom in an Atari 400

  1. #1

    Default This distinctive symptom in an Atari 400

    Hi Guys,

    Just wondering if any one has a comment of this Atari 400 issue I posted on the Atari forum. The symptom is fairly distinctive. The bottom 1/3 of the screen is black and the top two thirds has a bank of random characters that change very rapidly.




    I’ve checked power and have also swapped out most of the logic chips. I also swapped the RAM ICs with good 4116s.


    The ROM is always a possibility I guess but my suspicion falls on the ANTIC or CTGIA ICs, Would this be a fair guess, and would anyone know of a definitive test? Could a faulty CPU cause this? I wouldn't have thought so, but if you think it might I could disassemble my Atari 800XL and swap it over.


    This is the manual I’ve been using. It’s pretty low-res unfortunately.

    The fault developed suddenly after the unit was on for about 3 hours.

    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
    Join Date
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    Quote Originally Posted by tezza View Post
    The bottom 1/3 of the screen is black and the top two thirds has a bank of random characters that change very rapidly.
    Tez,
    The rapidly changing characters suggest that something is wrong in the area that generates the /write enable signal to the video RAM. Perhaps it is shorted (stuck on)?

    Actually from your documentation, I can't even find the video RAM. Is it embedded in one of the special chips? The schematic is difficult to read.
    -Dave

  3. #3

    Default

    Hi Dave,

    Thanks for taking the time to check out the docs and reply. I'm glad I'm not the only one who struggles to make sense of the low-res schematics. I can't seem to find anything better on the web though.

    Yes, video RAM was my first thought. Atari seems to be different from the old PETs and TRS-80s we are used to. The video RAM seems to be managed by one of the special Atari chips namely the ANTIC chip (as far as I can gather).

    The following is a description of the ANTIC IC from an Atari FAQ website...
    ANTIC ("Alpha-Numeric Television Interface Circuit") is a microprocessor
    dedicated to the television display. It is a true microprocessor; it has an
    instruction set, a program (called the display list), and data. The display
    list and the display data are written into RAM by the 6502. ANTIC retrieves
    this information from RAM using direct memory access (DMA). It processes the
    higher level instructions in the display list and translates these
    instructions into a real-time stream of simple instructions to GTIA.
    There is a website which sells these Atari chips (http://www.myatari.com/) . However before I go and order, I wanted a second opinion as to whether it's likely to be this one. The replacement ICs are not that expensive considering the rarity of the Atari 400 in NZ, but of course the shipping is.

    Power supply is ok. I've swapped out and checked all the logic chips associated with the video circuity and I'm pretty sure it's not them. The RAM is ok. That leaves the CPU (unlikely, right?), the CTIA chip (Unilikely as the screen image and colours seem clear enough), the ROM chips (possible, but I would assume a frozen screen rather than a rapidly changing one, right?) or the ANTIC chip (my chief suspect).

    I have a logic probe and a digital meter. The ANTIC is certainly getting power and the address and data lines are all working, but who knows if they are working properly. Am I right in thinking the data and address lines should all be pulsing? If one is not pulsing and is HIGH, would that be unusual, yes? I seem to remember 5v on some of these lines so I'll check with the logic probe.

    From what you know of electronics (far more than me) do the deductions above sound sensible? Could it actually be ROM?

    I can open up one of my Atari 800XLs and try it's compatible CPU in the 400. Do you think that's worth doing though?

    Any advice from anyone most welcome.

    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
    Join Date
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    Location
    Southern California, USA
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    Default

    Quote Originally Posted by tezza View Post
    The following is a description of the ANTIC IC from an Atari FAQ website...
    Tez,
    Based on this description and the fact that you swapped RAM, your analysis seems correct. It looks like the DMA controller or something else in the ANTIC is not working properly. Of course the blank portion of the screen is also trying to give us a clue especially if the raster scan is done in the ANTIC.

    I would replace the ANTIC chip.

  5. #5

    Default

    The general consensus on the Atari Age forums also points the craggy finger of suspicion at the ANTIC chip.

    I've ordered one. I'll let you know the result when it arrives.

    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)


  6. #6

    Default

    Hi just closing this up.

    The replacement ANTIC chip arrived, I plugged it in, reassembled and the Atari 400 is now OK. Problem solved.

    Actually the ATARI 400 is not FULLY ok. The SELECT and OPTION keys dont work. The RESET does and so does START. And so does every other key in on the keyboard! I guess those two would have been hammered in daily use. I suspect they have always been faulty. I've never really given this machine a good going over until this screen problem.

    I've had a look at the ribbon cable (which is a bastard to get out and put back in..and probably brittle with age) but can't see any cracks. Oh well, something to look at more closely at another time.

    Thanks for your help with this Dave.

    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)


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