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Spectrum 48k boots but small black dots start appearing

MaccMacc
edited January 2011 in Hardware
Anyone any ideas where I should look first?
Post edited by Macc on

Comments

  • Alien 8Alien 8
    edited January 2011
    Macc wrote: »
    Anyone any ideas where I should look first?

    Are they in approx 4 blocks, top and bottom? When it happened to mine, it was a new motherboard, not sure if it can be fixed or not.
  • MaccMacc
    edited January 2011
    little dots mid screen appearing from the left to right, its like an invisible ant is walking across leaving footprints!
  • spiderspider
    edited January 2011
    I'd start by checking the regulator heatsink is OK and not coming loose.

    It does to my (non technical) mind sound like signs of starting to overheat. At least this is free and easy to check. :)


    After that I would start to say ULA or the start of ram failure.
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  • MaccMacc
    edited January 2011
    Certain its not ULA, tried a known good one but good advice about the regulator, ill have a check on that
  • Alien 8Alien 8
    edited January 2011
    Macc wrote: »
    little dots mid screen appearing from the left to right, its like an invisible ant is walking across leaving footprints!

    Not seen that before, different from mine.
  • rokoroko
    edited January 2011
    I have seen such on one of my Spectrums where the memory was not 'refreshed' due to hardware experiments. The little dots started to appear after a few seconds.
    In general: If there are chips on sockets then lift them up a little and re-insert firmly. Otherwise the first step would be to try another Z80 and ULA, or test yours in a known good board.
  • MaccMacc
    edited January 2011
    Cheers Roko, thats exactly whats happening, I know its not the ULA, ive tried a good one in there and same results, the z80 and all memory is soldered in, is there a way to test these chips with a multimeter to save me desoldering everything?
  • rokoroko
    edited January 2011
    In my case it was not a faulty part, just bad practice that could be changed.
    If your dots appear at regular intervals of 8 pixels then one of the lower memory chips could be suspect. Theoretically...
    Can chips be tested 'in situ' with a multimeter... Not really, I tend to say.
  • MaccMacc
    edited January 2011
    So far ive managed to reduce the dots to two top middle and top right, this was achieved by putting some thermal grease between the 5v regulator and the heatsink and tightening thoroughly, but theres still two dots, better than 20 odd of em but they are still there, just about to reload fruitcakes diagnostics onto the cart and retry this one
  • MaccMacc
    edited January 2011
    nah, still the same, wont go to boot screen, black screen with some red writing that appears at the top followed by flashing green blocks and back again.
  • MaccMacc
    edited January 2011
    also, typing any kind of instruction results in the screen going completely garbled, ie load "" then enter usually resets the machine
  • rwaprwap
    edited January 2011
    You could check without the case screwed together - sometimes, if the felt pad on top of the modulator is worn (or missing), then this can cause the resets when you enter an instruction!
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  • MaccMacc
    edited January 2011
    Felt pads looking good, hardly a blemish on it
  • guesserguesser
    edited January 2011
    check for shorts around all the ram chips and on the edge connector.

    I had a 48k that gave all sorts of weird freezes and garbage booting up which turned out to be a tiny flake of solder shorting between two pads on the edge connector.
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