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48K specs boots, but THEN blinking blocks!

georgedbgeorgedb
edited May 2008 in Hardware
Of all ZX problems I have seen, the 'blinking blocks' is the most common... Today I picked up a 48K, it boots and THEN it locks filling the screen with blinking blocks. That is weird... When the lower RAM i broken, it will never get through booting, right? What can it be? When I touch a key, "R" for example it sometimes DOES display run and then blocks, or when I tap another key it can come up with "Start tape then press enter...". What can this be?
Post edited by georgedb on

Comments

  • WinstonWinston
    edited May 2008
    It still could be bad memory - if fails intemittently or fails in such a way that it tests OK, but then the machine stack gets corrupted, you can make it to the (C) message and then have it die.

    Of course, there are other things that could be a problem - weak power supply, bad capacitors (causing excess ripple on the power rails). If it were mine I'd check the power rails with a scope first, and if ripple is seen replace the electrolytic caps. Then start looking at other things after that.
  • georgedbgeorgedb
    edited May 2008
    OK, I will check that. It would be a good explanation, as the guy I got it from says he stored it working, 17 years ago... That the membrane would be gone, is not so weird.

    Which capacitors to check first? Can I see they have been leaking?
  • WinstonWinston
    edited May 2008
    Anything electrolytic. They tend to have limited lives, and it won't hurt to replace them all anyway.
  • CookieCookie
    edited May 2008
    I would also say lower 16K ram, there are many ways they can be broken.
    It can also be ULA, have seen this but have such little experience with
    broken ULA's so I just mention it for you.
    On those 8 ram chips, feel if any of them gets hot alot faster than rest of
    them, usually one or 2 broken ram chips, 2 minutes running time max
    would reveal it.

    Reading your error again: caps, small ones around memorychips, if you have those
    that look like diodes they usually get shorted.
    But never seen that error you have...

    s.
  • ZX-MADZX-MAD
    edited May 2008
    When you power it up does it make any sound? Listen for the buzzing/hissing noise. If its silent you can bet your life on it that TR4 is hosed (ZTX 650) its the most common part to die ive found. Next culprit is TR5 (ZTX313) which kills TR4 as it dies. Try these 2 first.
  • csmithcsmith
    edited May 2008
    Definately check the power circuit outputs (+5v, -5v and +12v).

    I had a problem with my replacement ULA project than showed itself as soon as you did a beep or something.. the very fast switching of signals to do that caused "ringing" which, well, messed things up.

    If your power is not stable, then you might encounter the same symtom - probably with the 4116.

    Just a hunch of course..

    Chris
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