48k issue 2 video issue

edited January 2015 in Hardware
Hi folks. I am currently tring to fix an issue 2 board. I was getting the yellow greenish screen. (also there there is a lower ram fault with it i think) I have been adjusting VR1 and VR2 and got the picture almost perfect but I am still getting some slight yellow banding on the display. I suppose just keep adjusting or could be something else?
Post edited by Firefox11 on

Comments

  • edited January 2015
    Keep adjusting. :)
    Then try changing the capacitors.
    Then change VR1 and VR2.
  • edited January 2015
    ok. thanks. Upon closer inspection it looks like VR 1 and 2 and the variable caps have been changed already. I did the composite mod and changed some of the lower ram and still getting the odd black pixels on-screen. Ill change the caps tomorrow and see how that goes.
  • edited January 2015
    What do you mean by "odd black pixels"? If the display has bad pixels, check your -5V and +12V lines....
  • edited January 2015
    Rule #1 - thou shall check voltages!
    Oh bugger!<br>
  • edited January 2015
    ^^^ What he said :)

    Do change the caps as soon as possible though.

    If you can work out which column within a character block the stray pixels are appearing in, you can work out with reasonable certainty which lower RAM IC is at fault. (See this post where I diagnosed a lower RAM issue the same way.)

    B
    The Spectrum Resuscitation Thread - bringing dead Spectrums back to life
    zx-diagnostics - Fixing ZX Spectrums in the 21st Century (wiki)
    Sinclair FAQ Wiki
  • edited January 2015
    Ok. I just checked the voltages on lower RAM IC 13 and they do look odd.

    Pin 1 0.12V
    Pin 8 9.73V
    Pin 9 4.86V
    Pin 16 0.00V

    Looks like this without the keyboard pluged in.

    http://tinypic.com/r/295qxw6/8

    When keyboard plugged in and start to type anything it freezes up and looks like this.

    http://tinypic.com/r/szimnb/8
  • edited January 2015
    (scratch what I previously wrote)

    Are you sure that the voltages were measured correctly? There shouldn't be colour present on the image if the +12v line is down at 9v....

    B
    The Spectrum Resuscitation Thread - bringing dead Spectrums back to life
    zx-diagnostics - Fixing ZX Spectrums in the 21st Century (wiki)
    Sinclair FAQ Wiki
  • edited January 2015
    The LM1889 can work with 9V, but it should be higher.

    Change the voltage transistors TR4 and TR5, and re-check your pin 1, which should be -5V (ish)
  • edited January 2015
    Spirantho wrote: »
    The LM1889 can work with 9V, but it should be higher.

    Change the voltage transistors TR4 and TR5, and re-check your pin 1, which should be -5V (ish)

    Good News! :smile:

    It was indeed TR4 and TR5. I replaced the transistors from an old non working issue 3 board and it's fully working now. :smile:

    Yellow banding is gone and I just need to adjust to get picture perfect and replace the old caps. Thank you so much all for your help!:smile:
  • edited January 2015
    Firefox11 wrote: »
    Good News! :smile:

    It was indeed TR4 and TR5. I replaced the transistors from an old non working issue 3 board and it's fully working now. :smile:

    Time to resurrect the old non working issue 3 board now :smile:
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