Image Map Image Map
Page 2 of 3 FirstFirst 123 LastLast
Results 11 to 20 of 21

Thread: Apple ][+ white screen issues

  1. Default

    I have an ESR meter, a newer and a few older multimeters, a couple of old 2 CH oscilloscopes (Hitachi V209, BBC D-8600) and a Hantek 6022BL PC Oscilloscope & logic analyzer (none of which I have yet learned to use, but this seems like a good project to start with if they are up to the task).

  2. #12
    Join Date
    Sep 2016
    Location
    Melbourne, Australia
    Posts
    280

    Default

    Arm yourself with one of the oscilloscopes and the schematic here...
    http://www.freeinfosociety.com/elect...ew.php?id=1472

    In the top left of the schematic you will find A3.

    Check pin 15, you should see a signal mostly high with short duration evenly spaced low pulses. This is the signal that latches each line of character data into the latch.
    Check pin 13, this should be mostly low but with very sporadic very short high pulses. These are the lit pixels of the characters. If it is mostly high (white screen) then we need to look at why.

    That will get you started in the right direction.

    Quote Originally Posted by Juror22 View Post
    I have an ESR meter, a newer and a few older multimeters, a couple of old 2 CH oscilloscopes (Hitachi V209, BBC D-8600) and a Hantek 6022BL PC Oscilloscope & logic analyzer (none of which I have yet learned to use, but this seems like a good project to start with if they are up to the task).

  3. Default

    It took a few days to get comfortable with the oscilloscope and pin 15 of A3 is as you described, with short lows followed by long highs. Pin 13 is basically all high. If I understand what is going on, then 15 is a (correct) input and combined with the other parallel inputs, outputs to 13. Since swapping A3 and A5 had no effect on the issue, an input(s) to A3, other than pin 15 is/are causing the issue?

  4. #14
    Join Date
    Sep 2016
    Location
    Melbourne, Australia
    Posts
    280

    Default

    I just double checked and I've given you bad advice. I incorrectly assumed the character generator (labeled 2516) on some boards had the same select lines as the 2516 Applesoft ROMs.

    The character generator uses the same select scheme as a standard 2716 unlike the ROMs. So a ROM in the character generator slot will give you the white screen. (I tested it in a working board here)
    I've just read the character generator from a board here and confirmed the selects are standard 2716. I've programmed a 2716 with that data and it works perfectly as a replacement.

    In light of that it appears your character generator ROM is faulty. If you have access to an eprom programmer here is an image file I copied from the ROM on my board.
    https://drive.google.com/open?id=0B8...nRHbXozczdsWHc

  5. Default

    Thanks for your continued help, for taking the time to re-check against your own equipment and for passing along the file. I will let you know how it 'burns out'.

    Looks like it will be a bit, I only have 27C256 eproms lying around (I searched through some old ones that I had stored, which were software revisions to a CAT scanner, those were 28 pin too). I will have to order up some, probably from eBay.
    Last edited by Juror22; August 4th, 2017 at 02:59 PM.

  6. #16
    Join Date
    Sep 2016
    Location
    Melbourne, Australia
    Posts
    280

    Default

    Quote Originally Posted by Juror22 View Post
    Thanks for your continued help, for taking the time to re-check against your own equipment and for passing along the file. I will let you know how it 'burns out'.

    Looks like it will be a bit, I only have 27C256 eproms lying around (I searched through some old ones that I had stored, which were software revisions to a CAT scanner, those were 28 pin too). I will have to order up some, probably from eBay.
    What area are you in, there may be a nearby member that can supply an eprom quicker than from ebay.

  7. Default

    I'm in central Illinois, (US) and I doubt that there is anyone nearby, but on the off chance there is, let me know.

    Otherwise, I should have something in hand by next week.

  8. #18

    Default

    Success! My ][+ lives!

    I received the 2716 chips today, erased and burned them, using the provided file (only took two tries to get the settings correct) and it now boots and runs correctly. I will put it all back together tomorrow and start giving it a proper workout, but the white screen issue is history! (Root cause was the bad chargen ROM)

    Thanks to all that wrote and of course a special "Above and beyond the call of duty", thanks to David_M for his expert assistance and sacrifice, just to help me sort this out. If there is ever anything I can do, let me know.

  9. #19
    Join Date
    Sep 2016
    Location
    Melbourne, Australia
    Posts
    280

    Default

    Good to hear its running again

  10. #20

    Default

    Made a couple of disks, after hooking it up to my Mac with ADTPro. Playin' Castle Wolfenstein on the A2+, duodisk and a color composite monitor, WooHoo!

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •