Original 1984 128k Macintosh display problem? duplicate screens

Discussion in 'Apple Collectors' started by 802.11mac, Oct 14, 2011.

  1. 802.11mac macrumors member

    802.11mac

    Joined:
    Mar 16, 2010
    #1
    I have this old original 128k Macintosh with ram upgrade. it's been a while since i powered it on and today i thought of taking some pictures of it. when i turned it on i noticed the startup sound is unusual. a bit distorted. and also the monitor displays two identical screens like this :
    [​IMG]
    http://www.flickr.com/photos/91379880@N00/6242620937

    Do you think its a problem with the board or with the crt monitor?

    you can see more picture of it at: http://www.flickr.com/photos/91379880@N00/sets/72157623523929099/with/6242620937/
     
  2. MacTech68, Oct 14, 2011
    Last edited: Oct 14, 2011

    MacTech68 macrumors 68000

    MacTech68

    Joined:
    Mar 16, 2008
    Location:
    Australia, Perth
    #2
    I would suspect the logic board.

    If you turn the brightness/screen control/cutoff adjustment on the Analog Board to increase the brightness, you should see the raster image is the full height of the screen.
    See the "incorrect" image here to see what a raster looks like when driven too high. Turn the control to it's original position when done.

    It's possible that the video circuit on the logic board is displaying both video pages simultaneously. Somewhere off the 6522 chip.

    Do you have the ability to remove the memory upgrade board replacing the chips and resistor packs? It could be associated with that board.

    It's probably worthwhile getting another 128K, 512K, 512Ke or Plus to make CERTAIN it isn't the discrete vertical circuit on the analog board.
     
  3. 802.11mac thread starter macrumors member

    802.11mac

    Joined:
    Mar 16, 2010
    #3
    Yes i see the raster image after turning the cutoff adjustment on the Analog Board.

    here:
    [​IMG]
    http://www.flickr.com/photos/91379880@N00/6245515394/

    Does this mean my logic board or the ram board is defective/problematic?



    Yes i can remove the memory upgrade board but I'm still researching on which chips, resistors or what parts i need to replace. I hope someone can help with this.

    Here's a shot after removing the memory upgrade board. But of course it didn't have the start up sound nor the folder icon.
    [​IMG]
    http://www.flickr.com/photos/91379880@N00/6244994067/in/photostream/



    So i can get my needed parts just from a 512k board?


    Here's the Mac board without the RAM upgrade:
    [​IMG]


    and here's the RAM/memory upgrade board:
    [​IMG]


    One more thing, do you think i can get the two 74LS244 chips from the memory upgrade board and placed it on the Mac logic board?
     
  4. MacTech68, Oct 14, 2011
    Last edited: Oct 14, 2011

    MacTech68 macrumors 68000

    MacTech68

    Joined:
    Mar 16, 2008
    Location:
    Australia, Perth
    #4
    Most likely the logic or ram board, yes.
    I would try to source new 74LS244 chips as using the ones from the upgrade board DOESN'T eliminate them from possible causes (unlikely as they are to be the problem).

    Secondly you'll need two SIL (Single In Line) resistor packs that go at the end of each memory bank on the main logic board.

    These may be harder to find. An alternative is to use five 47 Ohm resistors in each resistor pack socket (a total of TEN 47 Ohm resistors). A current Bournes part number would be "4610X-102-470" or "4610X-102-470LF"

    Each resistor is placed bridging two positions:

    1 - 2
    3 - 4
    5 - 6
    7 - 8
    9 - 10

    There is NO "common" for this type of resistor pack.

    Yes, you could source all of these from a working 512K logic board. :)

    The other thing you may need to do is to jumper the first two holes of the multiplexer pad socket (see attached pic). Your logic board already has 256K memory chips, but with the memory board removed, there is no 74AS253 to create the extra address needed for 512K of memory. Jumpering these holes will essentially use the 41256 as a 4164 and should not cause any harm, but you'll only have 128K of memory. IIRC the track is cut on the other side of the logic board that jumpers these pins on a 128K, but I'm unable to find a pic. :(

    Good luck! :)
     

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  5. 802.11mac thread starter macrumors member

    802.11mac

    Joined:
    Mar 16, 2010
    #5
    Thanks MacTech68! I will try your suggestion as soon as i can get a 512k board.
     
  6. MacTech68 macrumors 68000

    MacTech68

    Joined:
    Mar 16, 2008
    Location:
    Australia, Perth
    #6
    I notice you're trying to locate another board in another thread.

    I would still suggest getting the 74SL244 x 2 and the 47 Ohm resistors x 10. Much cheaper and may prove your board to be OK. There is a plethora of 74LS244s to be had on eBay.

    Getting the resistor packs off eBay doesn't seem possible, hence suggesting standard 1/4 Watt axial resistors, although you might get away with these 33 Ohm 10Pin 5 Isolated Resistor packs although at 50 pieces they're not so cheap: :(

    http://www.ebay.com/itm/330618580562

    If I may ask, what country are you in? It may help in locating the parts. :)
     
  7. 802.11mac thread starter macrumors member

    802.11mac

    Joined:
    Mar 16, 2010
    #7
    You're not gonna believe what happen. Last night, i was trying to look closely at the analog board particularly on the C1 capacitor at the top of the board coz someone from 68kmla.org told me that it could mostly be a problem with this capacitor. i tried reheating and turned the power on but i didn't know i plugged in the wrong voltage. :( I actually use a regulator with a 110v coz here in the Philippines we use 220v. i accidentally plugged in to the 220v. i heard a spark inside. visual damage include a fuse blown, two transistors cracked, several resistors and 1 capacitor with smoke. huhuhu..:( soo stupid of me!

    Well i'll just have to look for a replacement i guess.

    Do you think the analog board can still be fixed? How about the logic board, is it affected?

    will try to take some pictures and post them here.


     
  8. MacTech68 macrumors 68000

    MacTech68

    Joined:
    Mar 16, 2008
    Location:
    Australia, Perth
    #8
    C1 would not cause this problem since C1 is part of the horizontal deflection which is working perfectly.

    I've repaired many Switch Mode Power Supplies (SMPS) that were 110V plugged into 240V. The Mac 128-Plus power supply isn't as forgiving as many modern SMPS.

    It will be repairable, if you have plenty of experience repairing at component level, are handy with a multimeter and have access to spares. Miss one bad component, and you could destroy the replaced parts again.

    You might now be better off buying a complete MacPlus now. :(

    You MIGHT be lucky enough to find one cheap. :)
     

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