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Thread: Rescueing an Apple III

  1. #1

    Default Rescueing an Apple III

    Hello,

    I recently was very lucky to find a complete Apple III (serial 13198 ) with Profiler (serial 153461 ) for a decent price. We also want to show this machine hands-on in the HomeComputerMuseum and therefor I want to have the machine up & running. This is how I got it:


    I even got the original Apple III monitor too. The computer is not being used for a while and the previous owner said, it will turn on, but the FDD doesn't read nor does anything happen on screen.. right.. so I wonder how he managed to tell the computer was working then?

    Anyway, after his 'the powersupply is working' I just loaded it into my van and drove it to the museum. First thing, open it up and replace the filter cap. It was already cracked/open so it must've been smoking (perhaps that was considered working?). Anyway, after replacing that I tested the PSU for correct voltages, everything was fine on that part. Though I did put the connector the wrong way first. I really thought I put it right, since the connector was fitting perfectly. But no, it was wrong and it took another good hour of measuring to figure that out. The PSU doesn't power up then since it will short -12v and GND, so PSU powers down.

    After I discovered the mistake I kicked myself and put the connector on. It really shouldn't be like that, the plastic is all bend now.. very odd.. and I got a light on the mainboard!

    I connected a composite monitor and this is what I got:

    (some characters are flickering)

    Removing the 128K memory board:

    (no flickering at all)

    Both tests are barebone PCB, no FDD, keyboard or expansioncards (profiler card). Only PSU + mainboard.

    And because I can, here's the mainboard without memory:


    Can someone help me getting this machine back to a working state? Where do I start from here? Why is there such a difference between the screens with and without memory board?
    I have a few computers and only have 612m2 for computers.

  2. Default

    Hmm start with a reseat and check the memory. But I'm guessing you know that much already.
    -------------------------------------------------------------------------------------------
    The Lower Planes BBS - Telnet/ssh/www tlp.zapto.org

  3. #3

    Default

    Reseat done of all IC's (nearly first thing), memory card doesn't have reseatable IC's:


    This board has the full 256KB of memory. So that explains why the screen is white when this board is removed..:P
    I have a few computers and only have 612m2 for computers.

  4. #4

    Default

    At least you're lucky enough to have the 5V memory board. Unfortunately I can't help with the repair. Mine is in exactly the same state and I just don't have time to do anything with it.

    I do know that the computer will exhibit false signs of life (via changing video) even when the CPU isn't running.

  5. #5
    Join Date
    Mar 2012
    Location
    Augusta, Georgia, USA
    Posts
    663

    Default

    That's about all mine ever did, either - almost exactly. I sold it to another member here... Hopefully someone can repair them.

  6. #6

    Default

    Doesn't look like the CPU is even running. Any way of checking to see if the 6502 is ticking? Does it react when hitting the reset button? Do you have a logic probe that can check if the CPU is receiving clock? Apple II troubleshooting should apply to any 6502-based system. Upon power up, the 6502 should autostart on the Apple III, and it should immediately start running whatever is in ROM. But because the 6502 requires RAM because of page zero requirement, I'd start with looking at the lower banks of RAM, wherever that could be on the III motherboard.

  7. #7

    Default

    Strictly speaking, zero page isn't required at all for a 6502, but stack ($100 page) is. However, zero page is typically important for the kind of ROM programs this type of computer comes with.

  8. #8

    Default

    Quote Originally Posted by groink View Post
    Doesn't look like the CPU is even running. Any way of checking to see if the 6502 is ticking? Does it react when hitting the reset button? Do you have a logic probe that can check if the CPU is receiving clock? Apple II troubleshooting should apply to any 6502-based system. Upon power up, the 6502 should autostart on the Apple III, and it should immediately start running whatever is in ROM. But because the 6502 requires RAM because of page zero requirement, I'd start with looking at the lower banks of RAM, wherever that could be on the III motherboard.
    I agree, it must be the obvious steps when troubleshooting a computer system.
    Step 0: get the right schematic for it!
    Is the RESET signal behaving correctly? Does it stays low for a bit less than a second, then stay high forever?
    Does the clock input of the 6502 have the correct signal? Do both of the 6502 clock output have a signal (and are both signal with the correct phase?)
    Does IRQ and NMI (pins 4 and 6) stay high?
    After these basic checks, it's time to check for address bus faults, a NOP generator can help tracking all the address bus buffers and RAM's row and column address multiplexers.
    You need at the very least a logic probe and an oscilloscope is much better for this kind of troubleshooting.
    I should also mention that mT4264 DRAMs are very likely to go bad (not all of them, but statistics tells me that 1/8 to 1/4 of the chips of that era develops faulty bits), but I would never substitute all of them in a machine like that. It's quite easy (with the right tools) to identify the bad chips. Piggybacking could also identify all of them.
    I'd say it's an easy repair with the right experience and tools and can turn difficult without both of them
    (But we are here to help).

    Frank IZ8DWF

  9. #9

    Default

    Thank, I do have a logic probe, multiple oscilloscopes (analog and digital) and enough stuff to get everything disconnected from a PCB (and connected as well) :P

    I have the schematics of the board, so I can start poking around to see if the 6502 is running and next step will be piggybacking those memory IC's. I don't think I've seen memory on the mainboard and that matched the 32x 64kbit IC's (=256KB) .
    I have a few computers and only have 612m2 for computers.

  10. #10

    Default

    the CPU looks like it's working. It gets a 4.2v at the 5v line (which I think is a little low, other IC's get the correct 5v, like the memory). RSET is low, IRQ is pulsing, clocks are there..

    This is not a MOS 6502, but a SynerTek 6502A. Not sure if there's any difference in pinout. The CPU does get hot, so it seems to be doing something. Also IC's next to it are getting warm, so my next suspect will be the memory...
    I have a few computers and only have 612m2 for computers.

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