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Garbage In, Garbage Out

I’m beginning to suspect there’s something subtly wrong with my Apple IIe, which might explain a lot of strange intermittent errors I’ve recently observed with Yellowstone and Floppy Emu development. It’s hard to know what to conclude, but when you can’t trust your test equipment, it’s impossible to trust the validity of your test results.

Recently I used this Apple IIe to test my new Floppy Emu OLED prototype board. When connected to a standard Apple Disk 5.25 controller card, everything worked fine initially. But when I tried doing some ProDOS file copy operations, the Floppy Emu spontaneously reset to the happy face / self-test screen. DOH! I tried it twice, and the Emu reset itself during the copy operation both times. I concluded there must be something wrong with the prototype board.

Then I tried the copy test two more times, and got different results. The Emu prototype didn’t reset itself, but the OLED display went blank several times during the copy operations. Hmmm.

So then I tried a plain vanilla Floppy Emu Model B, the same hardware and firmware that I’ve been using successfully for more than a year. I found that when trying to boot the Apple IIe from a ProDOS v1.9 disk image, the Model B’s LCD went blank several times during booting. Huh? This happened in two consecutive test runs, but then mysteriously stopped happening. I also tried the same file copy operations I’d done with the OLED prototype board, and saw a similar behavior where the LCD went blank a few times during the copy. But as before, after reproducing the bug twice in a row, it stopped happening.

Finally I went back to the OLED prototype board, and this time everything worked fine. No more unexplained resets or display blank-outs.

Maybe there’s something wrong with the Apple IIe’s power supply, or some problem where it needs to warm-up for a while before it works reliably? My first OLED prototype board tests were the first time I’d powered on the Apple IIe in several days, so it was cold. During an hour of testing, the strange Floppy Emu problems I’d observed gradually disappeared. It doesn’t really make sense to me, but it’s the best explanation I can think of. This might also explain some strange unexpected resets of the Floppy Emu last month, when I tested it with the Yellowstone card. In fact, it casts doubt on all of my Yellowstone testing.

Read 9 comments and join the conversation 

9 Comments so far

  1. Avi March 19th, 2018 10:47 am
  2. Merlin March 19th, 2018 6:53 pm

    Steve, is there any way that you can emulate the computer itself, and send the appropriate commands to the user card while it\’s on the bench? And see how the card responds then?
    I know it\’s asking a bit, I just don\’t know how much emulation equipment you have.

    If not, then yes you\’ll end up troubleshooting the Apple computer itself. I don\’t know too much about them specifically, but power supplies and any distributed filter caps are high on the list to investigate.

  3. Steve March 19th, 2018 8:43 pm

    I would get another Apple IIe, if I could find something clearly and testably wrong with this one. But when it’s all mysterious intermittent errors, and especially when all the errors disappear after a while, it’ll be hard to test whether a second IIe actually helps anything.

  4. Fluffysheap March 20th, 2018 6:03 am

    Didn’t you test things on a IIgs too? I guess it could have flaky power too but that

  5. Fluffysheap March 20th, 2018 6:04 am

    That seems to imply that the //e hardware is ok. And also that my phone is horrible for posting.

  6. Steve March 20th, 2018 6:27 am

    I repeated the OLED prototype board tests on a IIgs, and didn’t encounter any problems.

  7. LEELAND HEINS March 20th, 2018 7:54 am

    I’m with Avi on this one… the PSU is the first place I’d look. Wacky voltage levels can cause all kinds of weird and often unpredictable behavior.

  8. Steve March 21st, 2018 9:43 am

    This morning I repeated the tests, after not using the IIe or the IIGS for two days. This time I tested with the IIGS first, to see if the suspected “needs to warm up” applied to the computer or the Floppy Emus. Then I did the same tests on the IIe. Everything worked fine on both computers, with both the Emu Model B and the OLED prototype board. So… whatever this problem is, it’s not easily reproducible. I agree it might be a power supply problem, but it might be something else, or something totally unrelated to the computer. Since I can’t reproduce it anymore, there’s no way I could tell whether replacing the power supply helped at all. Argh. But since everything is working OK for now, I guess I’ll just ignore it and move on.

  9. Ben Gillies March 28th, 2018 12:53 am

    I would strongly suggest load testing you PSU, out of the system.

    Set up an appropriate load bank to simulate 70%-80% max load of the PSU from cold and monitor the Voltage outputs as well as all the gnds(see if they drift apart..)

    The tests on yellow stone showed a gnd voltage of 1-2V, this could be easily explained with a high resistance on an gnd connection in the PSU, which s not all that unlikely to occur in something that vintage…

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