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Thread: Mac II jump starting

  1. #1
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    Default Mac II jump starting

    Tonight I finally got round to replacing all the SMD caps on my Mac II motherboard and was pleasantly surprised to see that when I jumpstarted it using the battery trick, I got the normal Mac chime.

    I did not get anything onscreen and did not see any evidence of boot activity... would that likely be because I do not have the PRAM or clock batteries connected at the moment?

    The original batteries were 3.0v. I got some 3.6v by mistake, however I note on multiple forums and websites they list the Mac II as accepting 3.6. I installed these following the orientation indicated on the board, but the computer did not respond to the power key press on the keyboard.

    Further, there is a bit of weirdness with the original batteries; I seem to recall one of them being installed in the reverse of the indicated polarity. I happened to still have one of the original VARTA 3.0v batteries from this machine.. and with positive probe on positive terminal of said battery and ground on negative, it shows -3.14v, unless I reverse probes. It's got me wondering if something is mislabeled. The DMM probes are setup correctly.

    Any thoughts? Should I first obtain 3.0v batteries for the PRAM?

  2. #2
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    One thing that is really confusing me - there are several sites i've perused about this, including one that has an excellent breakdown of the power supply. Many claim the original Varta batteries, although labelled 3.0V, are actually 3.6V ? Seems kind of weird to mislabel the voltage..

    And yeah, I'm mystified why the polarity on this one battery seems to be the opposite of its markings. I'm wondering now if it was mislabelled, and the factory simply turned it around, hence why it appeared to be installed backwards.

  3. #3
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    If both batteries are in series with each other, it makes sense how one could be reverse polarity, it happens in laptop battery packs all the time.

    Batteries are not 100% the same, they will always vary in capacity by a tiny amount. As they run down, one will invariably run down faster than another; And when the voltage difference gets significant enough, the stronger battery will start pulling current backwards through the other battery and reverse charge it. This usually results in the violent destruction of the weak cell, but if it's done at a very low current over a long period of time, the weak cell can survive and be significantly reverse charged.

    The reverse charge still irreversibly damages the battery and it will have basically no usable capacity, but it can report a high reverse charge voltage until something tries to draw current from it.

  4. #4
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    This fellow did (I think) a pretty good description of the power on circuit:

    https://www.downtowndougbrown.com/20...onoff-circuit/

    He suggests it has diodes in place to drop the voltage to about 7V even, and I've tested across the batteries from positive on one battery to negative on the other, and I get 7.2V before the diode, so if he's right that all seems legit.

    Nevertheless, still get nothing onscreen after the startup chime.. so it's got me wondering if it is broken down somewhere else. I can see some corrosion damage near the bottom right corner of the board where three of the old SMD caps were. Gonna have to trace this circuit out. I'm hoping I'm not dealing with a bad video card.

  5. #5

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    Did it have video before you recapped it? Do you have a known good bootable floppy disk? If you do, put one in and listen to see if it'll boot off a floppy. If it does, then it's probably booting to a flashing Disk icon because it can't find a bootable disk.

  6. #6
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    Well, video worked before whatever the original problem was happened. Didn't work with the initial jumpstart efforts. And no, the Mac definitely isn't booting anything. It won't even turn on using the back power button or keyboard power.. but it will turn off with the back button. Probably a bad trace somewhere.

  7. #7
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    Question: Does anyone know if a Mac II would try to boot with a bad/missing video card?

    I've really been at it with this thing, checking *everything* on the board to no avail so far. It powers up, it chimes, there's no error beeps.. but zero onscreen. The keyboard power button can turn it on, but not off. Back power button does both. CPU, RAM, etc all seem to be ok. I'm thinking this must be a trace, or something rusted somewhere, but I'm wondering about the video card. It was working when last used years ago, but who knows. I don't have a spare and don't have another NUBUS machine to test with, so I'm kind of stuck there.

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