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Macintosh Plus boot issue - help!
Posted by: FrenchMac on 2017-09-03 03:14:07 Hi everyone, I have received this july an old Macintosh Plus in very good condition, starting up allright, just a disk ejection problem. That was a perfect project for a week end. So I have done a full maintenance on the floppy drive and identified a broken gear in the ejection mecanism. I decided to remove the ejection motor and gearbox but I let the 2 retention screws of the motor+gearbox on the drive not to lose them. That was my first mistake... Once the drive was perfect, I connected it on the Mac for testing and strangely the Mac did'nt want to start normally! With the drive removed the Mac didn't any have problem (normal bong and screen). I was really upset and try several times with or without the drive, the drive cable was anormally hot and then my second mistake came : I disconnected the drive but forgot to first turn off the Mac! After that, impossible to have a normal startup even with the drive disconnected, arghh!! And most exasperating, at that moment I remembered this two little stupid screws I let on the drive : they were more pushed in because the ejection motor was removed and it was creating a short circuit. After removing the screws, the floppy drive behave well (no cable overheating), head moving. However the Mac still didn't want to start. It is so stupid, if I didn't disconnected the drive with power on I would have an almost perfect Macintosh, sniff 🙁 ( Here is the situation now : - power arriving on the logic board : OK - power measured on the external drive connector : OK - startup : no bong, a strange glitch on the screen (columns appearing, descending and disappearing) - test with RAM removed : only change the screen glitch at startup (a kind of horizontal bar appearing then disappearing) - Test with ROM & RAM removed : no more glitch on the screen - Logic board is clean, no trace of damaged component (idem for the analog board) I read different things and books on the Internet, many solutions are focused on the analog board. My analysis is the problem is not coming from the analog board (correct voltage measured on the logic board and the the external drive connector) but from some component on the logic board however I really don't know wich one... (and maybe this analysis is completely wrong!). Back from holidays I have check the capacitors on the motheboard with an ohmmeter. C5 and C6 had a strange behaviour but once they had one leg unsolder, the ohmmeter give the same result than with the others. I saw this forum was really hi-level on these old Apple technologies and I am sure you guys have maybe a little idea of what's hapening : what component has not survived to my bad manipulation and how can I test it and perhaps change it... Thanks for your help! PS : excuse-me for my english, I tried my best...
Posted by: mactjaap on 2017-09-03 09:03:09 What a bad luck. Those stupid screws. I guess you don't have a second plus to test? I always switch components to have an idea where to look.
Posted by: FrenchMac on 2017-09-05 09:02:14 Yes, its really too stupid... Indeed, I don't have a second logic board to test. I don't know if I can do any other component check on the board only with a multimeter...
Posted by: trag on 2017-09-05 13:38:33 I think you can download the Dead Mac Scrolls on line now somewhere. I would check for your symptom description in that book. I hope you have not killed the IWM chip, as that is a custom Apple chip and only replaceable by salvage from another Mac. I IWM is the floppy disk controller.
Posted by: mactjaap on 2017-09-05 15:06:19 Here it is: http://www.maccaps.com/MacCaps/Repair_books_files/The%20Dead%20Mac%20Scrolls.pdf
Posted by: techknight on 2017-09-10 05:18:01 Word of the wise: The Macintosh Plus will NOT start up without a floppy drive connected. Something with the ROM. Usually hangs on a frozen ? Just so you're aware when testing things. But to the point: You blew shit up. plain and simple, most likely one or more fried ICs. At minimum the IWM. But any other IC that is attached to the floppy connector was just as vulnerable to the mistake. Hot floppy cable = irreversible damage... You will need to start swapping ICs, but again you need another logic board, Would be easier just to get a new board and call it done. Then, there is the drive itself. it too, could be damaged as well. Just take it as lesson learned, and move on. I have been there, done that and its costly.
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