68kMLA Classic Interface

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Re-Capped Macintosh Classic II Analog Board Grounding Issues
Posted by: jmb2011 on 2022-05-09 12:36:44 I did a re-cap of the analog board on my Classic II as it was having some power issues with a wavy like screen (you could see a ripple roll down the monitor slowly). Replaced all radial electrolytic capacitors, put it back in the Mac and still having issues with it. Worth noting I am testing with a cleaned and recapped Classic Motherboard that works fine on another machine. When initially powered up, I got a glitched/ garbled checkerboard, and it wouldn't boot. Then after a few moments it would boot, but it would restart the moment you touched the chassis and you would here the boot up beep -- The screen also shrunk and dimmed when this occurred. Thought it was a grounding issue, so being I live in an older house - some outlets are not grounded properly. I plugged it into an outlet near the panel that I know for a fact has a working ground and the Mac refused to boot at all when grounded properly, and gave just a checkboard pattern. Checked the 5 volt and 12 volt rails through the floppy connection on the back and both were low, so I adjusted the P1 pot until they came up to 5v and 12v (within .01 volts). After this the Mac would reliably boot up and no longer had a wavy screen -- however power would drop on both rails if I touch the chassis / ground plane (fan would spin down, hard drive slowed, screen dims). On one boot, after it got to the desktop, I plugged a ADB mouse in and got a sad mac. Wouldn't boot after that, back to the garbled checkboard pattern. Put the good logic board back in its original Mac Classic, and after power cycling it a couple of times it worked again, and didn't have any issue with a sad mac or crash. I also checked and double checked and all capacitors are facing the correct way (polarity). Has anyone experienced something like this? Anyway to recover this analog board?
Posted by: OsvaL on 2023-11-05 06:30:55 I have a similar issue with my Macintosh Classic. The only way it will turn on is plugging it without the ground pin using an adapter. When I connect an ADB keyboard some keys don’t work, I think is related to this issue. I never tried to solve this situation since I don’t have the knowledge to do it so.
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