68kMLA Classic Interface

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LCIII rebooting?

Posted by: Fred1212 on 2023-02-05 14:55:59

I have a recapped LCIII that has been working well. It has just started to reboot itself just after it has booted into the desktop and a couple of times has locked and rebooted during startup. I've done all the usual swaps of known good memory drive and power supply from another working LCIII. Anyone know why a reboot is happening. Have had the death chimes once as well. Thanks in advance

Posted by: imactheknife on 2023-02-05 18:25:22

Did you check for shorts / solder balls? Possible broken trace? Believe it or not, recapping may work well for awhile, but sometimes if you don’t clean board well cap juice can keep working its magic unfortunately. I would give her a good clean in a tub of isopropyl, light scrub especially around where old caps were and let dry.

Posted by: Fred1212 on 2023-02-05 23:02:43

Thanks I'll have another go at it

Posted by: mitchW on 2023-02-12 18:50:41

From my memory, LCIII has one cap marked the wrong way on the logic board, so the polarity is incorrect. Electrolytic caps will survive that reverse polarity (and might even self reform to reverse polarity after time), but tantalum caps really hate it and may be shorting it.
Or it could be just a red herring.

Here found the link:

LCIII Recap - Apple design fault -47uF reversed!

Hi, Am I the first person to discover this? C22 (the 47uF capacitor in the corner beside the power supply connector) on our LCIII has the + sign on the PCB on the negative capacitor terminal. This capacitor is between -5V and ground. The original C22 electrolytic on this board, and on...

68kmla.org

Also, did you recap the power supply? I think TDK ones need recap AASP, Astec or other brands are better, but I still found few that had leaky caps after all

Posted by: Paralel on 2023-02-13 10:01:37

A tantalum in reverse will just blow won't it?

Posted by: Big Ben on 2023-02-13 11:21:38

It could, the reversed capacitor is on the -5V rail IIRC so if replaced with a 47µF 16V it will probably burn/blow at some point but in my experience it was just really hot.
Since a tantalum blowing can do severe damage, it was a relief, but still a bad thing tho, I did immediatly replaced it when I came aware of the problem.

With a lower voltage rated tantalum it would have probably blown immediatly. You can check on the capacitor datasheet, there is a part about reverse voltage limits.

Posted by: joshc on 2023-02-13 12:51:54

For posterity, this issue was solved in a different thread:

Rebooting LCIII

68kmla.org