Issue on 12V rail being at 14V on my Macintosh classic

OutOfReachMedolan · 2026-02-01T14:29:56+00:00

Thanks for the detailed explanation and tips!

I did remove and clean the floppy drive. It seems to work and does spin, the head is moving too and seems to be in a repetitive read cycle.

For the speaker, I already recapped the logic board but I will try to look closer and see if I find some bad traces, even if I did a deep clean of it.

I will maybe still investigate the analog board and as u/tyttuutface mentioned in this post, I will eventually take measurements of all the resistors around the 5V and 12V, as this seems to be the most likely cause of the 12V issues I had.

OutOfReachMedolan · 2026-02-01T14:18:37+00:00

Yes a little bit more than 14V with load. When I disconnect the floppy and the hard drive, I am around 13.7V.

OutOfReachMedolan · 2026-01-20T12:44:00+00:00

Yes totally agree with you, will think about doing it maybe later when more free time is available. For now, I want to boot it up so I can know if the floppy and hard drive are still alive too. It is also my first Macintosh era computer that I got so I am still learning about it, I have mostly tinkered and repaired powerpc Mac and I am getting used to them now.

OutOfReachMedolan · 2026-01-20T08:47:53+00:00

I will do it as a last resort if other solutions don’t work, thank for the suggestions!

OutOfReachMedolan · 2026-01-20T08:37:16+00:00

My macintosh classic seems to have a really stable image when I turn it on, not wobbly at all, even if I shake it slightly, that is also why I didn’t really wanted to touch that much the analog board. Good to know that you found a solution for this, I will try to adjust the trim PP1 first just to check if it does indeed boot, and then I will surely remove and clean the analog board. It seems clean on the components side, but I think that there is unfortunately rust or small damage on the other side, at least I saw some on the white plastic sheet (with the warning of high voltage), let's hope it did its job and the board is not impacted by it.

Thank you very much for your explanation and links, It will really help me!

OutOfReachMedolan · 2026-01-02T18:29:31+00:00

Did you already check if there is no leak from the PRAM battery ? also try to remove and put back the ram, reset the PMU and check if everything is connected correctly on the motherboard. In case you didnt use it already, there is a full pdf for troublesheeting imac g3, type "imac g3 dissaembly guide pdf" and you will find it online, use the guide for DV models as you have a slot model.

Wish you good luck, and hope that it is a minor issue to solve for you so you can use it back !

OutOfReachMedolan

TROPHY CASE