More II/IIfx power circuit troubleshooting

[outZider]

Hi, everyone. Finally de-lurking!

I'm the process of resuscitating a Macintosh IIfx. After an exploding C1, resulting in me going ahead and replacing all of the big tantalums, the only two electrolytic SMDs, and the remaining axial capacitors, replacing the batteries, and repairing a trace between battery 1 and C24, I am having one last little issue. From cold, the IIfx either takes quite a few starts (like cranking an old engine), or a direct connection of power to the on/off pin of the power supply for 5+ seconds to start up. Oddly, and maybe coincidentally, it seems like it takes fewer when starting it over ADB than the rear switch. I thought maybe there was a capacitor that just wasn't charging properly, but everything seems to check out okay, and if I shut down and start it up again within a few seconds, it'll fire right back up.

I'm hoping to get some ideas of what I should check from people who may have been down this road before. My current theories are to crack open the power supply to recap it, in case 5v is just slow to warm up, or potentially replacing transistors at Q2 or Q3 that may be slow to respond (though they seem to both be operating fine?). Anything else I should be checking?

Thanks for any ideas at all, even if they are just to spark some other ideas!

 

[ClassicGuyPhilly]

Hi @outZider welcome! I'm pretty new too, a IIfx fell into my lap and I joined hoping to get help with its restoration. There are many amazing IIfx owners here with an unbelievable amount of knowledge, you're in good hands! I've been learning a lot.

I highly advise you open the PSU and at least clean it out, this is what mine looked like before I had at it.

Best of luck, will be following along.

 

[outZider]

Resurrecting this old post in case the right Google search comes this way. I ended up tracing my way through the IIfx Startup Circuit sketch, finding that my battery connections were fine, 5v was coming up quickly, and all of my diodes were doing exactly what diodes should. Realizing that was all fine, I started working backward from R110 through the power off circuit, and found that C24 was struggling to power up. Turns out, it wasn't having difficulty starting up, it was in fact powering off as soon as it was powering on. A reflow to C24 solved the issue.