• Updated 2023-07-12: Hello, Guest! Welcome back, and be sure to check out this follow-up post about our outage a week or so ago.

Shutdown issues...


Well-known member
Hello, back again now that the autumn/winter has returned and I've put down my bike and picked up my Centris again...

Bit of an odd question here, but I'm a bit foxed... My Centris starts up okay, but when I come to shut it down again, it does that just fine, but then instantly restarts. I've had a look at the sprung power button on the back of the computer, and that seems to be mechanically fine, but before I take the whole computer apart again, I just wanted to check if there is something really obvious I'm missing here?

My guess was that the on-off button at the back of the computer was sticking, but I don't know if there is any other thing I should be looking for...

Any suggestions gratefully received...



Well-known member
Reset PRAM and replace the PRAM battery, first, before looking at any hardware issues (caps ...)



Well-known member
Thanks for the suggestion, i'll give it a try, but I recapped the whole thing last August, and it has a new PRAm battery in it...



Well-known member
It is a Centris 650, with a pile of other stuff tucked into it...

I've been doing a few tests, having stripped the whole thing down, checked the main circuit board (everything looks fine, but I gave it a clean anyway) and no peculiar sounds or smells when it powers up. 

HOWEVER, I realised that there was something not quite right with my keyboard (some keys were not working), so removed that and just connected the mouse to an ADP port, shut the mac down, and it was fine. AHA! So there is a stuck key or something mechanically wrong with the keyboard. I swap the standard keyboard for one that is on my classic (and which I know works perfectly fine) and lo and behold, the computer immediately starts up again without being prompted.

So, I am sure it must be something to do with the power-up key signal that comes from the adb keyboard that is triggering the mac to start-up: however, the thing is that it must be a fault in the Mac, as the same issue is repeated with both keyboards... 

Any further insight very gratefully received!



Well-known member
Dis you use different ADB cables with the different keyboards? Might as well just be the cable.



Well-known member
Thanks for the suggestions, but it was definitely a problem with the keyboard... I've tested the system again since my last post, and my observation about the same fault happening with a different keyboard hasn't been repeated, and the Centris has been shutting down just fine, and staying shut down. I'm just awaiting a new Apple Design Keyboard (and an ADB trackpad!) to arrive, and then I hope to be back up and running!





Well-known member
I ran into this same exact issue awhile back. My 8550 would spontaneously (or near spontaneously) reboot after shutting it down. After troubleshooting, dumb luck found that using a different keyboard fixed the issue. I am not sure what craps out in the ADB keyboard to allow it to happen. 




At a guess there's a short somewhere inside the keyboard between the Power On key and the PSW (power switch, pin #2) on the ADB port (or plug, if it's a permanently wired cable).

A quickie fix might be to make up a box to go between the keyboard and the Mac, with a NO (normally-open) momentary switch to short the PSW pin #2 to (ground or power? I forget), and don't pass line #2 through to the keyboard at all.

That would also be a handy box for testing soft power-on Macs without a keyboard.