Macintosh SE stuck in reboot loop

[bigmessowires]

Tried it, I get the flashing icon. Same with just the HDD connected. But it can't boot off it. The reboot behaviour only starts when it attempts to boot from the floppy. My assumption is that it is rather boot process related. But can confirm as I have no other device to boot off.

Have you tried it with the floppy drive installed, but no floppy disk in the drive? Or a blank disk or a non-Mac disk?

My first guess would be a power supply problem, specifically with 12V. As soon as the floppy drive tries to spin the disk or move the stepper motor, it will draw more power from 12V. If there's a problem with the PSU then it might drop out of regulation and affect the 5V supply as well, causing the computer to reboot. This could all happen so quickly that you wouldn't see it when watching the supplies with a meter - you'd need to capture it on a scope.

 

[hbanko]

Yes, it doesn't matter what disk I insert. As soon as it accesses it, the Mac reboots.

The 12V rail seems a bit weak. I have ordered in these 330uf 240 Volt capacitors that are still the old ones in the power supply and will replace them as well.

 

[hbanko]

The good news first, I got a working Macintosh SE booting from its old interal harddrive! Persistence paid off .

I went ahead and replaced the two remaining capacitors in the power supply. But that did not change anything. The next thing was recapping the whole mainboard. If I remember correctly there are only 13 capacitors - so its not the biggest job. After that the Mac went a lot quicker to the screen with the Disk Icon. But still would just reboot as soons as I insert a floppy.

So out of options I started to have a look again at the analog board and noticed one capacitor close to the logic board power connector, that was not in good shape. I replaced it and it also did not solve a thing. Instead the Mac was now stuck on a garbled screen.

As a last resort I had a look at the analog board and all the solder joints. Especially those for the socket that connect with the logic board. Wearing my mangifying glasses I noticed that the two outer pins (towards the screen side) got some corrosion from the leaky capacitor. I guess the Mac was stored for a long time on its side and acid made its way to it. I cleaned up these joints (one highly corroded) and resoldered them.

Boom, the floppy drive made noises it never made before. But now it seems it got mechanically stuck. Likely I got something wrong at re-assembly when I serviced it. So I hooked up the relubed and originally seized harddrive. To my surprise, after some seemlingly endless seeking back and forth, I got a happy Mac! It booted right away to the desktop and revealed the contents of the ancient drive. It still on system 6.07(?) and got a bit of Clarisworks and Office on it. The documents I found were created in 1995.

Now I am back on the floppy, the famous eject gear stripped and the resin printer behind is producing new ones as I type. In about an hour I can finally put it all back together.

It could have been easier. But I got a Mac with a dead screen and seized hard drive for free - saved at the tip. Now I have a fully recapped working machine that will be reliable for decades to come.

 

[bigmessowires]

Great job! Glad to hear you got it working.