Macintosh SE stuck in reboot loop

[hbanko]

Hi,

I picked up a poor Macintosh SE at the tip. The screen did not turn on but the issue was quickly indentified and fixes. One capacitor next to the high voltage generation was bulking up. After that it not only chimed but we had a working screen.

Now I am stuck with it being in a boot loop. It can stay forever on the "disk screen" but as soon as I insert any disk and it acesses it, it reboots and chimes again. THe only wait out is to turn it off and manually eject the disk. The internal harddrive seems to have some reading errors so I can boot from it.

Thought it might be worthwile to service the floppy drive. But that did not change anything. I have also recapped the power supply and replaced the battery on the mainboard.

Not a chance, just chime and reboot forever.

Any ideas where to look next?

 

[bibilit]

So you can boot from the hard drive but not the floppy drive and when trying to boot from the floppy, you get that loop thing ?

If so probably the drive is faulty and shorted.

 

[Phipli]

Now I am stuck with it being in a boot loop. It can stay forever on the "disk screen" but as soon as I insert any disk and it acesses it, it reboots and chimes again. THe only wait out is to turn it off and manually eject the disk. The internal harddrive seems to have some reading errors so I can boot from it.

This sounds like a couple of issues, the SE doesn't like the boot floppy, either because of the OS on it, or because it is struggling to read it, and the eject mechanism isn't working properly.

What OS are you trying to boot, and is it an OEM disk, or something you've made?

 

[Phipli]

If so probably the drive is faulty and shorted.

I don't think it is shorted, it wouldn't get this far if it had a short.

 

[bigmessowires]

How far does it get with the floppy disk before it reboots? Do you see the happy mac icon? "Welcome to Macintosh" text?

 

[ymk]

Check your 5V rail.

 

[cheesestraws]

Check your 5V rail.

Came here to say this - I have had machines that rebooted as soon as they tried disc access because the PSU was on the way out.

 

[hbanko]

So to clarify - right now I can't boot at all. The harddrive is likely corrupted (it never booted from it). The machine booted to desktop one time from floppy but then hung. From then on it only rebooted when accessing the floppy. I hear a chimme, then see the mouse cursor but no happy mac icon. Screen goes into this grey background that would usually have the happy mac icon. But only for half a second. And then again Chime, Mouse Cursor, Grey Screen, Chime. ....

To rectify that I have serviced the floppy drive and the power supply. I also replaced the battery on the mainboard. But the Mac behaves the same.

The 5V rail looks good measuring +5.04 Volts. I am using a System 6.04 disk I made myself and an Original Apple System 6.04. But the reboot also happens with disks that are not even bootable.

 

[bibilit]

For troubleshooting purposes, i will test with a minimum setup.

Just the boards and no drives.

If you get to the flashing icon with no reboot, either one of the drives is faulty or something is draining too much current.

You can then hook the drives one by one to locate the issue.
Try with an external PSU for the HD only

 

[68kPlus]

I had a problem with my SE that the drives were creating errors and the fans weren't consistent and I was able to figure out that it was a weak power supply. After recapping, it runs way better and the voltage is much more consistent.
Maybe something to look into?

 

[hbanko]

I think I can rule out the power supply. It has been recapped already and I measured the 5 volt rail on the (spinning) HD. It's at 5.06, the 12V is a bit low at 11.98.

The only other bootable device at hand would be an SCSI ZIP 100 drive. I got a USB version as well, but the OS images I dumped onto the ZIP floppy are just ignored by the Mac. (https://blog.rekawek.eu/2016/12/29/macos-images-for-iomega-zip/)

Time to get the G4 powermac out of storage and check the ZIP floppy in its internal drive.

I'll probably just go and order the 11 radial capacitors needed to recap the mainboard. Maybe that helps.

 

[Phipli]

The only other bootable device at hand would be an SCSI ZIP 100 drive. I got a USB version as well, but the OS images I dumped onto the ZIP floppy are just ignored by the Mac. (https://blog.rekawek.eu/2016/12/29/macos-images-for-iomega-zip/)

Time to get the G4 powermac out of storage and check the ZIP floppy in its internal drive.

G4 ear operating systems have a habit of... Breaking bootable zip disks for 68k macs, so there is a chance that won't help.

 

[Phipli]

I hear a chimme, then see the mouse cursor but no happy mac icon. Screen goes into this grey background that would usually have the happy mac icon. But only for half a second. And then again Chime, Mouse Cursor, Grey Screen, Chime.

It does this even with no floppy disk?

What does it do if you power up with the floppy drive not connected?

 

[Phipli]

I think I can rule out the power supply. It has been recapped already and I measured the 5 volt rail on the (spinning) HD. It's at 5.06, the 12V is a bit low at 11.98.

Did you use low ESR caps on the output stage?

 

[bigmessowires]

For troubleshooting purposes, i will test with a minimum setup.

Just the boards and no drives.

If you get to the flashing icon with no reboot, either one of the drives is faulty or something is draining too much current.

This is an excellent suggestion. Definitely try this!

 

[hbanko]

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.

Not sure how common a floppy drive is that has a short? Never heard of anyone having that problem. But there are likely some tantalums on it that can go short.

The caps used in the power supply are low ESR (for switching power supplies).

 

[Phipli]

Do you have other macs? Can you try another floppy drive to see if the behaviour is due to the drive or the host?

 

[Phipli]

Not sure how common a floppy drive is that has a short?

I'm not sure that this is the issue either. The behaviour suggests a weak PSU to me. I realise you recapped, and used low ESR caps, but perhaps another issue.

But, let's verify it isn't the specific floppy drive if possible.

 

[hbanko]

I took out the floppy drive again and found 6 electrolytic caps. Some measured a quite high resistance. Luckily I got some small capacitors here for Walkman repairs and swapped all of them out - same result.

There are two "large" capacitors left in the power supply that I have not yet replaced as I did not have them here. I believe they are two 330uf >200 volt caps. I do not see the voltage dip at any time. But these can be replaced as well.

 

[ymk]

I took out the floppy drive again and found 6 electrolytic caps. Some measured a quite high resistance.

How are you making these measurements?