imac shuts down after power on

[Andy Cuffe]

I'm working on an orange slot loading imac that powers on normally with
the turn on chime, but shuts down about 5 seconds later. The power
light stays on (green), but the monitor and hard drive shut down. The
5v and 3.3v supplies to the logic board drop noticeably when the monitor
shuts down, but this may be due to the sudden loss of most of the load
on the power supply (the B+ to the monitor remains constant). Has
anyone seen anything like this before? I suspect a monitor/SMPS
problem, not a logic board problem. It has HV and vertical deflection
before shut down. Apple's service manual is useless.

 

[Jeff Wiseman]

You might want to make sure that there isn't a shutdown script in your startup folder (a popular joke around some offices). If you have the original CDROM that came with the iMac, try booting from it. If it lets you boot and run, then go check out the startup folder on your hard drive. Otherwise, if it still shuts down then you have a hardware issue. Also, try starting it up with the keyboard disconnected--a faulty powerswitch on the keyboard might create such a problem as well.

BTW, if you don't have the original CDROM (or any other that you can boot the iMac from) I believe that there is also a way to do a minimal power-up by holding down certain keys during powerup but I can't remember the sequence.

- Jeff

 

[Andy Cuffe]

You might want to make sure that there isn't a shutdown script in your startup folder (a popular joke around some offices). If you have the original CDROM that came with the iMac, try booting from it. If it lets you boot and run, then go check out the startup folder on your hard drive. Otherwise, if it still shuts down then you have a hardware issue. Also, try starting it up with the keyboard disconnected--a faulty powerswitch on the keyboard might create such a problem as well.

BTW, if you don't have the original CDROM (or any other that you can boot the iMac from) I believe that there is also a way to do a minimal power-up by holding down certain keys during powerup but I can't remember the sequence.

I know it's a hardware problem. It never gets far enough to detect the
hard drive, or even have the monitor warm up. I suspect that the
monitor/power supply is at fault.

 

[Dbowey]

baltimore posted:

I know it's a hardware problem. It never gets far enough to detect the
hard drive, or even have the monitor warm up. I suspect that the
monitor/power supply is at fault.

Tell us what it does do. Do you hear the startup chimes? Then what?

What model Mac? What system do you have installed? Do you have a startup CD?
Have you checked the battery? My old 638 CDV failed to start when the battery
failed.

Is your "psu.edu" Portland State? If yes, I could loan you a startup disk and
some help. I have a copy of every Mac System since 6.0.4.

Don

 

[Andy Cuffe]

baltimore posted:

Tell us what it does do. Do you hear the startup chimes? Then what?

What model Mac? What system do you have installed? Do you have a startup CD?
Have you checked the battery? My old 638 CDV failed to start when the battery
failed.

Is your "psu.edu" Portland State? If yes, I could loan you a startup disk and
some help. I have a copy of every Mac System since 6.0.4.

Don

See my original post. The start up chime is normal, then about 5
seconds later (before the monitor warms up), the hard drive stops and
the HV shuts off. The power light stays green, but dims slightly. The
battery measures 3.6v. It's not my computer, so I can't say what's
installed, or how the problem started. It can't be software because it
does the same thing with the hard drive unplugged. Looks like a power
supply problem to me, but everything looks good until it partially shuts
down.

I'm at Penn State and have access to copies of the OS.

 

[Dbowey]

Andy posted:

It can't be software because it
does the same thing with the hard drive unplugged.

Unless you have a system on some device other than the HD, you must have the HD
plugged in because that's where the system resides. By the way, Macs I have
owned would boot and run without a monitor attached.

Don

 

[Andy Cuffe]

Andy posted:

Unless you have a system on some device other than the HD, you must have the HD
plugged in because that's where the system resides. By the way, Macs I have
owned would boot and run without a monitor attached.

Don

I know it won't boot without a hard drive. The problem is that the
computer partially suts off before trying to boot. I should display the
'?' icon untl it finds something to boot from. The monitor of an imac
is also the power supply, so if it fails, the rest of the computer won't
work. The 3.3v supply drops to about 2v when the monitor shuts down.

 

[Dbowey]

The monitor of an imac is also the power supply, so....

No, the monitor just happens to be in the same case with the cpu motherboard
and power supply.

Don

 

[Andy Cuffe]

No, the monitor just happens to be in the same case with the cpu motherboard
and power supply.

Don

The moitor's power supply also supplies power to the logic board in this
imac.

 

[rubik]

I'm working on an orange slot loading imac that powers on normally with
the turn on chime, but shuts down about 5 seconds later. The power
light stays on (green), but the monitor and hard drive shut down. The
5v and 3.3v supplies to the logic board drop noticeably when the monitor
shuts down, but this may be due to the sudden loss of most of the load
on the power supply (the B+ to the monitor remains constant). Has
anyone seen anything like this before? I suspect a monitor/SMPS
problem, not a logic board problem. It has HV and vertical deflection
before shut down. Apple's service manual is useless.
--

try this, open the back and unhook the video connect and connect it to

an external monitor if it powers and boots normall then its a problm
with the flyback transformer which would have to be unsoldered and
replaced, this was a common failing with a bunch of imacs.

rubik

 

[Andy Cuffe]

try this, open the back and unhook the video connect and connect it to

an external monitor if it powers and boots normall then its a problm
with the flyback transformer which would have to be unsoldered and
replaced, this was a common failing with a bunch of imacs.

rubik

I spent some more timeon it today and I seem to be making some
progress. For what ever reason, the monitor doesn't shut down now, but
there are still problems. As soon as the monitor warms up it displays a
raster with the pin balance badly misadjusted. The picture sides look
like this )). The picture also has poor white balance and smears a lot
unless the G2 is increased until the picture is much too bright. I
connected the display adjustment memory jumper, booted from a CD and ran
the display adjustment utility. As soon as the utility started (before
I tried to adjust anything) the picture returned to normal. Gray scale
and geometry were all reset to being perfect, so it couldn't have just
restored the factory defaults. The good settings hold until the
computer is restarted, then is returns to the way it was before (yes, I
did save the settings in the adjustment utility). Obviously there's a
problem with the monitor control circuit, or EEPROM. It seems that
some, or all the factory settings get read incorrectly from the EEPROM
on turn on, but they are still accessable to the display software. I'm
going to investigate the CRT board which seems to contain all the
control electronics. It can't be a software problem becuase it happens
before it's even started to boot.

 

[Laurence Dishman]

I spent some more timeon it today and I seem to be making some
progress. For what ever reason, the monitor doesn't shut down now, but
there are still problems. As soon as the monitor warms up it displays a
raster with the pin balance badly misadjusted. The picture sides look
like this )). The picture also has poor white balance and smears a lot
unless the G2 is increased until the picture is much too bright. I
connected the display adjustment memory jumper, booted from a CD and ran
the display adjustment utility. As soon as the utility started (before
I tried to adjust anything) the picture returned to normal. Gray scale
and geometry were all reset to being perfect, so it couldn't have just
restored the factory defaults. The good settings hold until the
computer is restarted, then is returns to the way it was before (yes, I
did save the settings in the adjustment utility). Obviously there's a
problem with the monitor control circuit, or EEPROM. It seems that
some, or all the factory settings get read incorrectly from the EEPROM
on turn on, but they are still accessable to the display software. I'm
going to investigate the CRT board which seems to contain all the
control electronics. It can't be a software problem becuase it happens
before it's even started to boot.

You can get a normal image on the monitor - Good - that indicates that
the hardware is functioning normally. But the software doesn't seem
to hold the settings - Hmmmmmm - Try patching the Apple Open Firmware.
There's probably an update at the Apple.com site.

 

[Andy Cuffe]

You can get a normal image on the monitor - Good - that indicates that
the hardware is functioning normally. But the software doesn't seem
to hold the settings - Hmmmmmm - Try patching the Apple Open Firmware.
There's probably an update at the Apple.com site.

I replaced several high ESR caps on the CRT board, went into the
adjustment utility and changed all the setting by at least one value to
force it to update all EEPROM settings and then I updated the firmware.
So far it seems to be working properly again. I'm not sure why, but I
think the firmware update was what fixed it. I can't imagine why this
would be the case though. It had an up to date firmware version for the
OS installed and it would have been impossible to do a firmware update
normally because it went into ABL shutdown unless the screen control was
turned way down. My only guess is that the firmware became corrupted.
The owner said it happend when he installed some software. A virus
maybe?

Thanks for everyone's suggestions.