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Reader-submitted feedback in regard to Giles Kennedy's iMac Firmware Problem Solver.

Go to Nathan's Submission

Go to Dave Doolittle's Submission

Disclaimer: We are not responsible for opinions presented, nor does sharing those opinions necessarily constitute agreement. Working on electrical equipment and/or around a CRT monitor can be deadly and should not be attempted without proper training.

This was submitted by Nathan, an Apple-certified repair tech, on November 19, 2005.

I have a little addition to make to your web page. Some more detail as to what goes wrong and why.

First, a little background to fill in the gaps...

The PAV board does everything video in the imac. (it also handles initial power input from the wall plug) It converts a VGA signal to the CRT. It has several modes it can be in, for various resolutions, scan rates, and color depths. Because it was designed for the iMac, it makes certain assumptions about the nature of the video signals coming from the MLB (main logic board). Normal monitors might take the signals with a grain of salt (a "sanity check" it's called) but the PAV board does not do a sanity check, it just goes with whatever the MLB says to do, since it never expects a bogus request to be made by the MLB.

The MLB uses the PRAM to tell it how to set up the playing field early in the boot process. PRAM holds settings that need to be changeable easily, but that also need to be maintained even when the computer is unplugged, such as video resolution, sound volume, etc. One thing it does is to set the MLB's video modes to the values the machine was at last time it was booted up. "Zapping pram" clears/resets many of the settings in PRAM, but not all of them. This is an important part of the problem.

When you boot an out of date firmware imac on jaguar (and maybe panther?) CDs, jaguar does a Bad Thing(tm). It displays a warning saying you need to upgrade your firmware, which is good, but unfortunately it also makes a minor adjustment in PRAM, changing video settings. These settings changes are meant to be interpreted (correctly) by the NEW firmware, and NOT the old firmware. It does not cause the display to adjust to the settings immediately, so all it's done is lit the fuse, to blow up on you later. This is Apple's mistake, it should NOT make this change without first checking firmware versions. I don't know why Apple let this issue go so long without patching their OS installer to behave properly.

Now, when it restarts, the PAV board resets, and receives video mode settings from the PRAM as its firmware directs it to do. Unfortunately, the old firmware does not know how to interpret these new video settings in PRAM, and sends bad settings to the PAV board. The PAV board then blindly tries to follow these settings, and that's where trouble starts.

Details are not entirely consistent, nor is the behavior of any given imac, but it appears that the initial video settings cause the display to go psychedelic, hard to say which setting for video is bad but something is way off base from what the PAV board can pull off, so it displays very badly. The good news is, the PAV board is usually ABLE to pull it off at least in a fashion, and the machine boots up. If you are lucky, you can reset your video back to a reasonable setting in the Monitors CP, or stumble your way through a firmware update right off.

Here's where trouble really starts. If you find your imac going psychedelic and decide to zap PRAM, only SOME video settings are changed by the zapping. Unfortunately, the resulting combination of old and new settings is beyond the capability of the PAV board. It blindly tries to follow them anyway, and as a result it creates an internal overload condition. (arguably not an immediate safety/fire hazard, but it's beyond the limits set in the hardware) Safety circuits on the PAV board trip, sending a signal to the PMU (power management unit) on the MLB, telling it to emergency shut down, which it does, basically telling the PAV board to kill power to the computer. This happens just as the video is being engaged, which accounts for the 7 second delay before shutdown. That's when the video is turned on, high voltage is applied, and the degaussing cables fire. (bong!)

I have read a procedure that someone developed to "hot-wire" the iMac's connector that goes between the MLB and the PAV board, to disconnect the safety trip signal. The result is a very psychedelic mac, that you may or may not be able to read enough to update your firmware with. Of course it may work better with an external monitor attached that's capable of coping with the flakey video signal. The important thing is the PAV isn't tripping its overload and shutting you down.

I doubt booting off an external hard drive (or changing out the internal drive) will help this problem in the least, since the shut-down occurs prior to anything being read off the hard drive. If you are still in the early stage of the problem, prior to zapping pram that is, it's possible the PAV board won't be able to pull off a readable display and you will have a black screen. (hopefully it's not shutting down as a result) Having an external monitor to attach at this point would probably let you see the screen and do the firmware update. (since those imacs lack sufficient VRAM to video span, you'd get mirroring instead, which would show you the desktop etc on the external display without any hacking or lucky mouse clicks required)

good luck!

- Nathan
(Apple certified repair tech)

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The following story was submitted by Dave Doolittle on November 29, 2005.

I do Mac tech support for a number of folks here on the Cape. Recently, a client asked me to run "some utilities" to speed up his graphite iMac which had been running rather slowly of late. I came over and inserted my Disk Warrior CD and rebooted the iMac. Too late did I notice it was the wrong version, the OS X version of Disk Warrior! It booted up into Panther and after rebooting, the iMac was dead... "Pantherized!!" Got a startup chime, then shut down after five seconds or so. Spent a few anxious hours surfing the web for tips and even drove to the Apple Store in Braintree to consult with their Apple Genius. He told me, "Sorry, you've fried the logic board and it's new computer time." Ouch! I went home and tried all of the usual stuff: yanked the battery, yanked the RAM, hit the PMU button, etc etc. Nothing worked. The iMac just would not boot up, even to a blank screen. It just chimed and shut down. So, finally, today, I was about to crack the case and remove its hard drive, when I thought, what the heck, it's good as dead already... why not try one more thing? So, I held down the Programmer's button on the side of the iMac while starting it up.. and bingo! It didn't shut down this time! The screen was black, but it was up and running to some extent. I hooked up a PC monitor to the iMac and saw the Open Firmware screen. This was pretty exciting, until I realized that it was not accepting any input from the keyboard or mouse. What now? I stuck my OS 9.2 CD into the slot, did the "3 finger salute" reboot, held down the C key and prayed harder than I have in a while.... and eureka! The CD booted the iMac successfully! I was able to do a clean install of the OS to upgrade it from 9.0 to 9.2, restart the Mac and then upgrade its firmware.


Either I'm living right or pressing that programmer's button was my lucky charm today. I have not seen any postings online about trying this method, so thought I'd share my story with you to add to your excellent page on this subject.

Dave Doolittle
Sandwich, MA
"The Mac Man"

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