On Feb 7, 2010, at 10:15 AM, Bruce Johnson wrote: > > On Feb 6, 2010, at 11:26 PM, Jim Scott wrote: > >> Try an Open Firmware reset of the video RAM. >> > snip >> set-defaults >> reset-nvram >> reset-all > > > Just to head off future confusion, I'll be pedantic here. > > This procedure dies NOT reset the 'video RAM'. That is volatile, its contents > go away when the computer shuts down. > > the 'nvram' stands for 'Non-Volatile RAM', that is, stuff that stays > remembered between shutdowns, and is stored in special memory chips on the > computer, kept alive by the PRAM battery. (PRAM, stands for 'Parameter RAM' > and is another name for NVRAM, as it stores the parameters the computer needs > to boot up to it's previous state, hardware wise, like video settings, boot > disk, and the current date and time, when they're set differently than the > defaults. If a Mac ever gets past the initial boot stage (you see the gray > apple or the gear) the PRAM is not the problem. > > This procedure WILL reset the video settings, so it'll work IF the problem is > that the display settings are set to something the display cannot handle; > however, I've never seen mis-set screens go all white, they usually switch > off...what typically happens is the computer 'bongs' the screen comes up, > then immediately switches off. Some monitors (the iMac screen does not do > this afaik) will display a warning that the signal is out of range. > > Having to do things like this is most often a sign that the battery that > maintains these settings is dying. The lower than nominal voltage makes it > easier for the data in the nvram to get corrupted. > > Some Macs will boot just fine without such a battery (at the cost of having > the boot volume, display settings and date and time reset to the default > every boot time) but most require that trickle of voltage to turn on. >
Bruce, Thanks for the clarification. Since you are a professional, your knowledge and experience, and especially your ability to express things clearly, are always appreciated. Thank you. My terminology wasn't correct, but you correctly state my real objective, which was to give an Open Firmware path to "reset the video settings," which aren't that difficult to corrupt. The causes of corrupted video settings depend on the computer's architecture as much as they do on things like improper shutdowns, faulty hardware and the presence of a clock/PRAM battery. White iBooks, for example, don't have a clock battery. They also tend to have video chip problems. That is why the Open Firmware resets on a dual USB iBook often will bring up video on the screen again when all else fails. In my hobby, which is resuscitating Macs and giving them to kids, schools, the needy, etc., I deal almost exclusively with "troubled" Macs whose histories are unknown. If a Mac doesn't chime and boot to the desktop the first time I push the power button, or if it does boot but has "problems", I automatically do the Open Firmware resets noted above. Many times, that's all that is needed to clear things up. Several times, it has let a slotloading iMac with a white screen hang boot normally on restart. (Yes, CRT iMacs do not display a "signal out of range" warning.) Since most of the Macs I work on currently tend to be first- and second-generation iMacs, the very first thing I do with a "new" patient -- after that initial power-on test and Open Firmware resets -- is to test the battery for proper voltage. I actually had one yesterday that showed absolutely no voltage whatsoever. Usually there's still a little bit left. If a battery replacement is needed, I put in a new one, push the cuda button (only once!), then try to start the iMac again. If it boots and runs, I then watch carefully for color shifts (usually the video board), snaps/crackles/screen flashes/etc. (usually but not always the flyback transformer), and screen geometry. After 15-20 minutes of running time, I adjust the screen geometry to factory default and readjust the focus. I do all this in OS 9 since OS X's video demands make it impossible to get a "crisp" screen focus. iMac DV models have a built-in external VGA port, which I've used many times as a diagnostic tool. If the external video is OK and the built-in video screen isn't, then I know the problem isn't in the logic board but is somewhere in the PAV circuitry. So if the iMac DV 600 in this thread can deliver OK video to an external monitor so that the Open Firmware screen can be seen, then that alone has narrowed the white screen hang reasons considerably. Jim Scott -- You received this message because you are a member of the iMac Group, a group for those using Apple iMacs and eMacs. The list FAQ is at http://lowendmac.com/imac/list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/imaclist
