[X4U] Re: Please save an iMac from the grave

[Jim Scott]

On May 20, 2007, at 10:42 AM, Kenneth Philbrick wrote:

> I have a G3 iMac/600 which has run perfectly since new, requiring  
> only a couple of hard drives. Now it is failing, and I would very  
> much like some tips and suggestions before removing the drive and  
> sending it off to iMac heaven.
>
> The symptom is that unannounced it will die completely—no screen,  
> no nothing; except the power light glows faintly. The restart  
> button does nothing; the only thing I have found is to press and  
> hold the power button for a few seconds and it will flash bright  
> and then go dark.
>
> If you leave the machine for at least ten minutes in this dark  
> state it will start up normally and run for an unpredictable length  
> of time; most often several minutes, but sometimes all day.
>
> After repeated attempts I kept it going long enough to run the  
> hardware check disk and it reported all systems to be functioning,  
> most importantly (to me) the logic board and memory.
>
> I truly do not want to trash this machine as it makes a perfect  
> backup for my type of use. Given its value little more than an  
> inexpensive part and my labor can be justified.  Any ideas will be  
> most welcome.
>
> Ken Philbrick

First, check the PRAM battery. If it's the original, it's probably  
dead or close to it. If it has 3.5 volts or better, it's OK.

Next, if the battery's OK or new, then unplug the machine and let it  
sit for a half hour or so. Then press the cuda button (it's inside  
the RAM door opening and to the left and down a bit) ONCE. Then plug  
the iMac back into AC power, and do a start and a PRAM reset. Let it  
chime/gong/whatever at least twice, then release the Option/Command/P/ 
R keys. All of this restores PRAM-keeping capability and resets the  
PMU/logic board.

If it starts and runs normally, you've achieved the inexpensive/free  
labor fix. If not, then you're probably looking at a replacement of  
the flyback transformer (relatively cheap but time-consuming and  
dangerous if done by yourself) or the PAV assembly (power/analog/ 
video boards). Either way, you're looking at total disassembly of the  
unit to fix the problem.

Your description of the symptoms point to the flyback transformer.  
What happens is that the fine wire windings inside the little black  
canister lose their insulation over time as a result of heat up/cool  
down cycles. In a failing unit, everything's OK until the heat  
buildup expands the wires enough to create a short, which shuts off  
power to the screen and shuts down the computer. Leave it alone for a  
while, things cool down, and it starts normally again. This also  
could be caused by a capacitor or two on the analog/power portion of  
the PAV, but your description fits my own experience that it's the  
flyback transformer.

Flybacks can be bought for about $35, shipped. But it's a dangerous  
job because you must discharge the potentially lethal voltage in the  
CRT as part of the disassembly process.

PAV boards can be bought on eBay for less than $50 these days. Just  
make sure you've got the right one for your iMac. There were two  
models: one without a switch for LG CRTs only, and one with a switch  
that accommodated both LG and the other monitor manufacturer, whose  
name I don't recall.

Do some Googling for iMac flyback transformer replacements, and  
you'll learn all you need to know.

Good luck.

Jim Scott