Repair manual

S4. iMac turns itself off 7 – 10 seconds after power-on, even if the

power cord was pulled and replaced. You will probably hear the

hard drive spin up briefly, and the screen (or external monitor)

may come on momentarily. You will probably not hear the

“thump” of the CRT coils loading up/relays switching nor the

crackle of static which can sometimes be heard as the high

tension voltage builds up in the CRT.

The symptoms may vary and be experienced in combination.

Some early iMacs (typically the Rev A – D slot loading models, 233 – 333

MHz CPU) may show a similar symptom to S4 whereby when the power

LED comes on it starts orange, turns green about 10 seconds later and

immediately turns off. There may also be a “zap” sound from inside. This

can be a symptom of monitor flyback transformer failure which is covered

in detail here. The procedures described in the linked document are

potentially dangerous so don’t to it unless you know what you are doing.

Some other screen problems (too bright or dim or out of focus) may be

resolved by referring to the “last resort” and “other tips” sections below.

What’s this firmware stuff?

Firmware is software loaded into flash memory on the motherboard. The

iMac firmware includes the code for fundamental processes in the iMac,

e.g. for booting up, for controlling the built-in PAV (Power Analog Video)

card.

The PAV card drives the CRT and includes the monitor flyback transformer

that steps-up the voltage to the many kilovolts the CRT needs and other

hefty looking capacitors etc.

On the logic boad (motherboard) there is a PMU (power management

unit). This is a microcontroller chip – a computer within a computer - that

controls all power functions for the computer, including the PAV board. It

includes firmware (some or all of the firmware described above).

The logic board also includes the PRAM (parameter RAM) that stores

certain system settings. The PRAM is maintained by the PMU.

We don’t know fully how all these parts work but suffice it to say that the

firmware, the PMU and the PRAM are all intimately connected.

We believe that when an iMac which doesn’t have the latest firmware

boots OS X 10.2 or higher (this happens when running the installer from

the CD, even before the new OS is installed) some of the settings in the

PRAM are somehow “scrambled”. As a result, the PMU loses the ability to

control the PAV board. In serious cases the PMU seems to think that the

PAV board is not working and shuts the iMac down several seconds after

the bootup chime. In lesser cases the screen may be blank or a strange

colour or dim. The symptoms may be seen immediately after the

encounter with OS X 10.2 (or higher) or some time later, depending on

the iMac model and possibly other factors.