A1311/A1312 Power Supply
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The unibody iMacs have an AC power supply that can fail due to heat over time. If you scope the 12V output you will see quite a bit of noise and ripple. Be sure to use a digital ground, the chassis ground might give unexpected results. Also be sure to scope with the screen connected, the additional load of the screen causes some of the instability. This instability can cause the disappear backlight to go out intermittently or total. You may also see bulging caps.
There are two types of power supplies.
TBD: list caps needed for both PS