Re: iMAC G5 ALS PowerSupply API4PC46-290 REV.A R37 near 3845B

Have you checked to see if there is a short in the power factor controller in the primary side. I've found about 10% of these have a blown PFC chip.

Also I found in some cases, that the 3.3V rail is pulled low. It appears the housekeeping circuit uses this as a reference voltage and the PSU increases the output, but by doing so over volts the 5v and 12v rails, which get detected and then the PSU instantly shuts down. This all happens in a split second, but can be viewed using a digital storage scope.

Re: AcBel 9841 problems

Hi,

There are effectively 2 PSUs in one with this power supply. The Gigabit G4's used a 28V start up because of them running an ADC monitor directly from the graphics card. The two small daughter boards on the primary side each have their own PWM on them. The larger board is for the 28V supply. The noise and shut down is caused by the SMD caps going out of tolerance in the RC circuit of the PWM making is ocsillate at too high a frequency. I've repaired hundreds of these over the years, but not much call any more. Are you in UK, I could send...