661-7170, ADP-300AF T, power supply troubleshooting

Hello all,

I understand the logic board (661-7156, 820-3298-A) has 4 LEDs, the first of which helps indicate the health of the power supply (661-7170, ADP-300AF T) by turning on and stay on when plugged into the mains power.

My LB has no LEDs on, indicating a bad PB. However, my PB's 8 PIN Molex to the LB correctly feeds 12V on 4 PINs and ground on the other 4 PINs, and that's testing on the LB.

There is also a 7 PIN ribbon cable connecting the PB to the LB. When testing on the LB, there is ground on 2 PINs, 3.3.V on 2, and then low mV on the other three. I'm trying to find info on what values are meant to be on each PIN, before and after the power button is pressed. I presume at least one PIN must change to trigger the MAC to turn on, one to indicate PB is good, and then some thing else for the third. However, I haven't found any info to verify each PIN.

When the power button is pressed it should go to 0 ohms, and it does indicating the button is working. From my research, there should be 3.3V to the power button when plugged into mains power, but mine does not.

I have also read there are 2 fuses on the board. One for the mains power input labelled "6.3A/240V" (In Australia) and it tests good which is obviously as there is 12V to the LB. But I haven't been able to find info on the second fuse's location. Does anyone know if there a second fuse and if so, where?

If in-fact there is a second fuse and it's blown, I reckon this is my culprit for a lack of 3.3V to the power button as well as indicating the PB is 'ok' and the LB to turn on LED #1. That signal would be via the 7 PIN ribbon.

Thanks for reading!