AGP Graphics --> Gigabit Ethernet upgrade possible?

[beachycove]

My G4 AGP Graphics server appears to be dead for some reason. I reset the Energy Saver panel to auto shut down and start up and, well, the shutting down part worked just fine. Now it won't start up, though it powers on and the l.b. LED is lit, disks are spinning.... I have reset the logic board several discreet times over a 10 day period, with long periods without any power, as detailed in the Service manual and as described in some old posts from eQuill, replaced the RAM, reseated cards, etc. — all to no avail.

I am assuming that the power manager "computer" in the computer is defunct. So I am wondering, if a replacement logic board must be had, could I pop in a Gigabit Ethernet logic board instead of an AGP Graphics? The processors, RAM, etc. are compatible. Is everything else?

 

[iMac600]

You can, although you do need to make a small change to the power supply to do so.

AGP Graphics Logic Board Power Connector

Gigabit Ethernet Logic Board Power Connector

The big difference is in the positioning of the trickle (TRKL) pin. On the AGP Graphics pinout, this pin is a +5v rail, second from the top on the left. On the Gigabit Ethernet pinout, another row of pins has been added to the top, and this pin is the +28v rail on the right. All you need to do is move the 5v wire from its current position on the Sawtooth PSU and connect it to the Gigabit Ethernet logic board where the 28v wire would normally go. Even though it's supplying a significantly lower voltage, it's still enough to start and run the machine, but ADC displays will not work, along with some higher voltage FireWire devices that draw power directly from the system.

The other wire that differs is the third pin down on the right. On the AGP Graphics pinout, this is a "POK" (Power OK) line. On the Gigabit Ethernet, this is a 3.3v line. I haven't modified this wire on mine and the computer still seems to work fine.

Of course, you could also just get a Gigabit Ethernet power supply to match and skip all the hassle completely.

Apart from the small power supply differences, the boards are otherwise interchangeable between models.

 

[beachycove]

Thanks for that excellent post. Now to find the part.