FW800 G4 MDD CPU woes

[cjtmacclassic]

Hi everyone,
I picked up this FW800 MDD for $50 thinking it only had power supply issues. I replaced the PSU with a mini ITX power supply through wiring diagrams available online, but to no avail. On boot, I get no chime, no RAM beeps, just fans at full blast after several seconds.

The serial on the rear indicates it's a single CPU G4 at 1GHz, built in Feb. 2003. The CPU I found installed is a dual 867Mhz. There were 3 sticks of PC2700 installed when I got it.
After more digging online, I found that this specific G4 has no CPU upgrade path. I bought a single CPU card at 1GHz hoping to restore the machine.
I installed the card and this changed nothing whatsoever.

I've tried PC2100 memory, PC2700 memory, no memory, different slots, reseating, detaching and reattaching components one at a time - all to no avail.

At this point I can't really identify what combination of logic board and CPU this thing needs to work, or if the logic board itself is even functional. The lack of a startup chime or beeps indicating the absence or incompatibility of the RAM isn't great.

Can anyone confidently identify what logic board I need to compliment either of these CPUs? Firewire 800 isn't essential if it comes down to it.

 

[trag]

You are in the unenviable position of not having a known working set of hardware with which you can trade components one at a time.

So your problem could be anything, really, including the PS replacement.

There wasn't much in the way of 3rd party upgrades, but Apple made a variety of processor cards for this machine, ranging up to a dual 1.4 GHz G4, if I remember correctly. I think there was 867 MHz, 1 GHz, 1.25 GHz and 1.42 GHz in single and double processor versions.

Anyway, any of those processor cards should work in any MDD logic board, except....

There were two logic board bus speeds, settable by jumper or resistor on the logic board. I think it was 133MHz and 167 MHz, but I'm not sure.

Anyway, if the logic board and the CPU card are mismatched as far as the bus speed they are expecting, you could be trying to run the CPU card faster than it is capable of.

Also, the PC2100 vs. PC2700 corresponds to the slower and faster logic board speeds, but the PC2700 should work in both. That's DDR memory, right? Not DD2 or DDR3.

So it seems unlikely that all the memory you have is bad, especially if you've tried one stick at a time.

But that still leaves PS, logic board and CPU. It sounds like you've tried 2 CPU cards, so both of those being bad is probably less likely than just PS or just logic board bad, but again, if the logic board is set to 167MHz, it would try to drive those CPU cards faster than they're meant to go.

Sorry I didn't provide more detail on some of the things, but I would have to look them up. Hopefully the info about the logic board bus speed is still commonly available out there somewhere.

 

[cjtmacclassic]

Is there any way to identify bus speed visually? DVWarehouse lists items by a section of the logic board serial, in my case "P3G", which would appear to be a 133Mhz bus for 867Mhz dual CPUs. (167Mhz for faster CPUs)

Is this the correct choice for a theoretically compatible motherboard for the dual 867 CPU card?

https://www.dvwarehouse.com/661-2781-apple-logic-board-133mhz-for-power-mac-g4-fw-800-820-1474.html