Ello there, this is my first post here. I've just made my first attempt of overclocking an FW800 MDD 1.25GHZ.

First thing I did was modify it to run at 1.5GHZ powered by 1.6V, it hung after only 15 minutes. Second attempt at 1.42GHZ (still 1.6V) hung after 38 minutes. Now I'm down to 1.33GHZ, 1.6V running it without problems for 1h20min. Needless to say I'm somewhat disappointed, especially after reading that the 'bulletproof' 7455 should be able to run without any problems at 1.5GHZ.

What I've noticed though, is that I've got dual 7455A's instead of the 7455B's found in all MDD overclocking guides on the net. Besides a different layout of te chip and a 3- instead of 2-phase voltage regulator, I couldn't seem to find any significant differences. Maybe I'm just unlucky or maybe it's the 7455A, what do you guys think?

WELCOME to MacMod! Good to see another mac user out there modding his system. Maestro (i think) has a lot of experience modding that system so he might chime in here in a little bit. Can you check the temp of your processor (not sure if MDD has those sensors) and also what kind of additional cooling have you used?

In fact, I've built my own independantly controlled air cooling system. The CPU is being cooled by a quiet 120mm fan and a modified 80mm fan to cool the upper part of the heatsink.

Temperatures °C (idle/load):
On-board CPU sensor: 44/47
Manually installed sensor: 47/49,5

Ambient temperature is about 24°C. My own sensor is in between the 2 cores and is secured to the heatsink.

Thanks for the welcoming by the way

Post edited by: yann86, at: 2005/12/18 17:57

Maybe someone can correct me if I'm wrong, but that's nothing out of the ordinary. My 1.6 G5 runs at 160 F/ 71 C OR MORE under load. Granted I don't think thats a very healthy temperature and am none too happy about it, but I can't see a G4 processor crashing due to temp at only 116 F/47 C.

I'm pretty sure it's not the temperature as it ran 50/55 before my mod and still on 1.25GHZ. I really haven't got a clue as to what is the cause of those crashes. Probably the CPU just can't handle anything above 1.33GHZ.

Some (relatively) good news: I've been running full load for about 4 hours now, no problems sighted.

you're stable @ 1.33?

It seems stable, I'm gonna let it run for 24hours straight just to make sure though.

Its the 7555A I bet. The A signifies it is an older chip than the ones in the guides you used which were fine to 1.5GHz. The longer a production run continues, the more the process gets refined and the higher the yield will be. Higher yield = more chips which will run at higher speeds. The fact that they changed the revision implies that they made a larger change to the process of manufacture than just the usual day-to-day tweaking.

Waragainstsleep wrote:


It's weird though as the FW800 is the most 'advanced' version of the powermac G4 (the MDD 2003 was actually the same as the 2002 version). The chip also has the 3-phase voltage regulator just like the 1.42GHZ model which is definately a 7455B.

I'd create a major load on the system. Play the demo of Return to Castle Wolfenstein that you can download for free.