Hi,

I think the CPU will work at 33MHz. If you like to make it work at 40MHz
you have to do the "Quadra Chipping". Info about this you can find here:

http://lowendmac.com/chip/quadra.html

On this page they say you can go with your Q650 as far as 44MHz with
proper cooling  (by changing the original 16,67MHz oscillator with a 22MHz
version). Using higher CPU speeds give instabilities or unusable serial
ports.

The LC575 will not work at 40MHz (http://homepage.mac.com/schrier/lc575.html)

Regards,

Johny.

P.S. IIRC am running a 33MHz 68040 instead of an 25Mhz 68LC040 version.


On Wed, 6 Sep 2006, Bryan Vyhmeister wrote:

> Since I can't seem to find a reasonably priced 68060-based system out
> there, I am looking at the CPU upgrade possibilities for 68k Macs. I
> have been reading FAQs all over the place about upgrading Mac CPUs. I
> have read that you can easily swap a 68LC040 out and put a 68040 in
> its place. I have not found a clear answer on one thing though. Can I
> put a faster 68040 chip in place of a slower 68LC040 chip? I just
> purchased an LC 575 motherboard for my Color Classic (which now has a
> Color Classic II board in it) and I want to replace its processor
> with a full 68040. It now has a 33 MHz 68LC040 and I want to put in a
> 40 MHz 68040. Would that work? I am thinking it will not but I am
> hoping it will. If that would work than I assume I could upgrade my
> Quadra 650s to 40 MHz 68040 chips as well.
>
> Bryan
>