68040 33mhz versus 25mhz

[luxor]

I'm ready to switch out the 68LC040 33mhz chip in my Mystic CC mobo for a 68040. There seem to be a few 25mhz chips on ebay but I can't find a 33mhz or 40mhz for a decent price.

Compared to the FPU-less 68LC040, would there be much of a noticeable performance or speed difference between the 25hz and the 33mhz 68040 versions?

 

[Gorgonops]

The speed that's stamped on a 68040 CPU is what it's rated to handle, not the speed it will attempt to run at; that's completely dictated by the logic board. If you put a "25mhz" 68040 in place of a 68LC040 that's running at 33mhz it will run at 33mhz, it just might not be reliable doing it.

Chances are a 25mhz chip will be "okay"; the speed ratings they stamp on the chips are based on very conservative testing and the actual silicon is the same. But there does exist the possibility that the chip won't be reliable at that speed, and if that turns out to be the case your only option will be to buy another one, unless someone here knows how to adjust the clock speed of the LC575 motherboard.

EDIT: Okay, here it says you can clock down the LC 575 motherboard by changing some resistors:

http://www.applefool.com/clockchipping/lc575.html

Note that they're itsy bitsy surface mount things.

The other possibility if it's not reliable might be to try putting a heat sink+fan on the 25mhz chip overclocked to 33mhz.

 

[Byrd]

I've never had any problems clocking an '040 25Mhz part to 33Mhz in several overclocked Macs, just put a small thin heatsink on top.  I didn't think you could adjust the LC575 board by resistor relocation - you needed to replace the oscillator - I suspect this is for an LC580 board?

 

[Gorgonops]

I didn't think you could adjust the LC575 board by resistor relocation - you needed to replace the oscillator - I suspect this is for an LC580 board?

No clue, just regurgitating the fact that a page saying you can do it exists. (It's also claimed that the pizza-box 475's use a resistor-adjustable clock generator and, well, they're differently shaped boards but the guts should basically be identical.)

 

[uniserver]

yup usually the full 25mhz 040's run fine at 33mhz... however, you will notice it will get quite warm, so you should get a 486 heatsink on there, or something to heat sink some of the heat away from the cpu,   I'v ran lc475's with full 25mhz 040's @33mhz, many times  but there is a fan right there next to the cpu, and i am sure that does help.   With a 575 in a CC or even a 575 in a 575,   you might want a heatsink.  with your IR temp gun you will see temps between F 128 and F 145 depending on what you are doing,,, witch are considered safe temps more then likely.  Just with all this stuff being vintage and all, might make it last longer... ?

 

[Paralel]

You might want to try to find a Chinese chip company that still sells the 68040's, yes, there are still come companies that make new chips of this old design.

If you want to stick to strictly vintage chips, I would imagine you'll have a hard time finding a 40 Mhz at a reasonable price, from what I know they didn't make as many of them and they are significantly harder to find than their 25 Mhz & 33 Mhz siblings.

 

[luxor]

Thanks all for your comments!  I realized that the LC575 board bus is 33mhz, so I best use a 33 and avoid clock problems. After much searching I found a seller on ebay with reasonable prices.  He's got 25, 33 and 40 mhz chips.  I ended up ordering this one:
 

http://www.ebay.com/itm/371085061798?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I'll put it in with a 486 heat sink (hopefully fan wont be needed).  Hope it works!