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Overclocking PM 6500

MultiFinder

Well-known member
Does anybody know if there's any way to overclock this machine? It's a wonderful machine, but it tends to lag a bit on some of the CPU-intensive tasks. Any pointers from anybody? I've googled a bit, but alot of the links that I've been finding are dead :(

 

alk

Well-known member
Get an L2 G3 accelerator.

That's about the only help you're going to get for your 6500.

Given that you've got a 6500/225, I you could probably get to 250 MHz without too much trouble. But anything more than that would probably be a crapshoot. I doubt 50 MHz will be possible, and I'd bet good money that 75 MHz will be a non-starter.

The 603e supports up to a 6x bus multiplier giving you 300 MHz on a 50 MHz bus. That also happens to be the same speed as a production model 6500, so there's not much point in overclocking yours when you could just buy a 300 MHz board. They cost about $20 - $30 if you are patient. If you are still considering overclocking, the only other thing you can do is increase the bus speed by swapping oscillators.

You could conceivably get a 55 MHz bus giving you up to a 330 MHz 6500, but anything more than that would be severly pushing the Alchemy board's various components that were designed to work at 50 MHz.

Go here for ideas: http://www.bekkoame.ne.jp/~t-imai/pm55e1.html

I've never been one to poo-poo anyone wanting to overclock his/her Mac, but you do have to be realistic about it. Overclocking a 6500 just isn't practical when an L2 G3 accelerator is so much faster and relatively inexpensive. Also, the 6500 has poor cooling, so overclocking it may not be a smart idea. Finally, you'll only get a few MHz out of the deal, so it's not really much of a bargain if you are already struggling with CPU intensive tasks.

So in short, just get a new CPU. Don't bother overclocking yours.

Peace,

Drew

 

tomlee59

Well-known member
I'll have to second Drew's recommendation. The speedup that you can practically expect is too small to make a real difference. Assuming that you've already put the maximum amount of RAM into that thing, you've probably done about as much as you can, realistically. And if you haven't loaded up on RAM, then do that. It's the biggest bang/buck/minute ratio thing you could do.

Then go find a G3 accelerator. :)

 

Bunsen

Admin-Witchfinder-General
Pretty comparable. The L2 cache slot is basically a pin for pin shortcut to the CPU slot.

 

Bunsen

Admin-Witchfinder-General
And if you do OC the motherboard, you'll speed boost the G3 as well [:D] ]'>

 

Quadraman

Well-known member
If you're going to buy a G3 accelerator, you might want to try to find a beige G3 for local pickup instead. The upgrade path is better and you can run Tiger on it using XPostfacto, which doesn't currently work on a 6500. Any of the beige PCI Powermacs from the 7x00, 8x00, 9x00 generation would also be a good choice as the expandability options and ability to install Tiger are also there. The only one I wouldn't really buy would be the 7200 with the 601 soldered to the motherboard as that really hinders upgrading unless you swap a 7300/7500/7600/G3 motherboard into it.

 

Quadraman

Well-known member
The only one I wouldn't really buy would be the 7200 with the 601 soldered to the motherboard...
the 8200 also has the proc soldered to the mobo. ;)
That's true. I forgot about that one (I never remember it, probably because it's the only one of the PCI Powermacs I have never actually seen up close). It is pretty much the same motherboard used in the 7200 only in an 8500 case.

Oh, geez, no wonder I never saw one before, I just did some checking and it wasn't sold in the USA. I'll probably never find one now. :'(

 

Bolle

Well-known member
i have one :p , but its not really worth having one. the 7200 would be a much better choice.

 

TylerEss

Well-known member
but 7.{5,6} would fairly fly on a Newer 300MHz in a 6500. :)

You might consider the following mod if you want the greatest G3 performance: crank the CPU multiplier down as far as it will go (for stability and cool running) and then get a few 4-pin oscillators and start overclocking the system bus.

I bet you could get the bus speed up to 60MHz if you were lucky, and I'll further wager that the PCI speed is fixed at 33MHz, since 33 isn't a good multiple of 40 or 50 (the bus speeds of the Alchemy / Gazelle machines).

The 6400 Zone has the info you're looking for about overclocking: http://www.zone6400.com/6400clocking.html

If you wanted to really be sure, you could get several speed grades of oscillators, and benchmark the Mac as you slowly overclocked the bus.

NewerTech put out a whitepaper talking about how PowerLogix and XLR8 were slowing peoples Macs down by overclocking the bus. They claimed that the memory controller added additional wait states to keep the memory accesses within spec as the bus was raised. If Apple really overengineered the chipset then that might be true, but it smells like so much FUD (Fear, Uncertainty, Doubt) to me. A test like that would put the rumor to bed, at least for the case of the 6X00.

 
NewerTech put out a whitepaper talking about how PowerLogix and XLR8 were slowing peoples Macs down by overclocking the bus. They claimed that the memory controller added additional wait states to keep the memory accesses within spec as the bus was raised. If Apple really overengineered the chipset then that might be true, but it smells like so much FUD (Fear, Uncertainty, Doubt) to me. A test like that would put the rumor to bed, at least for the case of the 6X00.
This is actually interesting. Today I was experimenting on one of my Beige G3s with overclocking. It's a 233 MHz model with 66 MHz bus. I was able to overclock it to 300 MHz / 66 MHz bus without any problems. At 333 MHz / 66 MHz bus, it froze up during the boot screen.

Then I decided to mess around with the bus speed. I set it to 292 MHz / 83 MHz bus and it booted fine but programs kept crashing. Turns out it had a 64 MB PC66 chip so I took it out. The other 2 chips are 256 MB PC100 so it was not a great loss. That fixed the stability but when I ran Xbench and stuff, I noticed that memory was slower, threads were slower, and overall it was slower and just not as good as 300 / 66.

So it may be true that for some reason overclocking the bus causes some kind of slowdown at least on Beige G3 and possibly some other Macs.

 

MacJunky

Well-known member
IIRC it has to run the RAM slower at higher bus speeds. Perhaps madmax can chime in in this with his experiences in this exact issue?

 
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