PowerMac 7500 Speedup

The daughter card in the Apple PowerMac 7500 is built around a PowerPC 601+ processor running at 100MHz. Unlike many other Macs where the processor is driven by an oscillator or clock generator, this daughter card uses a crystal. It comes with a 25MHz crystal which generates the 50MHz bus (BCLK), and 100MHz processor speed (PCLK). If you remove the 25MHz crystal and replace it with a faster one, everything will be fine up to around 28.735MHz which gives a 114.94MHz processor.

Above that the card appears to run at about half the expected speed. So a 30MHz crystal gives a 53MHz processor. I do not completely understand the results, and I'll look into it next time I get my hands on one to play with.

One thing to keep in mind is that most boards are not designed to run much faster than 50MHz (BCLK), so unless the bus: processor ratio can be changed from 1:2 to 1:3, this is probably the max for the card even though we have seen that 100MHz PowerPC 601+'s can often make it to 120MHz.

Another little tidbit is that the mach gestalt reported by a 7500 is 68. If you pull the LED/jumper the mach gestalt returned is 69, the one reported by a 8500. This is just like the LED/jumper in the Q800/Q650. This does not appear to have any other effect.

This information is based primarily on e-mail correspondence with Rory Christianson (rmkc@portal.ca) who first did the modification.