[Success] Trayloader iMac G3 100MHz bus clocking

[pete1]

I just wanted to show how I’ve successfully modded an iMac G3 trayloader to run a 100MHz bus.

Now I don't actually own an iMac but I acquired a Rev. A logic board and 233MHz daughtercard and rigged up some harnesses to run it headless.

The first step once I got it up and running was to swap the 233MHz PPC750 for a 350MHz 750L along with its cache chips, and then swap the 66MHz rated XPC106 chip with a 100MHz rated part. This went smoothly.

Next, I adjusted the MPC932 clock generator to set the bus multiplier to 3x. It takes a 33.33MHz reference from the SC608 clock generator on the main logic board, so I knew that a 3x setting should drive the CPU and RAM at 100MHz.

Move R10 to R11:

This change to 3x needs to be reflected in the Grackle PLL configuration. The four 1K resistors in the following photo are linked to GRKLPLL(0) to (3) on the XPC106.

Move R19 to R18 and R16 to R17:

In my case, no change was needed to the CPU clock multiplier, because the 3.5x stock setting runs the 350MHz replacement chip at its stock speed with a 100MHz bus.

Here’s a Gauge Pro screenshot of the modded machine up and running:

I (and another user who carried out the mod) noticed that at 100MHz the iMac is more picky about booting with certain larger RAM modules, particularly in its top slot. Sometimes, it would chime but the display wouldn't come on. However, using two matched Crucial PC133 modules with low density chips I seem to have no problems running stable at 512MB:

Here are the modules I used:

Just for fun, I bumped the PCI clock to get to a bus speed of 110MHz, which is pretty impressive for a machine originally designed to run only at 66MHz. (Note: You can get to 105MHz just by setting the SC608 logic board clock generator to 35MHz PCI, but beyond that you need to install a faster reference crystal. This introduces problems with video and USB.)

I reckon the same mod could be applied to the Wallstreet and Lombard PowerBooks, whose CPU card is similar.

 

[TheShortTimer]

That's really descriptive overview of the work that's involved.

How much of a performance boost is gained from increasing the bus speed to 100Mhz?

 

[pete1]

That's really descriptive overview of the work that's involved.

How much of a performance boost is gained from increasing the bus speed to 100Mhz?

I haven’t benchmarked the setup yet and you can’t really tell during casual use.
What sort of performance testing would you like to see? Maybe Norton System Info & MacBench 5 under OS9 and Xbench under OS X? I don’t think Quake3 will run on the lowly Rage IIc chip, so no timedemo possible.

 

[TheShortTimer]

I haven’t benchmarked the setup yet and you can’t really tell during casual use.
What sort of performance testing would you like to see? Maybe Norton System Info & MacBench 5 under OS9 and Xbench under OS X? I don’t think Quake3 will run on the lowly Rage IIc chip, so no timedemo possible.

Whichever takes your fancy.

If you're willing to test all/some of those then please do so. I'll be interested to seeing whatever you share. I'm also curious as to what tempted you to experiment with increasing the bus speed?

 

[pete1]

Whichever takes your fancy.

If you're willing to test all/some of those then please do so. I'll be interested to seeing whatever you share. I'm also curious as to what tempted you to experiment with increasing the bus speed?

I'll do it, since I'm interested to see the difference too. I'll run the CPU at 300MHz at both 66 and 100MHz bus speeds in order to keep the test fair. My 350 chip won't do 400.

As to why, well like many of us here, I enjoy unlocking the full potential of my favourite vintage machines. I'm not a special fan of the original iMac, though I recognise and appreciate its status as an iconic machine. I got involved as a kind of side quest of the [so far unsuccessful] 100MHz bus mod experiments for the Beige G3. The architecture is really similar, so I was interested to see what I could learn from modding the iMac to advance the Beige project.

 

[TheShortTimer]

I'll do it, since I'm interested to see the difference too. I'll run the CPU at 300MHz at both 66 and 100MHz bus speeds in order to keep the test fair. My 350 chip won't do 400.

Thanks, I appreciate this and I'm sure others will too.

As to why, well like many of us here, I enjoy unlocking the full potential of my favourite vintage machines.

Same here and I've bought gear a few times just for the challenge of seeing what I could with them/how far they can be pushed beyond their original design remit. To paraphrase Walter Bagehot, the greatest pleasures in life come from doing what others say cannot be done.

I'm not a special fan of the original iMac, though I recognise and appreciate its status as an iconic machine. I got involved as a kind of side quest of the [so far unsuccessful] 100MHz bus mod experiments for the Beige G3. The architecture is really similar, so I was interested to see what I could learn from modding the iMac to advance the Beige project.

I bought a 350Mhz iMac G3 for similar reasons and I've great fun pushing it beyond what Jobs and Apple's engineers would've ever conceived. Oh and by the way, it's always nice to encounter a fellow Londoner.