Quadra 800 "Wombat" overclocking - options

phipli said:

Good morning @AlpineRaven. If you don't mind, I'll have a go at answering. I have a soft spot for Wombat family machines.


That's fun - my dad has a Quadra 650 in a IIcx case in a similar manner. One thing to start with - I've heard that DOS cards work in over clocked machines - but I don't have any personal experience of this. Keep that in mind. Hopefully someone else with experience can answer that, but I have always kept my Performa 630 DOS at stock bus speed just in case (I have another 630 that happily runs at 40MHz, so I'm not too fussed).


I'd add that there are more options within option C) (my site).
Option C-1 : Basic 40 MHz Overclock - Just Bridge J29 and nothing else. Either with solder, or a jumper.
Option C-2 : Adding 40 MHz Timings - Bridge J29 and remove resistors R151 and R152.
Option C-3 : The Ideal Setup - I believe this is what you described as Option C.

So to go through the options...

Option A - This modification is more difficult to do than option C-1, and provides no advantages. In fact - Option C-1, if you fit a jumper, is completely reversible by just reaching in and removing the jumper while the computer is off (keep in mind your DOS card), and so could be said to be better, without the difficult space envelope for soldering and risk of damaging the Nubus slot (it isn't a soldering skill issue - it is really tight up against plastic and hard to do the rework without damaging the Nubus slot - trust me - I've done two or three). The only advantage is that you can install a socket (again, tricky) to make it easier to swap the clock to higher values again if you want to go above 40MHz, or if you are starting from a 20MHz Centris 650 and want to turn it into a Quadra 650. Note that changing resistors R151 and R152 is just as much needed here (i.e. it is optional in that if you don't change them, you'll be overclocking your RAM / ROM / VRAM timings - which you might or might not want, and which might or might not be stable (it is usually stable).

TLDR - Option A is harder to do than option C (including C-1) and doesn't provide any advantages if all you want is 40MHz.

Option B - This provides flexibility - you can pick exactly what speed you want, higher or lower. As before, you still need to consider resistors R151 and R152 (their impact is independent of the method you use to change the speed). If you only want a 40MHz, I personally don't see why you would spend money and wait for shipping to do this.

TLDR - If you only want 40MHz, Option C-1 is cheaper, faster, easier.

Option C - As described above, there are three option Cs really. For your first step, what I'd suggest is soldering a 2 pin header (option C-1, but with the header, not just a blob of solder). This will mean by fitting a jumper (2.54mm - like an old fashioned hard disks SCSI ID jumper) you'll be able to select booting at 40MHz (with an overclock on the RAM, ROM and VRAM that will give you PowerPC 6100 like video performance)... but also allow you to just reach in and remove it if it is unstable, or doesn't work with your DOS card. If it is unstable, do the resistor mods I list on my website. That will give you a second jumper that lets you pick your RAM, ROM & VRAM timings between the ones for 33MHz and the ones for 40MHz. This should give you stable performance at both 33MHz and 40MHz as you've not actually overclocked the machine, but instead created an unreleased 40MHz Q800.

TLDR - I'd honestly just suggest doing C-1 for now, and then move to C-3 later.


Option C also lets you switch between 33 and 40MHz, just by removing a jumper if you fit one. C-3 also lets you switch between 33MHz and 40MHz timings.


This is only a problem if you move resistors to select 40MHz timings (i.e. remove the overclock from the RAM, ROM and VRAM). If you just clock up the CPU and run the RAM, ROM and VRAM faster than spec (due to still using 33MHz timings), the computer reports the same machine ID as normal - it will just report the same Machine ID as a 33MHz Quadra 800.

Note that when you have timings set for 40MHz, the 7.1 System Enabler wont recognise the computer... except... I recently made a modified version of the Enabler, I haven't shared it yet because I haven't had a chance to test it, but all I have done is added the extra computers, so it should be fine. Feel free to test it. I'll fetch it off my development emulator and upload it here shortly.

All later operating systems are fine running on a 40MHz reporting machine with the resistor mods, except that sometimes you need to use "Wish I Were" to run the installer, or hold option while launching the installer to bypass the machine check.

Hope this helps.

Edit - added the modified system enabler that should work with all unreleased wombat machine IDs. I didn't just add one - I added all the 650 and 800 variations, and even 610 variants. Note, this wont allow you to use 40MHz timings on a 610 as the ROM doesn't recognise 40MHz timings as a valid config (same as the LC 475) - you're stuck with 33MHz timings at most on a 610 (meaning I've struggled to get my 610 stable at 50 MHz, so I tend to only run it at 40 at most).

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