The basic idea of Modification #1 is removing the onboard oscillator, and replacing it with a faster one. This is the mod most people use on the IIsi, IIfx, Q700, Q900, & Q950, and more recently on the PB and Duo's.
First find the crystal oscillator by referring to the previous
table and description of its physical characteristics. Be
careful when you remove the oscillator. Most people just use a
normal soldering iron, and are fine. A grounded (three prong
soldering iron) would be a bit safer. Use copper wick to soak
up the solder from all four pins, and pop out the proper
oscillator. Because the boards are multilayer, be careful not
to damage anything; be gentle. There was recently one report
of a guy who damaged his IIsi board doing this. But that was
the only incident I have ever heard of, and lots and lots of
people have done this. I use a "desoldering iron". They melt
the solder, and have an electric pump to suck out the solder
while you swirl the pin from the oscillator around to get all
the solder out. After you have done all 4, if you have done a
good job, the oscillator just pops out. If you have access to
one of these desoldering irons, I highly suggest you use it as it
does a cleaner job, and there is less risk of burning
(discoloring) the board.
Instead of putting a new oscillator straight onto the board, it is nice to use a socket so you can test your individual Mac, and see what the cutoff frequency is, and you can always put the original oscillator back in the socket.
Take a 14 pin IC socket, remove all the pins but 1,7,8, and 14, and solder it into the board. Make sure you put it in so pin 1 will go into pin 1, 2-2, 3-3, 4- 4. And the notch in the socket should face the same way the dot on the old oscillator was facing. Now just put in a faster oscillator.
I have done this to a few IIsi, and the highest frequency we could get to work without problems was 27.5MHz. Thus a speed increase from 20 to 27.5MHz. The actual crystal is 55MHz (double the frequency). 55MHz TTL crystal oscillators do exist, but they are rare. The thing most people seem to do is get a 55MHz CMOS oscillator, and they work just fine. At 58.9 and above, there are problems with the floppy drive; you cannot boot the Mac from a floppy, but other than that it is fine until just over 30MHz. I recently had a IIsi at 28.3MHz and it was fine. Be warned that some NuBus cards may not work after this modification. Most will work at 25MHz, but will not at 27.5MHz, so just stick with 25MHz if that is the case.
The IIsi and IIfx do not come with heatsinks, so to reduce the heat in the processor, get a small heat sink to attach to the 68030 to cool it down; any heat sink will do; the more surface area the faster heat will be dissipated. Be careful when you put on the heat sink. Typically you'll use some heat transfer grease, but the heat sink can slide off if the Mac is moved, and the heat sink might short something out. The best thing seems to be to get a heat sink with a hole in the middle, or drill one yourself, use the heat transfer grease, but also put a small drop of super glue through the hole in the heat sink onto the chip or put a drop on the side, and this should hold it in place. Super Glue is brittle enough that you can pop off the heatsink later if you need to. Fry's sells nice heat sink/fan combo's. They run $20 and up, and should keep the processor cooler. They come with a Y cable to tap into your hard drive power cable to power the fan. A more complete FAQ on this modification for a IIsi is available via anonymous ftp from sumex-aim.stanford.edu in info-mac/info/hdwr (iisi-25mhz-upgrade-faq.txt).
For the Quadra 700 and 900, you can get 70MHz TTL crystals from Fry's. The 70MHz may not work, and you may have to back down to 66.6666MHz, the next most common frequency. The Q700, Q900, and Q950 come with a heatsink installed. A more complete file on this modification for a Quadra 700 is available as quadra-700-clock-mod-145.txt.