Keep in mind that this will void your warranty. Dealers or Apple may refuse to accept the board as an exchange in or out of warranty with this modification. And there is really no reason to do it.
Unlike the modifications outlined in the Mac Crystal Oscillator Speedup History File I maintain where those computers use a TTL crystal oscillator to clock the processor, the LC575 uses a Clock Generator. Depending on the surface mount resistors used, this part will either put out a signal for 20, 25, 33, or 40MHz operation. Unfortunately the computer will not work at 40Mhz (in 040 or PPC mode)
| R111 (low) | R118 (high) | R119 (low) | R112 (high) | Pull | Speed |
|---|---|---|---|---|---|
| 301 | 301 | low, low | 20MHz | ||
| 472 | 301 | high, low | 25MHz | ||
| 301 | 472 | low, high | 33MHz | ||
| 472 | 472 | high, high | 40MHz |
The mach gestalt is a number that is specific to each type of Mac Apple makes. There are several unassigned mach gestalts, and this modification will explain a few of them:
| Speed | Gestalt |
|---|---|
| 20MHz | 87 |
| 25MHz | 91 |
| 33MHz | 92 |
I take it few of you will really want to drop down from 33MHz to 25 or 20MHz. At 40MHz, the Mac is unable to lock onto the frequency, and will not work. If you must try it to see it for yourself, you will need a 4.7k Ohm resistor. If you send a self addressed stamped envelope (SASE) to Output Enablers (oe@well.com) at 1678 Shattuck, Suite #247, Berkeley, CA 94709, they will send you one of these SMT resistors.