LC575 (P575, P577, P578) Slowdown

This is a modification that can be performed on a LC575, Performa 575, Performa 577, or Performa 578 to slow it down to 20 or 25MHz.

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)

There are four resistor pads used in this modification. Surface mount (SMT) resistors are placed in these pads to pull "low" or "high". The pads are on the bottom of the motherboard, and are in sets of two, so you need to have a resistor on one of the first two and one of the second two. The resistors are either 300 Ohms (301) or 4.7k Ohms (472). They are the "0805" surface mount package.

Resistor Pads

R111 (low) R118 (high) R119 (low) R112 (high) Pull Speed
low, low 20MHz
472 301
high, low 25MHz
472 low, high 33MHz
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:

mach Gestalts

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 ( at 871 Industrial Rd., Suite K, San Carlos, CA 94070, they will send you one of these SMT resistors.

There are some Speedometer 4.0 Files for a LC575 at 20, 25, and 33MHz on the Clock Chipping Home Page you may want to check out.