Q605 (LC475, P475, P476) Speedup


This is a modification that can be performed on a Quadra 605, LC475, Performa 475, or Performa 476) to speed it up to 33MHz. The modification was uncovered by Alexander W. Chin (s9393063@cumulus.csd.unsw.oz.au) and I have confirmed his work prior to writing this up.

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.

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 Quadra 605 uses a Clock Generator. This clock generator is located at position U17 on the motherboard. Depending on the surface mount resistors used, Pin 6 of this part will either put out a signal for 20, 25, 33, or 40MHz operation.


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 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 300k Ohms (301) or 4.7k Ohms (472). They are the "0805" surface mount package.

R22 (low)  R21(high)  R25(low)  R24(high)
301 --- 301 --- low, low 20MHz
--- 472 301 --- high, low 25MHz
301 --- --- 472 low, high 33MHz
--- 472 --- 472 high, high 40MHz
If you monitor Pin 6 of U17, you will get the following:

low, low	9.982MHz	so 19.964MHz
high, low 12.377MHz so 24.754MHz
low, high 16.425MHz so 32.85MHz
high, high 19.964MHz so 39.928MHz

The Quadra 605, LC475, P475, and P476 use the same motherboard. A jumper in the front right hand corner, J118, differentiates the Quadra 605 and LC475. The Performa 475 and 476 are the same as the LC475. If the jumper is installed, it is a Quadra 605. If the jumper is not, it is a LC475. The system and other software know this from what is called a gestalt. The gestalt is a number that is specific to each type of Mac Apple makes. A Quadra 605 is gestalt 94 while a LC475 is gestalt 89. There are several unassigned gestalts, and this modification will explain a few of them:

     J118   Installed   Removed
20MHz 93 **TBA**
25MHz 94 89
33MHz 95 90
It makes no difference if the jumper is installed or not.

Prior to performing this modification you will need to update to system 7.5, or replace your system enabler with System Enabler 065, version 1.2.

I take it few of you will really want to drop down from 25MHz to 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 (oenabler@netcom.com) at 1678 Shattuck, Suite #247, Berkeley, CA 94709, they will send you one of these SMT resistors.

So for the rest of you who want to go from 25 to 33MHz, a 32% speedup, you just need to swap some resistors. The resistors have been glued onto the board prior to soldering, so you need to heat both sides of the SMT resistor at the same time with two soldering irons, and lift the resistor off with the irons. This will break the glue's bond to the SMT resistor. To solder on the resistor, place it on the pads (it does not matter which way it faces), hold it there with a small screw driver, and solder one side to the board at a time.

The modification:
1: remove the SMT resistor from R21, and solder it onto R24
2: remove the SMT resistor from R25, and solder it onto R22

That's it.


Keep in mind however that the 25MHz 68LC040 will now be running at 33MHz. The part is not rated to run at these speeds, but my experience with other Mac's and two Quadra 605's shows that this will not be a problem.
If you need the math coprocessor you may also want to replace the 68LC040 with a full 68040. I have done this, and it works fine.
You may also want to place a heat sink on the 68040. You have about 0.7", so any of those heatsinks will do, but if you mount a fan on it, get the 0.250" one.
There are some Speedometer 4.0 Files for a Q605 at 20, 25, and 33MHz on the Clock Chipping Home Page you may want to check out.

At 20, 25 and 33MHz I have checked the serial ports, video, floppy drive, SCSI drives, and RAM, and have observed no problems.

Important System 7.5 Updater note The system 7.5 installer will NOT work on a Q605 or LC475 accelerated to 33MHz. It will say:

The System Software 7.5 Installer Script does not recognize this Macintosh. Please use the original disks that came with your computer.
So if you plan to update to System 7.5, you MUST install it prior to performing the modification. If you have already installed System 7.5, that's fine. If you have already performed the modification, you MUST undo the modification, install System 7.5, and then redo the modification (or swap hard disks, and do the install on another Mac).