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 (alex@gerulf.acsu.unsw.edu.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, Apple p/n 343S1135 -a N, is located at position U17 on the motherboard. On at least one LC475, the clock generator is Apple p/n 343S0161 -a. 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 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

R22 (low) R21 (high) R25 (low) R24 (high) Pull Speed
low, low 20MHz
472 301
high, low 25MHz
472 low, high 33MHz
472 high, high 40MHz

If you monitor Pin 6 of U17, you will get the following:

Pull Pin 6 Speed
low, low 9.982MHz 19.964MHz
high, low 12.377MHz 24.754MHz
low, high 16.425MHz 32.85MHz
high, high 19.964MHz 39.928MHz

The Quadra 605, LC475, P475, and P476 use the same motherboard. A jumper in the front left hand corner, J18, 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:

New Gestalts

Speed J18 Installed J18 Removed
20MHz 93 86
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 (sales@outputenablers.com) at 871 Industrial Rd., Suite K, San Carlos, CA 94070, 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 should 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 can undo the modification, install System 7.5, and then redo the modification or hold down the option key while you launch the installer, and you will be able to do a custom install.

Just a small unrelated note...
The battery in the Q605/LC475 typically lasts just 2-3 years, and since the Q605/LC475 have now been out about that amount of time there have several cases where the battery goes dead. The symptom is that the date does not hold, and the video dies. If you run into this independent of having clock chipped your machine, get a new 3.6 volt lithium battery. The battery is used to store the settings in the PRAM.

At the normal 25MHz, you can use both 640x480@67Hz and 832x624@75Hz on a 15" multiscan monitor, and 640x480@67Hz, 832x624@75Hz, 1024x768@75Hz, and 1152x870@75Hz on a 20" multiscan monitor without any problems. When you accelerate your machine by the above modification you will still be fine at 33MHz. The current clip-on modifications however for the 605/475's only allow you to use 640x480@67Hz.