Mac Crystal Oscillator Speedup History 2.6

June 1998

There has been a great deal of interest expressed over the net about these simple and inexpensive Macintosh modifications that yield 20-60% speed increases. Over the last five years I have been doing a fair amount of crystal oscillator swapping/acceleration on Mac's, and gathering information from others. I've made several posts to comp.sys.mac.hardware.misc with the bulk of this info and as new machines come out, and new concerns surface, I will try to add them to this history of the modifications, and post them on comp.sys.mac.hardware.misc. Included in this version are some more stats on the PowerMac's and on the Apple PPC PDS card in the Centris' and Quadra's. I've also started a Clock Chipping Home Page which has the latest history file, and several other useful files.

A little background:

All computers operate at a certain frequency with which operations are performed. Within a certain class of computers, for example Mac's with a 68030 processor, the higher the frequency, the higher frequency of operations processed, and the faster the computer provided there is no other speed effecting hardware like a cache or slow data path. The designer of the computer, Apple in this case, will use components that are rated at the same frequency or faster than the final computer will be. In this case the 68030's are made by Motorola. All 68030's are generally alike in what they do, but they are not alike in how fast they can do it. Motorola sells several 68030 processors rated at 16, 20, 25, 33, 40 and 50MHz for Mac's, accelerators and such. A large frequency difference will require a different mask during production of the processor, but small changes may not. Motorola only needs to guarantee that the chip they mark as 20MHz will function properly at 20MHz under a variety of conditions. Some chip vendors will test parts at different frequencies and sort the chips accordingly while others may just label the them at will and sell the chips at the different price as long as they are within the specifications. Because of this it is reasonable that the 20 and 25's actually come from the same batch, are separated on demand, and tested to make sure they will withstand that frequency. And thus it is reasonable that a 20MHz processor will function fine at a higher frequency, say 25MHz. Running the 20MHz part at 25MHz will generate more heat than at 20MHz, but no more than the 25MHz part if they came from the same production mask.

Many of the components in the computer need to be synchronized, so a fundamental frequency is generated by a crystal oscillator to synchronize them. Other parts like NuBus cards and video do not have to be the same frequency, so they may have separate crystal oscillators. A typical computer may have three crystal oscillators to clock different groups of components on the motherboard. Provided the components that are clocked by a particular crystal oscillator are capable of a speed increase, that crystal oscillator may be replaced with one of a higher frequency. How much a specific Mac can be sped up by this method depends on how the motherboard was designed, the components used, and what things the crystal oscillator that controls the processor also controls. With some of the newer Mac's, there are a few MHz differences in the top stable speeds reported for the same model, so part of this is luck of the draw. See the Clock Chipping Statistics page for more info on typical speeds attained.

This crystal oscillator swapping has been done for years, and some early computers even had jumpers that made it really easy to disable one oscillator and enable another higher frequency one. The first Mac's to be modified were the IIsi's. A stock IIsi's runs at 20MHz, and IIci's at 25MHz, and since the architecture of these machines was so similar it seemed reasonable to run a IIsi at IIci speeds. Another important factor was that earlier Mac's had just one crystal oscillator that controlled everything, and if you replaced it you would mess things up. The IIsi was different as some noted through its frequency deviation from its 8 and 16MHz precursors where the main frequency was halved and quartered to run the CPU, serial ports, video... The IIsi was different, it had 4 crystal oscillators, only one of which controlled the processor speed.

The Crystal Oscillator:

The type of crystal oscillator in the early Mac's is a full size, 14 pin package, TTL type crystal oscillator. It is a rectangular metal can, with approximate dimensions of 2.0 x 1.3cm and typically about 0.3-0.6cm high. All crystal oscillators have at least 4 pins. Some are numbered 1,2,3,4 and others 1,7,8,14. Pin 1 is always the pin next to the pointed edge (the others are rounded), with the dot, or next to the indentation on the newer CMOS, or surface mount crystal oscillators. With the pins facing down, put the dot, or indentation to your left, and the pin on the left, closest to you is pin 1. Going counter clockwise, pin 2 (or 7, depending on what numbering scheme) is to the right, pin 3(8) right side and further away, and 4(14) left side, and further away. Pin 1 on all the newer Mac's with surface mount crystal oscillators, and some of the older ones is an output enable/disable pin (OE). On some of the crystal oscillators you purchase pin 1 will be OE, yet on many it will not be used (no contact (NC)). It is not important which you get as you will not be using the output enable feature on the new oscillator. If you ground pin one with a jumper to pin 2(7) on an OE oscillator you disable the oscillator. Pin 2(7) is a ground. Pin 3(8) is the output. Pin 4(14) is the supply voltage, +5 VDC.

I've checked a few of the older type Mac's, and the oscillator on Mac Plus's is not OE, while the ones on the IIsi's and Quadra 700's are OE. Apple uses these Output Enable oscillators so they can disable the oscillator and input their own signal during testing. The Output Enable feature is not used in normal operation. Printed on the crystal oscillator will be its manufacturer, model number, and frequency.

The more recent Mac's use surface mount crystal oscillators that also have 4 pins, but they are in positions 3, 5, 10, and 12 if you follow the above 14 pin package notation.

On the early Macs, the processor runs at half the speed of the oscillator, so a 20MHz Mac IIsi has a 40MHz crystal oscillator. The more recent Centris, Quadra, and PowerMac computers use a crystal oscillator running at half the frequency of the computer, so a PowerMac 6100/60 comes with a 30MHz oscillator.

There are several different modification techniques. They will all give you the same final max speed. Some are just easier or more elegant than others. As with all these modifications, even though there may be no visible sign that you modified your Mac, you have voided the warranty on the Mac. As Apple states:

"This warranty does not apply if the product has been damaged by accident, abuse, misuse, or misapplication; if the product has been modified without the written permission of Apple; or if any Apple serial number has been removed or defaced."


I have done several PowerMacs, and their top speeds vary quite a bit. The max I ever got with a PM6100 was 90MHz, but that was a rare case. At 84MHz the PPC601 overheats quite rapidly. With a cool hairdryer cooling the heatsink on the 601, it worked fine, but was a bit noisy :-). A 12 volt DC 40mm x 40mm brushless fan will fit snugly inside the heatsink on the PPC601 chip.

Be very careful when you insert the fan into the heatsink on the PPC. If you press down too hard you may damage the processor itself, and destroy your computer. Be very gentle; if the fan will not slide in rest the fan on top of the heatsink, and gently separate the fins of the heatsink to allow the fan to drop into place.


The oscillator you need to grab onto in the PM7100 is located below the power supply and there is only a small amount of room. You will need to decapitate the clip prior to wiring it up to get it to fit.


You will need a similarly short clip for the PM8100 You will also need to shave off some of the plastic clip on the computer that holds the motherboard to the case if you use one of these clips.

Testing the Modification

See the Machine Specifics Page for information on the typical problems when you are going too fast. If the problem is with the serial ports, there is little you can do other than slow down a little. A problem with the serial ports will be obvious as your computer will hang up or crash when you access your modem or printer ports. There is a modification to fix the serial port problem on the C650, and with a little poking around one might be able to come up with modifications for the other machines to fix these serial port problems.

The video problem on the 660's typically appears as video redraw problems where the cursor leaves a trace, or the screen does not properly redraw when a window is closed. James Wang (, maintainer of the AV FAQ 1.6.1, recommends placing small heatsinks on the onboard VRAM to help cool them down as they get quite warm.

The speed of the memory is also important, so if you plan to boost your Mac very far, you may need faster SIMM's. It is said that composite SIMMs slow you down by about 10ns, so if you must get composite SIMMs, get them 10ns faster than you would have.

To test out the modification, the best thing to do is just use it a while. You can run Speedometer 4.02 (available via anonymous ftp from in /pub/info-mac/cfg) to see the changes. I use Snooper with the serial port loopback plugs to check the serial ports to find their limits; Snooper also tells you what frequency you are running at in round numbers. Snooper is no longer sold, but parts seem to have been incorporated into the latest version of Norton Utilities, 3.2.1. The last version of MacCheck, 1.0.5, properly reports the computer frequency, but Apple has pulled it from all their sites "due to high support call traffic". TattleTech 2.52 is available via anonymous ftp from in /dms, and it properly reports the speed of the processor.

If the clip is only half on the computer will not start up. It may be disabling the surface mount oscillator, but not replacing its signal. If this is the case, just remove the clip, reposition, and try again.

If the clip missed the surface mount oscillator the computer will start up, but at its normal frequency. If this is the case, remove the clip, reposition, and try again.

If the jumper on the clip is broken you will be feeding two frequencies into the PLL, and it will not be able to lock onto the frequency, and the computer will not start. To check this, remove the oscillator from the clip, and clip the clip onto the surface mount oscillator. Try to start the computer; it should not start. If it does, your jumper wire is broken, or you put the clip on wrong.

If your Mac does not give the standard chime at startup it means your clip is only half on, or the oscillator is too fast.

C650 Serial Port Modification:

Using the above clips, the max frequency for a Centris 650 is about 30MHz before you encounter serial port problems. Marlin Prowell ( following up on a hunch by James McPhail ( looked into the differences between the C650 and Q800 motherboards in hopes that a simple modification might enable the serial ports to function properly at 33MHz (Q800 normal frequency) or higher. On the bottom of the motherboard, under the IOSB chip, Marlin found two differences.
R151 is installed on the C650's, and is missing on the Q800's.
R152 is missing on the C650's, and is installed on the Q800.
R151 is a 300 ohms resistor and R152 is a 1.2k ohm resistor. Looking at the bottom of the board, with the back away from you, R151 is 3 3/4" from the right, and 3" down. The tabs for R152 are 4" from the right, and 3" down. R151 is black, and says 301 on it.

Marlin felt that R151 was glued to the board, and just using solder braid he was unable to remove the resistor for fear that prying it off may damage the two traces that run under it. Heating the resistor with a soldering iron Marlin was eventually able to soften the glue and remove the resistor. Or you can use James McPhail's two soldering iron Western technique with a soldering iron in each hand to heat each side simultaneously and flip the resistor off the board. Now just add the R152. Marlin suggests holding the surface mount resistor in place with a small screwdriver while soldering it to the exposed pads on the board.

Marlin has since used both the serial and modem ports error free while running his C650 at 40MHz, and MacCheck reports no problems. He has also checked to make sure the ethernet works, and it does. Since Marlin's initial modification, it has been confirmed by at least one hundred people. On some of these Mac's the CPU overheats after a while, so Marlin suggests you add a fan to dissipate the heat faster from the heatsink. You can also just run a bit slower, say 38MHz. You can purchase these 1.2k resistors from Digi-Key, but the minimum order is 200 of them. If you e-mail your US mailing address to Output Enablers at, they will send you a free resistor left over from Marlin's extra 199. OE has obtained many more as over 200 people have performed the modification with Marlin's spare resistors.

This modification makes your Mac think it has become a Quadra 650, and the Quadra 650 did not exist when most of you purchased your Centris 650. The System Enabler 040 that came with your Centris 650, version 1.0, will not work after this modification, and your Mac will not start up unless you have already updated the System Enabler 040 to version 1.1, the current version. The System Enabler 040 version 1.1 is available from your local Apple Dealer, or you can ftp it from in /dts/mac/sys.soft/7.1.system.enablers. Marlin suggests you also put the new Enabler on all your recovery utility disks as well so you will be prepared next time something goes wrong. The System Enabler is incorporated into System 7.5, so you need not obtain the new Enabler if you are running System 7.5.

If you have any questions or comments that should be added to this, feel free to e-mail me.
And a big thanks to everyone who has contributed to this file.

Marc Schrier

Disclaimer: I have been providing this info for quite some time, and I am now associated with Output Enablers. I would like to continue to update and maintain this unbiased information as long as there is interest on the internet. Any opinions represented here are mine, not necessarily those of Output Enablers.

Copyright © 1993-2012, Marc Schrier
Please contact me prior to distributing or reproducing this file.