November 1994

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 speeds reported for the same model, so part of this is luck of the draw.

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."

Mac IIsi

Processor: 68030@20
Gestalt: 18
Initial Oscillator (MHz): 40
Max Oscillator (MHz): 50-55
Probs: Floppy Drive Controller

Processor: 68030@40
Gestalt: 13
Initial Oscillator (MHz): 80
Max Oscillator (MHz): 92.9405-100

Processor: 68LC040@20
Gestalt: 52
Initial Oscillator (MHz): 10
Max Oscillator with Serial (MHz): 14-14.31818
Max Oscillator w/o Serial (MHz): 15
Final Oscillator with PPC card & Serial (MHz): 14.2857
Probs: Serial Ports

Processor: 68040@25
Gestalt: 30
Initial Oscillator (MHz): 12.5
Max Oscillator with Serial (MHz): 14.31818-14.7456
Max Oscillator w/o Serial (MHz): 20
Probs: Serial Ports

Processor: 68040@25
Gestalt: 36
Initial Oscillator (MHz): 12.5
Max Oscillator with Serial (MHz): 19-20
Final Oscillator with PPC card & Serial (MHz): 19.44755
Probs: Processor (crash)

Processor: 68040@25
Gestalt: 60
Initial Oscillator (MHz): 12.5
Max Oscillator (MHz): 16-17.496
Probs: Video

Processor: 68LC040@25
Gestalt: 94
Gestalt at 20MHz: 93
Gestalt at 33MHz: 95
Q605 (LC475, P475, P476) Speedup

Processor: 68040@25
Gestalt: 53
Initial Oscillator (MHz): 12.5
Max Oscillator with Serial (MHz): 14.7456-15
Max Oscillator w/o Serial (MHz): 18.5
Final Oscillator with PPC card & Serial (MHz): 16.96
Probs: Serial Ports

Processor: 68040@33
Gestalt: 36
Initial Oscillator (MHz): 16.6667
Max Oscillator with Serial (MHz): 21-22
Max Oscillator w/o Serial (MHz): 24
Final Oscillator with PPC card & Serial (MHz): 19.6992
Probs: Serial Ports

Processor: 68040@25
Gestalt: 60
Initial Oscillator (MHz): 12.5
Max Oscillator (MHz): 16-17.496
Probs: Video

Processor: 68040@25
Gestalt: 22
Initial Oscillator (MHz): 50
Max Oscillator (MHz): 66-70-74
Final Oscillator with PPC card & Serial (MHz): 74

Processor: 68040@33
Gestalt: 35
Initial Oscillator (MHz): 16.6667
Max Oscillator with Serial (MHz): 20-21
Max Oscillator w/o Serial (MHz): 24
Final Oscillator with PPC card & Serial (MHz): 19.286
Probs: Serial Ports

Processor: 68040@40
Gestalt: 78
Initial Oscillator (MHz): 20
Max Oscillator (MHz): 23- 24
Probs: Serial Ports

Processor: 68040@25
Gestalt: 20
Initial Oscillator (MHz): 50
Max Oscillator (MHz): 66-70-74
Final Oscillator with PPC card & Serial (MHz): 74

Processor: 68040@33
Gestalt: 26
Initial Oscillator (MHz): 66
Max Oscillator (MHz): 80-92.9405
Final Oscillator with PPC card & Serial (MHz): 80

Processor: PPC 601@66, but runs at 60
Gestalt between 19.6608-60MHz: 75
Gestalt between 60.48-72.7272MHz: 100
Gestalt over 73.028MHz: 101
Initial Oscillator (MHz): 30
Max Oscillator (MHz): 40-42
Probs: Processor

Processor: PPC 601@66
Gestalt: 112
Gestalt at 60MHz: 111
Gestalt at 80MHz: 113
Initial Oscillator (MHz): 33
Max Oscillator (MHz): 40-42
Probs: Processor

Processor: PPC 601@80
Gestalt: 65
Gestalt at 60MHz: 61
Gestalt at 66MHz: 64
Gestalt at 100MHz: 66
Initial Oscillator (MHz): 40
Max Oscillator (MHz): 42-48
Probs: Processor

Processor: 68030@16
Gestalt: 25
Initial Oscillator (MHz): 33
Max Oscillator (MHz): 50

Processor: 68030@25
Gestalt: 134
Initial Oscillator (MHz): 50
Max Oscillator (MHz): 66

Processor: 68030@25
Gestalt: 29
Initial Oscillator (MHz): 50
Max Oscillator (MHz): 66

Processor: 68030@33
Gestalt: 32
Initial Oscillator (MHz): 66
Max Oscillator (MHz): 80

Processor: 68030@25
Gestalt: 27
Gestalt at 33MHz: 62
LCIII (P450) Speedup

Processor: 68LC040@25
Gestalt: 89
Gestalt at 20MHz: TBA
Gestalt at 33MHz: 90
Q605 (LC475, P475, P476) Speedup

Modification techniques:

Machine      Mod-1      Mod-2      Mod-3
IIsi          yes        yes        no
IIfx          yes        yes        no
C610          yes        no         yes
C650          yes        no         yes
C650-mod      yes        no         yes
C660av        yes        no         yes
Q605          no         no         no
Q610          yes        no         yes
Q650          yes        no         yes
Q660av        yes        no         yes
Q700          yes        yes        no
Q800          yes        no         yes
Q840av        yes        no         yes
Q900          yes        yes        no
Q950          yes        yes        no
PM6100        yes        no         yes
PM7100        yes        no         yes
PM8100        yes        no         yes
PB140         yes        no         no
PB160         yes        no         no
Duo 210       yes        no         no
Duo 230       yes        no         no
LCIII         no         no         no

Mod-1

The basic idea of Modification #1 is removing the onboard oscillator, and replacing it with a faster one. This is the mod most people use on the IIsi, IIfx, Q700, Q900, & Q950, and more recently on the PB and Duo's.

First find the crystal oscillator by referring to the previous table and description of its physical characteristics. Be careful when you remove the oscillator. Most people just use a normal soldering iron, and are fine. A grounded (three prong soldering iron) would be a bit safer. Use copper wick to soak up the solder from all four pins, and pop out the proper oscillator. Because the boards are multilayer, be careful not to damage anything; be gentle. There was recently one report of a guy who damaged his IIsi board doing this. But that was the only incident I have ever heard of, and lots and lots of people have done this. I use a "desoldering iron". They melt the solder, and have an electric pump to suck out the solder while you swirl the pin from the oscillator around to get all the solder out. After you have done all 4, if you have done a good job, the oscillator just pops out. If you have access to one of these desoldering irons, I highly suggest you use it as it does a cleaner job, and there is less risk of burning (discoloring) the board.
Instead of putting a new oscillator straight onto the board, it is nice to use a socket so you can test your individual Mac, and see what the cutoff frequency is, and you can always put the original oscillator back in the socket.
Take a 14 pin IC socket, remove all the pins but 1,7,8, and 14, and solder it into the board. Make sure you put it in so pin 1 will go into pin 1, 2-2, 3-3, 4- 4. And the notch in the socket should face the same way the dot on the old oscillator was facing. Now just put in a faster oscillator.

I have done this to a few IIsi, and the highest frequency we could get to work without problems was 27.5MHz. Thus a speed increase from 20 to 27.5MHz. The actual crystal is 55MHz (double the frequency). 55MHz TTL crystal oscillators do exist, but they are rare. The thing most people seem to do is get a 55MHz CMOS oscillator, and they work just fine. At 58.9 and above, there are problems with the floppy drive; you cannot boot the Mac from a floppy, but other than that it is fine until just over 30MHz. I recently had a IIsi at 28.3MHz and it was fine. Be warned that some NuBus cards may not work after this modification. Most will work at 25MHz, but will not at 27.5MHz, so just stick with 25MHz if that is the case.

The IIsi and IIfx do not come with heatsinks, so to reduce the heat in the processor, get a small heat sink to attach to the 68030 to cool it down; any heat sink will do; the more surface area the faster heat will be dissipated. Be careful when you put on the heat sink. Typically you'll use some heat transfer grease, but the heat sink can slide off if the Mac is moved, and the heat sink might short something out. The best thing seems to be to get a heat sink with a hole in the middle, or drill one yourself, use the heat transfer grease, but also put a small drop of super glue through the hole in the heat sink onto the chip or put a drop on the side, and this should hold it in place. Super Glue is brittle enough that you can pop off the heatsink later if you need to. Fry's sells nice heat sink/fan combo's. They run $20 and up, and should keep the processor cooler. They come with a Y cable to tap into your hard drive power cable to power the fan. A more complete FAQ on this modification for a IIsi is available via anonymous ftp from sumex-aim.stanford.edu in info-mac/info/hdwr (iisi-25mhz-upgrade-faq.txt).

For the Quadra 700 and 900, you can get 70MHz TTL crystals from Fry's. The 70MHz may not work, and you may have to back down to 66.6666MHz, the next most common frequency. The Q700, Q900, and Q950 come with a heatsink installed. A more complete file on this modification for a Quadra 700 is available via anonymous ftp from sumex-aim.stanford.edu in info-mac/info/hdwr (quadra-700-clock-mod-145.txt).

Mod-2

The basic idea of Modification #2 is to disable the onboard oscillator with a jumper and feed in a new signal on the back of the board. There are very few who have performed this mod, but I feel it is more elegant and safer since you don't have to remove the onboard oscillator. This newer, and less evasive method has been performed on IIsi's & Q700's by myself and others, and should work fine on the IIfx, Q900, and Q950.

The most difficult and risky part of "Mod-1" above is the removal of the oscillator, and this is an alternative procedure that gets around that since the crystal oscillators Apple uses have pin 1 as OE. On a crystal oscillator with pin 1 as OE, if you ground pin 1, you disable the output from pin 3(8), and you can feed a new signal into pin 3(8) without removing the original crystal oscillator.

Several months ago I performed this modification on a Quadra 700 by tacking (soldering) a jumper on the back of the motherboard between pins 1 and 2(7) of the 50MHz oscillator, and ran wires about 8 inches long each from pins 2(7), 3(8), and 4(14) to a 14 pin socket attached to the inside of the Q700 with pins in positions 7, 8, and 14. Into this we placed a 70MHz crystal oscillator and later a 74MHz oscillator. This modification is nice in that it is a bit less risky as far as damage to the motherboard, but you have to be careful to use thin wires in order to make clean solder joints. With this modification you could remove the wires at a later date to return to the original configuration more cleanly.

Mod-3

The basic idea of modification #3 is building a clip that disables the onboard oscillator, and feeds in a new, faster signal. The beauty of this modification over the others is that you do not have to do any soldering on the motherboard itself, just on the part you clip onto the surface mount crystal oscillator in your Mac. This is the modification most people use on the C610, C650, C660av, Q610, Q650, Q660av, Q800, Q840av, PM6100(av), PM7100(av), and PM8100(av). It will only work on machines with surface mount crystal oscillators.

The really neat thing about this came into play in February 1992 when Apple released the Centris 610, 650, and Quadra 800. In these machines and since, Apple has been using surface mount crystal oscillators. Now that Apple was using surface mount crystal oscillators, there was plenty of accessible area on the metal tabs of the oscillator. In June '93 Guy Kuo reported the first crystal swap of sorts on a Centris 610 to the net. He soldered pins 3, 5, 10, and 12 of a 14 pin socket directly onto the surface mount crystal oscillator. Because the pins on a standard 14 pin package TTL crystal oscillator are at positions 1, 7, 8, and 14, he made jumpers between pins 5-7, 8-10, and 12-14. He disabled the on-board surface mount crystal oscillator with a jumper between 3-5. Then put the new crystal in the socket. The complete text of his work is available via anonymous ftp from sumex-aim.stanford.edu in info-mac/info/hdwr (centris-610-clock-mod-11).

I was a little hesitant about soldering onto my new Quadra 800, so wrote to him a few days later about using a 3M Surface Mount Test Clip, and asked his thoughts. He suspected I could not find a reasonable test clip, but otherwise agreed it would work. A few days later the 3M SOIC test clip arrived, and the test clip worked perfectly. I was running my Quadra 800 at 40MHz with no problems, and best of all the modification was all contained in a simple little clip that could be removed without trace at will. And thus the removable test clip approach was born. My Q800 even worked at 48MHz as long as I did not access the serial ports. A few days later I got several crystals, and found the highest frequency on my Quadra 800 to be 42.0MHz. Since then I've tried it at 42.1052MHz, and the serial ports did not work, so the cutoff for my Q800 was at 42.0MHz. If you never use your serial ports, 48MHz worked fine for me, while at 50MHz my Mac was not happy and would not boot.

So if you are still interested, you will need a surface mount test clip; 3M and Pomona make them, and I prefer the 3M ones. Make sure you get a surface mount test clip. The I.C. test clips also work, but I prefer the surface mount SOIC (small outline integrated circuit) ones. A 10, 12, 14, 16, or 18 pin clip will be fine. I'd say go with a 14 or 16 pin narrow or wide clip. I used to recommend the gold coated ones, but the resistance/corrosion effect is minimal over the alloy ones. You will also need a 14 pin IC socket, there are plenty of types. The machined pin ones are nice because you can pop out the pins that are not needed to get them out of the way since you only need three pins in the socket. (Yes, just three, pin one on the new oscillator is not going to be used) You will also need an oscillator (more on this later), a little wire, soldering iron, solder, and possibly a heat sink and or fan depending on the machine. For a C610, C660av, Q610, and Q660av you should add a heat sink. And you will want a fan with the PM's.

The others already have heat sinks, and do not get too hot. I had an extra fan with my Q800, but removed it, and it has been fine. The heat sinks come with the clips needed to attach them to the chip. These are a bit of a pain, you just have to work at it for a while. There may be several ways to do it, but I just slide the clips on from the side. Sometimes they fall off half way there, but eventually it works. Some people have been using the heat sink/fan combo's.

The new Q610 and Q660av computers are based on a new mask of the 68040 (There is an "H" after the '040 and before the "RC") that runs cooler at 25MHz, so it comes at 25MHz without a heatsink. This is the same mask as the C660av and Q840av uses. If you do the modification on them it would still be best to add a heat sink.

How to put it all together:

Stand the clip so it's jaws are facing down, and the rows of pins go from left to right, and call the closer row A and the further row B. Number the pins from left to right 1 through 7 (for the 14 pin clip). Next place the 14 pin IC socket with the pins down, and the notch to the left, and number the pins as 1, 2, 3, 4, 5, 6, 7 in the row closest to you, going left to right. The other row is numbered 8, 9, 10, 11, 12, 13, 14 as you go right to left (back towards the notch).
Now starting with the test clip, remove all the pins but four, leaving pins in positions A2, A6, B2, and B6. Next solder a little jumper wire between pins A2 and A6. Now get the 14 pin IC socket, and remove all the pins but 7, 8, and 14. Solder a jumper wire from pin 7 on the IC socket to the jumpered pins on the clip, either A2 or A6 will do. Also solder a jumper wire from pin 8 to pin B6, and pin 14 to pin B2. If you get the narrow clip, you may want to replace the spring with one with less tension; they are like $0.30 at hardware stores, and I cut them into two springs. This way you don't have to push so hard, and it is easier to position on the motherboard. Now put the crystal in the socket with pin 1 in 1, 2 in 2, 3 in 3 and 4 in 4.

There are several surface mount oscillators used on the motherboards. The proper surface mount crystal oscillator on the mother board will have a frequency on it half that of your computer and can be determined from the previous table.

That is it, now you just clamp it onto the surface mount crystal oscillator with the notch on the socket facing the same way as the surface mount crystal oscillator. And watch to make sure the little pins clamp onto the surface mount chip. You may want to use a flashlight for this. These clips hang on very, very well, I've never had mine move in the last year and a half, nor any of the other ones I've done.

Now just turn on your computer and enjoy the speed.

Centris 610 Ethernet Problems:

Those Centris 610's that have onboard ethernet capability share the 10MHz oscillator with the CPU. If you replace that oscillator with a different one, your ethernet will no longer work. In January, Eckart Hasselbrink (Hasselbrink@fhi-berlin.mpg.de) posted a fairly simple hardware modification to fix this to comp.sys.mac.hardware. So if you plan to use your ethernet on your Centris 610 and speed it up, you will need to perform Eckart's C610 Ethernet Modification first.

PowerMac's:

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.

PM7100(av):

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.

PM8100(av):

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.

Apple's PPC PDS Card

If you have an Apple PPC PDS card installed in one of the above machines that can be accelerated with these modifications, the card will run at double the speed of the 68040 processor. The upper limit however is often a bit lower as the card typically cannot go above approximately 76MHz. See the above table for exact frequencies. If you accelerate one of these cards, you should put some extra cooling device(s) on the giant heatsink unless you use it as a frying pan to cook on. Two fans mounted on extruded heatsinks and mounted on the card work wonders (no more burns when you pull out the card).

Testing the Modification

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 (jwang@soda.berkeley.edu), maintainer of the AV FAQ (ftp://ftp.csua.berkeley.edu/pub/jwang/av-faq-15.rtf.sit.hqx), 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 4.0 (available via anonymous ftp from cag-www.lcs.mit.edu in HyperArchive/Archive/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, 3.1. 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 2.15 and the updater to TattleTech 2.17 are now available via anonymous ftp from cag-www.lcs.mit.edu in HyperArchive/Archive/cfg, 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 (mbp@janus.com) following up on a hunch by James McPhail (jmacphai@cue.bc.ca) 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 oenabler@netcom.com, they will send you a free resistor left over from Marlin's extra 199. OE has obtained some 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 bric-a-brac.apple.com 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.

PowerBook 140 Modification

Duo Clip

Address' for some parts suppliers:

Fry's Electronics

12 volt DC 40mm x 40mm brushless fan

about $10-15


14, 18.432, 19.6608, 25.175, 28.322MHz Oscillators

$2.99


70MHz Oscillator

$6.49

Digi-Key

14 pin IC socket w/tin pins

Part# ED3114
$0.57


3M Surface Mount Test Clip


14pin
Part# 923650-14-ND
$7.83


16pin
Part# 923650-16-ND
$8.28


68040 Heat sinks


0.250
Part# HS159-ND
$3.84


0.350
Part# HS147-ND
$3.94


0.600
Part# HS160-ND
$3.98


Crystal Oscillators

14.31818MHz Oscillator
Part# CTX115-ND
$3.25


14.7456MHz Oscillator

Part# X127-ND
$3.38


16MHz Oscillator

Part# CTX116-ND
$3.25


16.257MHz Oscillator

Part# X128-ND
$3.38


18MHz Oscillator

Part# X117-ND
$3.38


18.432MHz Oscillator

Part# CTX118-ND
$3.25


19.6608MHz Oscillator

Part# CTX124-ND
$3.25


20MHz Oscillator

Part# CTX119-ND
$3.25


24MHz Oscillator

Part# CTX125-ND
$3.25


25MHz Oscillator

Part# CTX126-ND
$3.25


25.175MHz Oscillator

Part# CTX127-ND
$3.25


28.322MHz Oscillator

Part# CTX128-ND
$3.25


30MHz Oscillator

Part# XC316-ND
$4.02


32MHz Oscillator

Part# CTX129-ND
$3.25


32.514MHz Oscillator

Part# X133-ND
$3.38


35MHz Oscillator

Part# X134-ND
$3.38


40MHz Oscillator

Part# CTX120-ND
$3.25


48MHz Oscillator

Part# X135-ND
$3.38


50MHz Oscillator

Part# CTX121-ND
$3.25


50MHz Surface Mount Oscillator

Part# SE2325
$6.53


55MHz CMOS Oscillator

Part# SE1509
$3.60

64MHz Oscillator

Part# X136-ND
$3.88


66.6666MHz Oscillator

Part# CTX137-ND
$4.45


80MHz Oscillator

Part# CTX138-ND
$10.01

Active Electronics

33MHz 68882

Part# MC68882-FN33A
$76.95 plus S&H

Output Enablers

Output Enablers sells kits for the C610, C650, C660av, Q610, Q650, Q660av, Q800, Q840av, PM6100(av), PM7100(av). They also stock other supplies including those for the IIsi, Q700, Q900, Q950, PB140, PB160, Duo210, Duo230, C610 ethernet, and C650 serial port modifications.

Marc Schrier
schrier@garnet.berkeley.edu

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.