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Mac IIsi - Clock Chip to 25 Mhz - How to ?

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The processor on that thing is factory-rated to 20MHz, it wasn't an underclocked 25MHz chip thrown in by Apple.


Anyway, that's one of the Macs where a clock crystal would need to be replaced (like most 68k and early PPC models).

I don't have a IIsi in front of me, so I can't say exactly where it would be, but there's a 20MHz oscillator somewhere on the logic board (either a small two-lead can type or, more likely, a larger rectangular type) that will need to be replaced with a faster part. There are likely a couple of different oscillators on the board, so make sure you replace the correct one.

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this says http://lowendmac.com/ii/macintosh-iisi.html


The IIsi was designed as a less expensive, less expandable alternative to the Mac IIci. Cost saving measures included eliminating NuBus expansion slots, soldering 1 MB of RAM to the motherboard, and using a slower CPU (20 MHz vs. 25 MHz). Although the IIsi was marketed as a 20 MHz computer, users quickly discovered it used parts rated at 25 MHz. (Apple had intended it as a 25 MHz computer, but chose to scale back the speed to avoid cutting into IIci sales.) Chipping the IIsi to 25 MHz -- or even 28 MHz - is not unusual.


at http://en.wikipedia.org/wiki/Macintosh_IIsi


Because of its heritage as a cut-down IIci, a simple modification carried out by many owners was to substitute a new clock crystal to increase the speed to 25 MHz.


As you say Franklinstein I need to get a faster clock crystal.

Hum... where can I get that one...

Edited by Guest
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At www.apple-history.com I found


The IIsi came in a streamlined case used only for this model. It was originally designed for a 25 MHz 68030 processor, but shipped at 20 MHz, so it wouldn't compete with the IIci.


Do you think I can add a 25 MHz 68030 processor to speed it up ?


Nope, it is soldered to the board(bottom left, smallish square chip):




See the four silver rectangular canisters near the top, under the large black RBV chip? IIRC, the third from the left is the cpu clock, but I'd have to double check. It is 40.000 Mhz so to make the cpu run at 25 Mhz you need to replace this with a 50.000 Mhz crystal. I'd socket it so you can try different crystals.

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You won't have to replace the CPU, most likely. I've never run into a IIsi that didn't "overclock" (actually, re-clock) to 25MHz. And this is a case where the speed difference is very noticeable. At 20MHz, the IIsi barely gets everything done. In imprecise terms, an extra 25% clock speed is all used to accelerate user tasks, so it will feel much zippier.

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  • 1 month later...

Is this information of any use (cut and pasted from an old text file I obtained a long time ago):




Mac Crystal Oscillator Speedup History 2.5b


November 1994




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 two 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 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, post them on comp.sys.mac.hardware

and make them available for anonymous ftp on sumex-aim.stanford.edu and its

mirrors in /info-mac/info/hdwr. 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. The URL is:





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."




This is what I seem to be finding. These numbers vary from Mac to Mac, so these

are just averages. Some machines will go faster than this. These are close to

the oscillators that Output Enablers ships in their kits. The problems noted are

those when above or near the top of the range listed. These problems are not

permanent, slowing down the computer will fix them.


Mac IIsi


Processor: 68030@20

Gestalt: 18

Initial Oscillator (MHz): 40

Max Oscillator (MHz): 50-55

Probs: Floppy Drive Controller


Mac IIfx


Processor: 68030@40

Gestalt: 13

Initial Oscillator (MHz): 80

Max Oscillator (MHz): 92.9405-100


Centris 610


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


Centris 650


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


Centris 650-mod


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)


Centris 660av


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


Quadra 610 (DOS)


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


Quadra 650


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


Quadra 660av


Processor: 68040@25

Gestalt: 60

Initial Oscillator (MHz): 12.5

Max Oscillator (MHz): 16-17.496

Probs: Video


Quadra 700


Processor: 68040@25

Gestalt: 22

Initial Oscillator (MHz): 50

Max Oscillator (MHz): 66-70-74

Final Oscillator with PPC card & Serial (MHz): 74


Quadra 800


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


Quadra 840av


Processor: 68040@40

Gestalt: 78

Initial Oscillator (MHz): 20

Max Oscillator (MHz): 23- 24

Probs: Serial Ports


Quadra 900


Processor: 68040@25

Gestalt: 20

Initial Oscillator (MHz): 50

Max Oscillator (MHz): 66-70-74

Final Oscillator with PPC card & Serial (MHz): 74


Quadra 950


Processor: 68040@33

Gestalt: 26

Initial Oscillator (MHz): 66

Max Oscillator (MHz): 80-92.9405

Final Oscillator with PPC card & Serial (MHz): 80


PowerMac 6100(av)


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


PowerMac 7100(av)


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


PowerMac 8100(av)


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


PowerBook 140


Processor: 68030@16

Gestalt: 25

Initial Oscillator (MHz): 33

Max Oscillator (MHz): 50


PowerBook 160


Processor: 68030@25

Gestalt: 134

Initial Oscillator (MHz): 50

Max Oscillator (MHz): 66


Duo 210


Processor: 68030@25

Gestalt: 29

Initial Oscillator (MHz): 50

Max Oscillator (MHz): 66


Duo 230


Processor: 68030@33

Gestalt: 32

Initial Oscillator (MHz): 66

Max Oscillator (MHz): 80


LCIII (P450)


Processor: 68030@25

Gestalt: 27

Gestalt at 33MHz: 62

LCIII (P450) Speedup


LC475 (P475, P476)


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







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



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



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








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



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.







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



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


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.







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.


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


See the table of machines 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 (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


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


A few companies have been replacing the crystal oscillator in PowerBooks to

speed them up, and add a 68882 math coprocessor. If you are good with a

soldering iron you might give Glenn Nelson (gwnelson@tddcae99.tddeng00.fnts.com)

and Jerry Cupples' (jcupples@iphase.com) PowerBook 140 Modification a try.




Duo Clip


If you are interested in speeding up your Duo 210 or 230 without any soldering

on the motherboard you might give Ronald Leenes' (r.e.leenes@bsk.utwente.nl) Duo

Clip a try.




Address' for some parts suppliers:


Fry's Electronics


340 Portage Ave

Palo Alto, CA

(415) 496-6000


12 volt DC 40mm x 40mm brushless fan

about $10-15


14, 18.432, 19.6608, 25.175, 28.322MHz Oscillators



70MHz Oscillator





(800) 344-4539


14 pin IC socket w/tin pins

Part# ED3114



3M Surface Mount Test Clip


Part# 923650-14-ND




Part# 923650-16-ND



68040 Heat sinks


Part# HS159-ND




Part# HS147-ND




Part# HS160-ND



Crystal Oscillators


14.31818MHz Oscillator

Part# CTX115-ND



14.7456MHz Oscillator

Part# X127-ND



16MHz Oscillator

Part# CTX116-ND



16.257MHz Oscillator

Part# X128-ND



18MHz Oscillator

Part# X117-ND



18.432MHz Oscillator

Part# CTX118-ND



19.6608MHz Oscillator

Part# CTX124-ND



20MHz Oscillator

Part# CTX119-ND



24MHz Oscillator

Part# CTX125-ND



25MHz Oscillator

Part# CTX126-ND



25.175MHz Oscillator

Part# CTX127-ND



28.322MHz Oscillator

Part# CTX128-ND



30MHz Oscillator

Part# XC316-ND



32MHz Oscillator

Part# CTX129-ND



32.514MHz Oscillator

Part# X133-ND



35MHz Oscillator

Part# X134-ND



40MHz Oscillator

Part# CTX120-ND



48MHz Oscillator

Part# X135-ND



50MHz Oscillator

Part# CTX121-ND



50MHz Surface Mount Oscillator

Part# SE2325



55MHz CMOS Oscillator

Part# SE1509



64MHz Oscillator

Part# X136-ND



66.6666MHz Oscillator

Part# CTX137-ND



80MHz Oscillator

Part# CTX138-ND



Active Electronics


(800) 228-4836


33MHz 68882

Part# MC68882-FN33A

$76.95 plus S&H


Output Enablers


1678 Shattuck Ave. Suite # 247

Berkeley, CA 94709




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.




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 © 1993-4, Marc Schrier

Please contact me prior to distributing or reproducing this file.

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