[From: Stephan_Budach@hh2.maus.de (Stephan Budach)
CAUTION!! ALL STATEMENTS ARE DONE WITHOUT ANY WARRANTY ! I ONLY REPORT HOW I DID IT ON MY AND TWO OTHER 475'S. THIS INTERVENTION WILL VOID ANY GUARANTEE POSSIBLY STILL EXISTING !
Apple has installed a programmable CPU clock in the 475 and in the Quadra 605, that have the same circuit boards. You'll find this part on the upper side of the circuit board in the rear right corner (U17, close to the power supply connector). Interestingly, on the bottom side of the board, at the front end of the PDS-slot, you'll find four SMD solder pads, of which two are used at present : the number R21 with a 4k7 resistor and the number R25 with a 300 ohm resistor. These resistors pull the leads to high (4k7 ohm) resp. low (300 ohm). There are two free pads, each one corresponding to one of the other ones. Depending on the frequency we wish to achieve, R25 and/or R22 will get a 300 ohm resistor (low), R21 and/or R24 will have a 4k7 ohm resistor (high).
|Let's first look at the different possibilities :|
In this table, code 301 means a 300-ohm SMD-resistor, 472 a 4k7 SMD-resistor. As you can seem to get 33 mhz, you only have to move the resistors to the other pads : R21 goes on R24 and R25 on R22.
When manufacturing the board, the resistors were first glued onto the circuit board and then soldered. Moving them needs some tact and a bit of patience. I used a pointed pin, with which I carefully heated the resistors, while pushing them aside with it, switching sides every try. This will soften the glue and the resistors will loosen.
Next, I used a bi-directional switch and 6 wires. I soldered the resistors to the common contacts (middle) and pulled a wire from the resistors to the common connectors of the solder pads (R25/22 on mass, R21/24 on 5V). The two other wires go at the corresponding contacts, in such a way that at R25/22 ground is switched with the resistor.
I have installed the switch because without it, p.e. the System 7.5 Installer will not work, since the Mac generates a gestalt not assigned by Apple. (About the gestalt: jumper J18 near the front left corner switches the identification to "Quadra 605", but only at 25 mhz, otherwise an unassigned figure will be generated.
The whole affair also works with a 68040, with FPU. In that case however, you should add a heat sink after rebuilding the CPU since it will become considerably warmer. This is valid especially for the CPU with FPU, that gets hotter anyway than the LC type.
If you have a 68040/33, you can also use the 40 Mhz variant. You'll have to use two 4k7 resistors in that case, and change the wiring a bit.
So, be careful with the rebuilding!!
This is the drawing for the switch:
(Note of the translator : this paragraph was removed from the most recent version of the german web site. It's not yet clear whether this is because the information was proved incorrect, or because the author found the information to be out-dated.)
According to the magazine c't (10/96, page 356), the Geoport will no longer be 100% compatible after this operation. Modems and printers will probably still run, but for example a Sagem ISDN adapter is reported to no longer work on these Macs.
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Last Modified 1/1/01