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Mach 5  CPU Card Ratios Explained
Created: 7/21/98


Introduction
Due to all the mail I've gotten asking for a clearer explanation of the Mach 5 CPU card speed modifications shown at: http://www.bekkoame.or.jp/~t-imai/ilcpce1.html, I spent last night creating this page as an attempt to more clearly explain the Mach 5 CPU card ratio resistor configurations.


Warning: This information is for reference only - I do not recommend anyone try this at home. Unless you have surface mount soldering experience you are likely to damage the card. Too much heat can lift the solder pads off the board. The resistors in question are very tiny, easily lost and the chances of solder bridges are high (inadvertent solder flows across connections causing a short). There is no guarantee this will work on your card and any modifications you do will void your warranty.

Mach5 CPU card image

Card Speed ID:
In the photo above of the back of the Mach 5 CPU card, the blue box outlines what I consider as a visual ID only of the CPU Card speed. Although there appear to be a selection of CPU speeds (250 MHz to 400 MHz), actually all the resistor pads in that area are already shorted, and the surface mount resistor that is installed is a 0 ohm (short). Since all positions are shorted it's obvious this does nothing as far as speeds. I also created a close up image (64k) of this area.

Ratio Control Resistors:
The actual speed control resistors (1k ohm) are in the area outlined in red. These resistors act like switches to set the Ratio of bus/cache speed to CPU speed. Moving these resistors are the key to increasing the CPU/Bus speed ratio and therefore the CPU speed. Since a picture is often worth a thousand words, I've scanned the card, zooming in where needed and also created configuration diagrams that clearly show resistor placement. Due to the tiny silkscreen reference designators (R12, R13...) on the pwb (printed wiring board) I had to touch up the lettering in photoshop. You might need to use a magnifying glass on the actual card, however these pictures and diagrams show the relative orientation and position of the resistors as an aid in identification.

Zoomed image of Ratio Resistors
10X Zoomed Image (250MHz config)

300 Mhz Settings350mhz setting400mhz setting450mhz setting
Diagrams showing resistor configurations for various speeds

The diagrams above should be a big help in clarifying the resistor positions for each speed. The setting are based on the information at the Japanese site linked in the opening paragraph. I have not verified all these speed settings.

As mentioned previously, the marking and resistors are very small. Very few cards will run more than 50 MHz over the rated speeds and one reader reported his 300 MHz card was not reliable after a month of use at 350 MHz. The spare 300 MHz card I have has run at 350 MHz for some time now with no ill effects so far, but there is never a guarantee of reliability (short or long term) any time that you run a component over the rated speed. My 350 MHz card is running fine so far at 400 MHz. One reader reported his 350 card did not run at 400 MHz reliably. I am working on a little surprise that I'll be disclosing details on as soon as it's complete if all goes well.

I'm not supplying soldering tips, etc. here as I do not want to encourage those not already experienced in surface mount soldering to attempt to try these mods at home. More than likely they would end up damaging the card. Although I've done several of these with only a 15w soldering iron and some solder wick, some say two small irons (one on each side of the tiny resistors) helped. This is definately a job for certified technicians who may want to experiment with their cards and are willing to accept the risks involved.

I hate to keep harping on this but unless you have experience in soldering/unsoldering components this small you should not attempt this yourself. All mods you do are at your own risk. It will void your warranty and there is no guarantee you will have a working card afterwords.

Other Mach 5 Info:
I also scanned images of the Mach 5 CPU (note it says PPC 760 - the G3 is a PPC 750) and inline cache chip in case you're interested. Mach 5 owners may also want to read my Mach5 page for benchmarks comparing a 350 MHz and 400 MHz Mach 5 system to the Apple G3/300.


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