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The Source for Mac Performance News and Reviews
7500 to PC ATX Case Conversion
by Paul de Haan
Published 1/18/2001

    Disclaimer/Warning: This page is for entertainment only and is not guaranteed accurate. Performing modifications or other work inside your Mac will void your Apple warranty, may cause damage to your computer or result in personal injury. Although Dick indicates he made these modifications successfully, neither the site publishers nor the author warrants any of the information listed here. This article is published for entertainment purposes only. The modifications listed here are beyond the capability of most end users and all repairs or mods should be made by qualified computer technicians. The author and site publishers DO NOT recommend attempting any of the mods listed here. You assume all risk from the use of any of the information in this article.

With thanks to all contributions on your site regarding the case conversion in the first place. I did the bold thing as well, although I had some hesitation initially to set it off.

The biggest problem of a case conversion is the amount of time it costs. Hours and hours. The end result is a case with lots of space!

There are some difficult parts as well:

  • A Molex Mini-Fit, Jr. 22-pin receptacle
  • A Molex Mini-Fit, Jr. 10-pin receptacle
The 10-pin unit is mainly use for the 3.3V supply. I used the available information on to figure out what wires were doing what.

Toolwise, I couldn't have done it without my Dremel:

This tool allows for grinding, drilling etc. Very handy to cut the holes for the backplane!

On the case. I bought a so-called "server-case" at the local PC shop. It has easily removable sidepanels, which makes access to the inside quick. Not as quick as the original 7500 case, but quick enough. The picture below is showing the machine operational.

One of the first parts converted was the floppy drive. I used a defective PC-drive to act as the donor for some parts: the front panel and the knob. No, it wasn't a Sony, so the frontpanel didn't fit. Cutting the edges and using superglue however makes things fit. The same accounts for the eject-button. Now I do have a functional manual-eject as well. It was easier than trying to find a piece of plastic to fit in the hole of the button and makeing it fit nicely.

(sorry, but some of the photographs are a bit lighter then I intended..)

One of the challenges was to make the original 6-slot backplane fitting the 3-slot layout of the TNT motherboard. Then again, the AV-connection module had to find a place as well. Cutting away some of the metal, leaving 3 slots intact gave room for fitting it. Next was the measurement of the spacing between the Mac connectors and how they should fit in. I used a sheet of relatively thin aluminum to do the job. Setting out the diameters of the various connectors and holes on the aluminum was time-consuming and a precise job. Looking at the result, it is not ideal, but it works. Some of the holes might have been smaller, others might have been just a little bit bigger. The main reason for this is that the aluminum sheet is lightly further away from the main board as it was in the original 7500 case, therfore demanding some holes to be bigger, otherwise the connectors wouldn't fit anymore.

The dilemma here was the choice to either have the sheet aluminum cover the cut-out ATX-backplane or not. A different approach could have been to use a layered approach with one sheet with the smaller holes on the inside of the cabinet and on sheet covering the original ATX-backplane, but with larger holes. If I have plenty of spare time and get really bored, I might just do this... :-P

As you can see, there is a metal bridge in the case, just left of the attached connector. Yes, that's the one from the mounted Voodoo1 card. Fortunately it hardly covered any of the connectors. The hardly-to-not covering has been corrected by using some hefty pliers and squeezing it until it was small enough. Advantage is that the construction of the case isn't compromised at all.

For mounting the motherboard in the case, I used the original stand-offs as supplied with the case (and some extra). The original slots for the stand-offs are nicely cut out. I used two drill sizes and the Dremel to cut out the ones I needed. Using mounted PCI-cards is a good help to get the holes marked in the spots where they should get.

Comparing the two shows the difference of manual labour :-)

These are not the most clear pictures, but at least the contours of the holes are visible ;-)

A look on the inside shows where I cut away the space for the three extra slots, making room for the AV connection assembly.

And indeed, it also reveals a Carrier-ZIF card with a G3 on it.

One of the nice features of this case is that it hase some spare room for the extra power cables.

The cable assembly I put together is a different story. Since I didn't want to take apart the original PSU, I decided to buy an extension cable for the ATX power supply. Next step was to get the 22- and 10-pin Molex connectors and mount them on the extension cable. This now is an ATX-to-PM7500 conversion cable and has also the advantahe of easily being able to swap the ATX-PSU when needed.

Soft power-on was a different story. Using a 7404 and resistors somehow wouldn't work properly. Tough. Looking at the inverse logic, a simpler solution came to mind: just use a plain NPN-transistor (I had a very old BSX-20 handy) and two resistors. The following schematic is simple and effective. The resistor value on the basis of the transistor can be higher if it loads the main board too much (a plain multimeter helps out here). Little experimenting doesn't hurt.

The next picture shows more of the inside of the machine. The extra 3.5" cage is not in place. This sits just beneath the PSU, close to the top fan. This means I can add two more 3.5" HDD's, next to the three already fitted. Enough expansion room here!

Some might have noticed that there are already a number of cork tiles in place to help minimising the noise. Sidepanels and onther large surfaces are the easiest to do first.

There is still a lot of blank metal left however - by example the area around the backplane. When I find the time I will put more of this nice natural sound silencer in place.

Things still to do (if they ever get done due to time constraints):

  • Apply more cork to diminish the noises.
  • Construct a different 3.5" cage, so that the drives can be hung in rubber mountings, diminishing mechanical noises even more.
  • Improve airflow in the cabinet to bring down the G3 temperature from 44 to some 34 degrees Celsius.

One conclusion can be drawn; the whole thing is a lot heavier than the original 7500 case. On the other hand, it has lots of space available for future expansion as well.
-Paul de Haan

Related Articles:

For other upgrades, mods, etc. see the main Systems page, CPU Upgrades, IDE, SCSI, Firewire and Video/DVD/Graphics Cards topics pages.


Copyright © Mike, 2001.

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