Using the A3640 in an A3000

The A3640 is Commodore's 25 MHz 68040 board used in the A3000T-040, A4000/040 and A4000T. Originally it was designed to fit in the desktop A3000, too (hence the name). Unfortunately, this did not work as good as expected. Several problems were found, so Commodore never recommended the A3000 installation of the A3640 officially.

Because used A3640s are often available at a good price from people who upgraded their A4000/040 with a faster CPU board, one might be tempted to use it in the A3000 in spite of the known problems. With a little work and a little luck, it might work very well.

For daring people who want to do such an upgrade of their A3000, I collected the following information about possible problems and how to solve them. Good Luck!

A3640 board revisions

You can find three revisions of the A3640 in circulation:

3.0

This board will not work in A3000's at all. If you need to use it in an A3000 then send it to a repair centre for upgrading to Rev 3.1 or 3.2. It does work in the A4000 although zorro cards may not always work reliably. The problem is that the A3000 has a DMA SCSI controller and the board can give bus access to both the scsi and a DMA device in a zorro slot causing the machine to lock up. There is also a problem with the STERM* signal that can cause sporadic crashes with some cards such as the Cybervision 64.

3.1

This one can work fine in the A3000, but has a bug that prevents certain Zorro-II DMA cards like the GVP PhonePak from working. In most cases, this revision (which is the most common one) is OK.

3.2

Like a 3.1 board, with the above mentioned bug fixed.

When checking the A3640 revision number, also have a look at the PAL or GAL chip at position U209. Its revision number should match that of the A3640 in the following way:

A3640    U209
  3.0     -01
  3.1     -02
  3.2     -03

Mechanical problems

Though the size of the A3640 board fits perfectly even in the small A3000 desktop case, you can have some problem with the height.

The earlier A3640s have a large heat sink on the 68040 CPU that would need some room in one of the two floppy drive bays of the A3000. Newer A3640s come with a flat heat sink that fits under the drive bridge without problems.

If the heat sink of your A3640 is too high, probably the best thing to do is to get a smaller one (you could also get a larger computer case or modify the drive bridge). Largely available are i486 CPU coolers that consist of a heat sink with a small fan and mounting clips.

Because the i486 is a little bit smaller than the 68040, you should get a i486 cooler with elastic clips that can be widened a bit. You also need a cooler where the little fan can be removed, to make it fit below the drive bridge.

If you want to buy an A3640 with the high heat sink and need to replace it by a smaller one, you should check how the old heat sink is mounted. Normally, it should be only fixed with a metal clip that can be opened to remove the heat sink, but I also heard of one case, where the heat sink was glued to the 68040 CPU and thus virtually impossible to remove.

Heat

One of the reasons why Commodore never recommended the A3640 for the A3000 is heat. The A3000 was designed to be upgradable with such a board, but the 68040 turned out to run hotter than expected and hotter than the A3000 was designed for.

If you use the A3640 in the A3000 desktop, you always risk overheating the 68040 or any other part in the computer. This could cause errors, crashes or shortended life of certain parts.

To avoid heat problems, it might be a good idea to install an additional fan in the A3000, avoid using hot hard drives inside the case and not using it with high room temperatures.

Operating sytem ROMs

There are two socketed ROM chips on the A3000 mainboard that contain the ROM part of the Amiga's operating system. There are three different pairs of ROMs available for the A3000:

V36 boot ROMs

The older A3000s have these ROMs. They are only used to load an image file of a newer ROM version from hard disk (or floppy disk) into RAM. These V36 ROMS are not compatible with 68040 processors, so your A3640 will not work at all with such ROMs. You need to get the A3000 version of the AmigaOS 3.1 upgrade kit and install the new ROM chips that come with it.

Kickstart 2.04 ROMs

These are 'normal' AmigaOS ROMs, that do not require booting a Kickstart file. The newer A3000s were delivered with these ROMs. These ROMs will work with the A3640, execept if you have the combination of a RAMSEY-04 chip (which almost any A3000 has) and static column Fast-RAM. In this case, you need to exchange the SCRAM at least in bank 0 with page mode RAM chips or (even better) upgrade to AmigaOS 3.1 ROMs. You can not upgrade the RAMSEY to the new rev. 07 (which would also fix this problem), because for this you would also need a DMAC-04, which is not available anymore.

AmigaOS 3.1 ROMs

This is the newest ROM version available for the A3000, and the best one to use with the A3640. No A3000s were ever sold by Commodore with these ROMs, but they are available as an upgrade. You can either get only the ROMs, or a complete AmigaOS 3.1 package that also contains a new set of Workbench disks and new manuals. If your A3000 still has an older ROM version, it would be a very good idea to upgrade to 3.1, regardless if you have problems or not. As some new software requires AmigaOS 3.1 and it is also much nicer to work with because of its new features and fixed bugs, I would recommend to get the full upgrade package.

If you have a very old A3000 mainboard revision, the ROMs might sit in a special adapter socket, called ROM tower. When upgrading to 3.1, you should exchange the ROMs, but continue using the sockets. I heard of some cases, where A3000s with ROM towers refused to work with certain 3.1 ROMs. In this case, you should probably contact the manufactuerer of the upgrade kit for a different set of ROMs.

When upgrading to newer ROMs, remember the orientation and the numbers of the old ROMs you remove! Insert the new ROMs in the same direction and with the same number in the same socket. Do not pay attention to the numbers printed on the motherboard, they might be just the wrong way around. If you confused the two sockets, your A3000 will refuse to boot, but nothing will be damaged. But do not confuse the orientation of the ROMs, I'm not sure if it's harmless to put them in the wrong way. For the orientation, pay attention to the mark in the chip case itself, not to the orientation of the printed text on the chips!