Using the A3640 in an A3000

The A3640 is Commodore's 25MHz 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 one will not work at all in the A3000 (though it might work in the A4000!). You need to give such a board to a repair center for an upgrade to 3.1 or 3.2.
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 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.

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.