FM Towns:CPU Upgrade for Desktop Models

Basic information

Desktops had a slot to place CPU upgrade cards with some later models requiring the CPU upgrade card to be present as there wasn't a CPU onboard the motherboard.
There was absolutely zero retail software that required anything more powerful than a 486DX2 CPU and the majority of software was designed for the base 386 model in mind.
The white desktops with 4 expansion slots are optimal as some CPU upgrade cards were taller and designed for them.
The white desktops with 3 expansion slots are shorter and thus more limited on CPU upgrade cards.
The 486DX4 cards for the Towns MX/MA type desktops will work in a smaller desktop (EA/ME). However, you won't be able to close the top lid.

Spotting cards in auctions

Sometimes sellers on Yahoo Auctions Japan will have the lid taken off of the computer.
This is a good opportunity to see if a CPU card is present or not.
It will also generally raise the price.

• 

  MX Model with no CPU card present.

• 

  A CPU Card is present in this Fresh ES. Likely a 486DX2.

List of CPU Upgrade cards divided by Desktop type

• FMT-361
  Official Fujitsu Product. For HR desktops only. 486DX2 @ 40MHZ
• PK-FM486
  By I-O Data Equipment. For HR desktops only. 486DX2 @ 40MHZ
• (Unknown Model number)
  By SoftBank Corp. For HR desktops only. 486DX2 @ 40MHZ

• FMT-3621/3621A
  Official Fujitsu Product. For ME desktops only. 486DX2 @ 50MHz

• FMT-3623
  Official Fujitsu Product. For MA/MX only. 486DX4 @ 100MHz
  
  
• FMT-3622/3622A
  Official Fujitsu Product. For EA/MA/MF/Fresh only. 486DX2 @ 66MHz
• ODBT486F
  By Urban Corporation. 486DX2 compatible CPU (5V) support. Should fit most desktops except HG/HR
• ODBT486Z
  By Urban Corporation. The Holy grail and best card as it supports 486DX4 CPU (3.5V) and can use a 5x86 CPU.
  Often sells for $100-300+ on auction. Fits all desktops except HG/HR.
• ODBT486
  By Urban Corporation. 486SX/DX/DX2 compatible CPU (5V) support
  
  
• FMT-3651
  Official Fujitsu Product. For HA Pentium only. 60mhz.
• FMT-3652
  Official Fujitsu Product. For HB/HC Pentium only. 90mhz.

In addition, the Pentium models can accept the following CPU upgrades.

• PentiumODP(120/133MHz)
  For HB Model only. Overdrive processor.
• PentiumODP(150MHz)
  For HC Model. Overdrive processor.
• PentiumODP(166MHz)
  For HC Model. Overdrive processor.
• MMXPentiumODP(166MHz)
  For HC Model.
• MMXPentiumODP(180MHz)
  For HC Model.

There are also reports of users putting a K6 CPU inside a Pentium HC model.

Good Information

Hota-san has a very good page explaining some of the CPU upgrade cards.