FM Towns:CPU Upgrade for Tower Models

The tower versions of the FM Towns use a 386 CPU. For the Generation 1 and 2 tower models, the CPU is in a socket.
It is simple to remove the CPU and place a compatible 486 CPU in its place.
For Generation 3 and 4 towers, the CPU is soldered and extreme patience and experience is required to desolder all 138 pins from the motherboard.

• 

  Deunan's 486 Upgrade for Gen 3 Tower

For Gen 3 and 4 models it is very strongly recommended to add a socket instead of soldering a new 486 CPU.
You will need a PGA socket with 138 pins.

Compatible CPUs to drop in

The CPUs must have 138 pins. There are occasionally variants that don't comply with this pinout.

• Cyrix Cx486DLC
  Cheaper, more available, and recommended
• TX486DLC
  Cheaper, more available, and recommended
• TX486DLC/e (lower energy model?)
• 486SXL
• Cyrix Cx486DRx2
  The best in the slot possible for towers as it also has a clock doubler although you need to write a custom program to turn it on.
  Turning on the clock doubler is known to break games as much as turning cache on.
  Also extremely expensive and difficult to find.

CPU and Coprocessor Upgrades

There were several coprocessor upgrades for the tower models. It depends on the tower model.
The model numbers are below. They're extremely difficult to find for sale let alone find photos.
Overall the performance increase is very minor and shouldn't affect 386 based games.

• FMT-351
  Official Fujitsu. Made for the Generation 1 towers only. Has a 386 CPU onboard and the coprocessor is also onboard. Confirmed by Cyo.
• FMT-352
  Official Fujitsu. Made for the Generation 2/3/4 towers only.
• CPIT-387DX
  By Urban Corporation. Made for the Generation 2/3/4 towers only.
• CPIF-387DX
  By Urban Corporation. Made for the Generation 2/3/4 towers only.
• TWIO-387T
  By Yoshiki Industry. Made for Generation 2/3/4 towers only.
• MCP-35DT
  By Nissei Electric Manufacturing. Made for Generation 2/3/4 towers only.