FM Towns revolves around the 386 architecture with the CPU taking 5 volts. Generally you have two different packages or chip sizes and types for CPUs which are the SQFP and PGA. Only generation 1 and 2 tower models have sockets with the remaining ones having their CPU soldered.
- -Models 1 and 2 (very first Towers), S1, S2, 1F, 2F, 1H, 2H
These Towns have a 386DX CPU. You can remove it and place a 486 processor in it without difficulty.
- Models Sf, Sh, Sf2, Sh2, 10F, 20F, 40H, 80H, CX10, CX20, CX40, CX100
These Towns while they have a 386DX CPU it is soldered. If you want to upgrade the processor to a 486 you must perform a total disassembly and desolder the CPU and insert a PGA 138 socket in it’s place. You may also socket the stock crystal near the CPU (should be 32mhz) and insert a 36mhz 5 volt crystal for a marginal speed boost. Clock divider is 2 which sets the computer from 16mhZ to 18mhz. Stability is questioned beyond 18mhz.
The following CPUs should be a drop-in upgrade from 386 to 486
- Cx486DLC, TX486DLC, TX486DLCe, TX486SXL and Cx486DRx2
All have Cache that can be enabled with a script. There is a Cyrix 486 cache enabler script floating around here:
However for the Texas Instruments CPU you will need to find the datasheet and write a cache enabling script specific for the FMT. The DRx2 also has a clock doubler that can be enabled. Generally enabling cache will break most software and clock doubling breaks software as well.
The following have a 386SX processor (100 pin Quad Flat Package) and can be upgraded to a 5 volt 486
- UX10, UX20, UX40, UG10, UG20, UG40, UG80 and Marty
Options for upgrade include the 486SXLC2, Cx486SRX2 and IMB486SLC (warning 3.3v CPU. Clip-on board required with voltage regulator). It is also advised to use a heatsink when you upgrade to a 486. Generally the 486SXLC2 is easier to find. No matter the CPU speed you’re limited to onboard clock at 16mhz except UG model with 20mhz.