Information for overclocking an FM Towns computer

[Cyothevile]

I have been exploring the FM Towns computer for over a year now and I thought I should share a few unconventional CPU-related upgrades I've successfully performed.  Some people like myself like pushing stuff to the upper limit. Some don't care which is fine.  One thing I will mention right away is there is a few titles...less than 5 that totally break with a 486 CPU upgrade most notably Brandish.  Can't think of the others.

Gen 1 towers are generally MODEL1, MODEL2, or generally the models that require a SCSI card for a hard drive.
Gen 2 towers have the CD light on the front of the tower below the disc drive bezel
Gen 3 towers have the CD and HD light on the front of the tower below the disc drive bezel
Gen 4 towers have the CD, HD and FAST lights on the front of the tower below the disc drive bezel.  Generally, these are the best types of towers due to FAST mode being available.

I've owned Gen 1, 3, and 4 towers as well as having both Car Marty types and all 3 Marty types.  All of them have either a 386DX (Towers) or 386SX (Marty systems) CPU clocked at 16mhz.  The resident crystal nearby that controls the CPU is clocked at 36mhz with the towns having a clock divider by 2. 

Replacing this crystal in a base 386 system with a 40mhz crystal will clock the CPU at 18mhz.  All apps and software I've tested run stable with no visible speedup. 

Primarily I would advise clocking the CPU crystal up only if you upgrade the CPU in a tower model.  Generation 1 and 2 towers have a CPU in a socket so it's much easier to upgrade it while generation 3 and 4 towers have it soldered.  Personally, I have a 486SRX2 with a 40mhz crystal in my CX20.  I would also advise a heatsink if you put a 486 in any base 386 Towns system as the CPU does get warm but I don't know if it's a risk.

Personally, I've tried upgrading the CPU in 3 different Marty and I can't get it to post beyond the IPL during the bios initialization sequence.  Kind of frustrating.  I know a few others were successful. 40mhz modifications to Marty and Car Marty worked totally fine.

Anyway if you upgrade the crystal and don't upgrade the CPU itself you won't notice any improvements in performance in my experience.  I will note that upgrading the CPU in a tower will make Tatsujin Oh, Mahou, and other 386 games that have some minor slowdowns radically improve.  It also opens up a few games such as Alltynex being playable on a tower. 

A crystal faster than 40mhz will result in the Towns not booting the IPL. I believe Rob Ivy posted this here many years ago.

Now an upgrade I've never done but I would like to know if anyone has performed it.

According to Hota-san's maintenance manuals, I posted a while back have radical discussions involving Towns.  The ATX mods I hope were helpful for other members here.  Hopefully, you guys can revive some desktops with fucked up power cards that are too difficult to repair. Anyway, there is an overclock performed where the 66mhz crystal is removed and a 75mhz crystal is added.  Apparently, this clock multiplier pushes 486DX systems clocked at 33mhz to 37.5mhz.  This also pushes CPU upgrade cards even further and the motherboard stably supports it according to HDBench tests by Hota-san. 

Anyway, I wanted to see if anyone else has performed any overclocking on base 486 models before I rip apart my HR100 and socket the motherboard crystal and run some stress tests to see the limit.  I have two desktops with 486SX CPUs (HR100, SI2), one with a 5x86 card but Hota-san's guide only shows 486DX desktops being overclocked.

[Jehuty]

The Grey Towers have a 32Mhz crystal not 36, or ? With divider from 2 they have 16Mhz.
I tried a 40Mhz crystal in my CX20 so it runs with 20Mhz. In galaxy Force II i notices the Speech was speeded up.
But with that overclock the floppy drives won´t work anymore.
So i got a MA, but it has a broken CD Drive, now i am waiting for a wizard for this modell.

[Cyothevile]

The Grey Towers have a 32Mhz crystal not 36, or ? With divider from 2 they have 16Mhz.
I tried a 40Mhz crystal in my CX20 so it runs with 20Mhz. In galaxy Force II i notices the Speech was speeded up.
But with that overclock the floppy drives won´t work anymore.
So i got a MA, but it has a broken CD Drive, now i am waiting for a wizard for this modell.

Thanks for correction. I wrote that when I was pretty sleepy. From my eBay purchase history you're right; 36mhz crystal is what you need to pull off 18mhz. The stock one is 32mhz.

Interesting how the floppy drives won't work when the computer is clocked at 20mhz. At 18mhz my goteks work but now I'm a bit concerned physical floppy drives won't work.

[Jehuty]

Yes, my drives doesn´t read disks when it was clocked at 20Mhz.

[Cyothevile]

Thank you for clarifying. Seems 18mhz is the safe point for the base 386 systems.

When I disassemble HR100 and socket the crystal I will report back what seems to hold stable.