Content created by Sandy. Edited by Major Tom.

Overclocking is, err... bad, especially for PS/2 and PS/55 machines. What we expect from them is stable operation and reliability. So, it is not recommended to replace the CPU clock oscillator with a faster one. IBM really didn't not account for clock modifications. Therefore it's probably better to keep your MCA box "stock". But I was MAD, so here we go...

Japanese Type 1 "J" with 25 MHz oscillator for 61-W0 will take a 30 MHz one without any trouble. But all other complexes will not work with faster oscillator in place of the originally equipped one (to be more precise, it is possible to use 35 MHz oscillator in place of the 33 MHz one for some complexes). If you change the base clock, you need to modify the BIOS as well, in most cases. Without BIOS modification, system will hung up during POST with error code 102.

I didn't (well actually I still don't) have any knowledge of how to read or write BIOS. Mr. Sugiura, one of my friends, who knows about BIOS reading/writing, examined BIOS of the 5561-W1x complex and he modified the BIOS so that the 61-W could boot without the 102 error. According to him, there are several lines indicating jump to the 102 error section during the POST routine. He couldn't determine which line exactly was triggering the error with the modified system clock. So he rewrote all lines that were jumping to the 102 error section. I don't know if his modification includes any bugs or not. But, so far, BIOS ROM with his modification works fine. At least in my systems. He kindly examined another models' BIOS ROM and burnt new EPROMs for me, and for other members of the forum.

List of models and complexes modified by Mr. Sugiura:

Model Original Oscillator Replacement Oscillator CPU Clock (before/after) BIOS ROM Type
5551-N 66.66 MHz 84 MHz 33 to 42 MHz PROM (write once)
5551-W0 40.00 MHz 60 MHz 20 to 30 MHz PROM (write once)
5561-W1/W2 50.00 MHz 62 MHz 25 to 32 MHz EPROM
5580-Y 33.33 MHz 40 MHz 33 to 40 MHz EPROM
Type-A Upgrade 66.66 MHz 80 MHz 33 to 40 MHz EPROM
Type-B Upgrade 66.66 MHz 80 MHz 33 to 40 MHz EPROM

More of this story is available HERE.

Content created and/or collected by:
Louis F. Ohland, Peter H. Wendt, David L. Beem, William R. Walsh, Tatsuo Sunagawa, Tomáš Slavotínek, Jim Shorney, Tim N. Clarke, Kevin Bowling, and many others.

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