Strange X68000 PRO Hardware Modification

[Cloudschatze]

Greetings,

I recently received an X68000 PRO system from Japan, and noticed an odd addition. There is a toggle switch that has been run out of the back of the unit. It attaches to an upside-down IC, which is then wire-attached to the memory-controller chip. Some sort of RAM-clearing toggle? With a game loaded, flipping the switch caused the system to hang.

Strange. If anyone would like to see a photograph, I can take one.

[Lawrence]

I certainly would like to see pics of this mod.  What chip has been added?  Where does it connect on the motherboard?  If you could detail the connections, perhaps we could figure out what it does.

[Cloudschatze]

Very amateurish soldering...

Hmm. So, I was wrong. The only physical attachment to the memory controller is that the apparatus is attached, by double-stick tape, to the top.

The components involved are an IC (I'm not exactly sure what it is; it's glued to the crystal), a resistor, and a 60.0000MHz oscillator. Flipping the toggle apparently bypasses the 40MHz signal, and inserts a 60MHz timing signal at that "FN2" filter.

To what end though? Does the memory-controller use that signal? I have no idea what this accomplishes.



And then there's this, at the opposite corner of the mainboard. Another crystal, at 5MHz...

[Lawrence]

That's an overclock mod.  FN2 is a good place to insert a new clock signal, it's used on several X68000 PCBs.

I tried to figure out the second 5MHz mod, but I can't find any of those ICs in my schematics (which doesn't cover PROs).  Are there any useful markings on top of the large black chip (IC234)?

...Aha!  It's an LH853DA serial controller.  It handles the I/O for the RS-232 port, as well as two signals from keyboard and mouse.  The clock doesn't seem to be connected to it at all...

[Cloudschatze]

That's an overclock mod.  FN2 is a good place to insert a new clock signal, it's used on several X68000 PCBs.

I tried to figure out the second 5MHz mod, but I can't find any of those ICs in my schematics (which doesn't cover PROs).  Are there any useful markings on top of the large black chip (IC234)?

...Aha!  It's an LH853DA serial controller.  It handles the I/O for the RS-232 port, as well as two signals from keyboard and mouse.  The clock doesn't seem to be connected to it at all...

Hmm. I couldn't find a datasheet for the LH8530AP, but what meager details I did come across suggest that the chip probably has an internal, 6MHz clock. I assume that pin 20 is an external clock input... but why 5MHz?

I guess it's a moot point now; I've removed both mods.

Thank-you for the help, Lawrence.

[sharpie]

overclock mods are very common on x68k's and often use toggle switches.