MC-10 8K Internal Mod

One of the big shortcomings of the MC-10 is the very small amount of ram available on a stock machine.  Only 4 kilobytes of ram is provided to the system, not much for programs, and not even enough to display a hi-res screen (256×192, B&W).

There are ram modules that plug into the expansion port of course, but these don’t provide for memory access to the MC6847 VDG chip in the computer.  Meaning that even with a ram expansion, video memory is limited to 4K.

The two pics below show available ram in basic, and the VDG displaying the hi-res screen.  You can see how the VDG displays the same video information lower on the screen as what is displayed up top.  This is because only 12 address lines (out of 13) are connected from the VDG to memory, so the address rolls over after 4K.

Available ram in basic, and hi-res video on a 4K machine

To remedy this, long ago someone came up with an 8K internal mod that provides more memory for the system, and enough to display a full screen of hi-res graphics.  An article in the September 1983 issue of Color Computer Magazine, by Dennis Kitsz, details just such a ram modification.

Though I believe this is the mod most people have used over the years to upgrade their MC-10 to 8K, there are several errors in the article to watch out for.  Most notably, the enable on the 74LS139 should be connected directly to ground rather than any E clock, or pin 8 of u12.  Pin 8 of u12 is the expansion port /SEL line, and while things will run ok as long as nothing is plugged in, any cart that uses /SEL will cause a garbled video screen.

Here is a .pdf of the article…

8K mod article

Anyway, I was recently asked by Steve Strowbridge (host of CoCoTalk) to modify his MC-10 to 8K as well as install one of my composite boards, so that he could make some videos on the MC-10.

I’d never actually done one before, though I had seen the article and noted the errors.  So I pulled out the article again and looked it over.  Thinking about how this mod was done for a bit, I decided there was a better way to do this.

The MC-10 has a perfectly useable system for gating address lines over from the MCU at the appropriate time, so rather than use a 74LS125 and 74LS139, why not simply extend the existing method by a bit (or more, for more memory).

The stock method uses two 74LS367A hex buffers to gate twelve address lines (A11..A0).  We can simply piggyback another LS367A atop one of these and gate additional lines.

To go with this I selected an 8K SRAM, getting the chip count on the mod down from four, to two.  To implement the mod, I removed the two existing static rams, placed a socket in place of one of them which I put the 8K sram into (a mostly compatible pinout) with several of the pins bent out of the socket as needed to make the proper connections.

Some pics of the finished mod

This, along with the piggybacked LS367A, and point-to-point connections made as needed resulted in what I think is a better mod that accomplishes the same goal.  8K of internal ram available to both the MCU and VDG.

 With the 8K mod installed

Testing the video ram with a full hi-res display

As this can easily be extended to a 32K mod, I have plans to draw one up with proper PCBs for a cleaner installation.  This was a fun little foray into modding the MC-10!  🙂


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