James Littlejohn has successfully overclocked his Apple IIc Plus to 10MHz with nothing more than an oscillator change and it works really well. Old news, right?

Overclocking the IIc Plus has been around awhile; hackers have been bumping the speed on the IIc Plus for years using various techniques. The most common procedure has been to replace the old oscillator, CPU and cache RAM with faster versions to achieve the desired speed increase (usually around 8MHz). You had to be handy with the soldering iron too.

Then earlier this year, we had a breakthrough; it was discovered that some IIc Plus boards overclocked with just an oscillator swap. It was still considered voo-doo though, because while some boards readily overclocked, there were others that refused to go any faster than their native 4MHz. No one knew why… until now.

I visited James recently in Chelsea Oklahoma, and left a couple IIc Plus boards with him to upgrade. While trying to get them to work at 10MHz, he made some interesting observations that he’s allowed me to post here.

While trying to overclock a couple of Sean Fahey’s Apple IIc Plus motherboards, I made a few interesting discoveries.

There are at least two versions of the Apple IIc Plus motherboard. You can identify them by the the silk screening which can be found under the power supply. The “A” motherboard is silk screened “1987/88″, and usually has socketed cache RAM. The “B” motherboard has “1987/88/89″, and features soldered cache RAM.

There are also two manufacturers for the ASIC chips used on these boards, NCR and UMC. The NCR chips appear primarily on the “A” boards (and occasionally on some “B” boards), while the UMC is found only on the “B” boards.

While experimenting with overclocking both boards, I’ve come to the following conclusions:

The “A” boards with the NCR ASIC are easily overclocked, as is. The “B” boards with the UMC ASIC will not overclock at all. However, some “B” boards shipped with the NRC ASIC.

Now here comes the kicker:

An “A” board with the UMC ASIC is able to be overclocked. So is a “B” board with an NCR ASIC. This suggests the NRC ASIC can be used in either version to achieve at least 10MHZ (maybe more — 40MHz crystals were the fastest I had on hand), without having to replace any stock chips. However, the UMC ASIC can only be accelerated in an “A” board.

I conducted these tests using 2 Apple IIc Plus “A” boards, and 4 “B” boards.

Wow, there you have it. I wouldn’t be surprised if we see an overclocking kit from ReactiveMicro at some point in the near future.

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