Overclocking the A3630 :
The A3630 is the standard 68030 CPU card supplied for A4000 Amigas. It takes it's clock from the A4000 motherboard, although a space is provided for a socket enabling the 68882 FPU (if fitted) to be clocked separately if desired. Overclocking involves removing the 50MHz oscillator on the motherboard and replacing it with one of a higher frequency. The motherboard divides the signal by 2, so to get 33MHz, you will need to fit a 66MHz oscillator. 33Mhz is the highest speed I could get reliably by simply changing the oscillator. This limitation is partly caused by the use of a delay line on the motherboard to generate a second system clock, delayed by 90 degrees. This delay is fixed, so that if the frequency is changed, there is no longer the correct amount of delay to provide a 90 deg clock.
The delay line chip provides several tappings to provide different amounts of delay. Selecting a tapping that provides a shorter delay will help provide the correct 90 deg delay when a higher clock signal is used, increasing reliability and allowing even more overclocking. With the delay line tap changed, I was able to get to 35MHz, using a 70MHz oscillator. Unreliable operation was available to 37.5MHz, using a 75Mhz oscillator. Note that as we are changing some motherboard timing so you may get better - or worse results than me. These variations could be caused by different Zorro III cards and different SIMMs etc. In other words, your mileage may vary. I recommend changing the delay line tap if you intend to overclock at 33MHz or over. Below 33Mhz, the standard delay tapping is still the most appropriate.
The 030 chip used on the A3630 is of a later version, and it still runs quite cool, even at 35Mhz, so a heatsink should not be required.
Thanks to "Dave's Amiga Hardware Page" :-)
