Overclocking the Commodore A3630.

The A3640 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.

How to do it - step by step.

1:- Referring to the picture of the motherboard on the left, remove the int/ext jumper, J100, as shown. Note the position of the centre pin of J100 and also note the pin labelled "2" on the delay line chip next to J100. These two items need to be connected together. While this could be done from the top, I prefer doing it from the underside of the PCB to avoid soldering and ruining the jumper pins, should you need to remove the modification later.

2:- The picture on the right shows the underneath of the motherboard at the same location. Note that it is rotated 90 degrees. The added wire can be clearly seen.

3:- Carefully remove the original 50MHz oscillator. Although it's not obvious, it is already socketed. Using a small screwdriver, carefully lever it off. Note carefully it's orientation before removal, as the new oscillatior has to go in pointing the same way to avoid damage. Pin 1 is marked with a dot and also has a square corner on the base of the case. The position of the oscillator can be seen in the left picture. Disregard the foreign looking hack. The oscillator will look just like the other one labelled "do not touch". Do not change the "do not touch" oscillator as the Amiga will not work correctly if it is changed.

4:- Install the new oscillator. Note that the actual 030 frequency is half that quoted on the oscillator, ie, for a frequency of 33Mhz, you will need a 66Mhz oscillator. Be careful as the holes for the oscillator pins are very small and it's not easy making sure all pins go in easily.

5:- If you are using a 68882 FPU, it will be clocked at the same speed as the CPU. If you wish to clock it at a different rate to the CPU, there is a jumper on the A3630 board for this purpose. There is also a space for an oscillator. Solder in a socket with all but pins 1,7,8 and 14 removed. You can then plug in the desired oscillator. Ignore this step if you don't have a 68882.

Should you later wish to upgrade to a different CPU board, you will need to remove the wire added in step 2 and replace the jumper removed in step 1. If you don't want to do the delay line hack, you can omit steps 1 & 2. You will be limited to a maximum frequency of under 33MHz (66MHz oscillator).

Points to watch.

* Pay attention to correct crystal orientation. Getting it wrong will destroy the oscillator module!

* You will need to remove "speedRamsey" if in use - it won't work on overclocked systems, unless you have 40nS or better SIMMs!

* Make sure the 200 pin CPU connector to the motherboard is clean and in good condition.

* Thoroughly check all parts of your system, including any Zorro III cards, especially when overclocking at at higher frequencies - this is due to changes in system timing.

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Introduced 9th July 2000. Updated 9th July 2000. Version 1.0