Overclocking the Commodore A3640.

The A3640 is the standard 68040 CPU card supplied for A3000 and A4000 Amigas. It is easy to overclock, there being a single 14-pin DIL form-factor crystal rated at 50MHz in the centre of the board. The A3640 also provides some of the motherboard timing, which may cause problems at certain overclocked frequencies. Note that the A3640 halves the oscillator frequency in use so that at 25MHz it actually uses a 50MHz oscillator.

I have overclocked three of these boards, by replacing the 50MHz oscillator by one rated at 60MHz. This gives 30MHz for the 68040. All systems work reliably at this speed, and it's an ideal way to get some extra performance out of this board. I have heard of other people getting more. One user has been able to clock this board as high as 40MHz, and another I have heard of, has managed to get 37MHz. These two users were not able to get a lower speed of 33MHz, possibly because of the design of the A3640, which uses delay lines for part of it's clock circuitry. These delay lines provide a fixed amount of delay which becomes inappropriate when the clock frequency is changed too much. I suspect a "wrap around" effect occurs when the frequency is increased still further, the amount of delay once again becoming more or less correct over 37MHz or so. Recently I have become aware that it is possible to change the amount of delay from this delay line, so that it is now possible to overclock to frequencies such as 33MHz, and possibly more.

I recently had an opportunity to try this delay line modification at a recent Overclocker's workshop, held by the Melbourne Amiga Users Group. Unfortunately, I did not have a lot of time to really test it out. After performing the modification, and fitting an IC socket for the oscillator module, I first tried it at 40MHz. There were no signs of life. At 33MHz the Amiga booted perfectly and ran for about 10 minutes before locking up. It would not reboot at all after that. I then tried 31MHz, and it worked perfectly for the remainer of the night. I suspect overheating was the problem, although nothing on the A3640 got unusually hot. If I had the time, I would have liked to try it with a fan on the 040, and the A4000's case modified for better airflow. Although these results seem ambiguous, I had noted that the A3640 wouldn't even boot at 33MHz with the unmodified delay line on other A3640's I had overclocked previously. I also noticed that the A3640 would still work at the standard 25MHz after this modification, although I didn't have the time to test for proper reliability. Since then I have heard of someone who has successfully overclocked his A3640 to 33MHz, using the cut and jumper mentioned above. He also found his board would not work at 33MHz if the cut and jumper was not done. It seems that the particular 040 in my friend's Amiga could not cope with 33MHz - a classic example of "your mileage may vary".

How to do it - step by step.

1:- Referring to the picture above, cut the track from R101 to pin 4 of the delay line. Disregard the numbers printed on the top of the delay line. Note that not all pins are there too, however, on the PCB you can count the vacant pads until you get to 4, which is connected to the delay line. Unfortunately, that pin is not visible in the picture above, however you can see R101, and the small patch as indicated, which is the cut.

2:- Using a short length of insulated wire, connect one end to the end of R101 that had been cut. The other end of the wire goes to pin 12 on the delay line. This is clearly visible in the picture.

3:- Carefully remove the original 50MHz oscillator. Take care not to damage the plated through holes. Note the pin 1 position, marked with a dot on the oscillator (see picture)

4:- Using a good quality 14 pin IC socket, remove all pins except for pins 1, 7, 8, 14. (only the ones in each corner of the socket should be remaining).

5:- Solder in the new socket, with the notch at the pin 1 end.

6:- add a fan to the 040 heatsink. Also ensure the A4000 case is well ventilated.

7:- Install the new oscillator. Note that the actual 040 frequency is half that quoted on the oscillator, ie, for a frequency of 33Mhz, you will need a 66Mhz oscillator.

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 about 30MHz (60MHz oscillator).

Points to watch.

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

* Use a heatsink fan if you are using a desktop A3000/A4000 and/or you are overclocking it past 30MHz.

* 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 25th July 1998. Updated 8th August 1999. Version 2.1