Saturday, 22 October 2016

Overclocking the Macrosystems Draco

article from Dave's Amiga Hardware Page

The Draco is an Amiga compatible computer intended for high quality video production. It is well known for it's efficiency and reliability as well as the excellent software supplied with it (after all, it IS Amiga compatible!). The Draco was one of the first Amigas to have a 68060. Due to the fact that the 060 is only clocked at 50MHz, and that many of it's tasks are CPU intensive, there is a real need to get more speed out of it. I was recently given the opportunity to overclock one at a recent Amiga User's Group meeting, and viewed the task as a new challenge due to the fact it is quite different in design to a standard Amiga, and is one I am quite unfamiliar with. Please don't ask me any further questions about the total knowledge of it is on this page!

The Draco is housed in a large tower case (I understand there was a later version in a smaller case). It contains a backplane board with Zorro II slots and special Draco slots inline with each other. One of the slots is occupied by the CPU/SCSI/RAM board. A display card, (based on the Retina ZIII) and a video card, (based on the Vlab motion) are also supplied. The special Draco bus provides high speed interfacing between these cards.

The CPU board contains a number of oscillators and stand alone crystals. A 25MHz oscillator supplies a clock signal for the CPU (refer to pics 2 & 3 below). This 25MHz clock is doubled by circuitry in the board to provide the 50MHz clock required by the 68060. I assume a PLL (Phase Locked Loop) circuit is used to double the 25MHz to 50MHz. This of course, means that there will be a limitation imposed on how much overclocking can be achieved. The PLL circuit will cease functioning correctly long before the 060's limit of about 66MHz is reached.

A problem was revealed on initial inspection of the board. The 25MHz oscillator is actually a tiny surface mount type, which was physically incompatible with the standard type oscillators I have. Fortunately testing revealed that they were electronically compatible, so it was really only a problem of physical accommodation. The second problem was that the existing oscillator could not be removed with the tools I had with me. Presumably, like most surface mount components, it was glued to the board. To solve this I isolated the 25MHz oscillator by cutting the track leading from it's output pin. Of course, if you have the appropriate tools to allow removal of the surface mount oscillator, and you have the appropriate replacement oscillator in surface mount format, then it is simply a matter of removing the old one and soldering the new one in.

Draco with it's cover removed and the location of the CPU board

The relevant section of the CPU board and the oscillator

Note that a yellow dot has been added which corresponds with pin 1 on the oscillator. Although tiny, this oscillator has the same pinouts as the standard type oscillators and these have been labelled in the picture. Also note that pin 1 is not connected. Also shown is the track to be cut to isolate the old oscillator, and the point on the PLL IC (labelled "OUT") where the output of the new oscillator is to be connected.

Entire CPU board and the location of the enlarged view

Pictures 4 & 5 show the socket with flying leads soldered to the appropriate points on the PCB. To make up this socket, start with a high quality "machine pin" IC socket - in either 14 pin or 8 pin format, depending on what oscillator you are using. Remove all pins except those at the extreme corners of the socket (For a 14 pin socket as I used, only pins 1, 7, 8 and 14 should remain). Solder a short length of insulated wire to each of pins 7, 8 & 14. About 30mm should be plenty. Use black wire for pin 7, which is connected to the point labelled "GND" in pic 2. Use orange wire for pin 8, which is connected to the point labelled "OUT" on the PLL IC - be very careful not to short to the adjacent pins. Finally use red wire on pin 14 to connect to "+5V". Note that all pins are insulated with heatshrink tubing, including the unconnected pin 1, which is more clearly shown in the top left of pic 4.

The socket will allow you to experiment to find the best frequency oscillator to use. The PLL circuit in the Draco multiplies the oscillator frequency by 2, so for 50 MHz, a 25MHz oscillator was used. In my experiments, I found that a 32MHz oscillator would not work, but a 28.6MHz oscillator did work. This corresponds to a CPU frequency of 57.2MHz. I did not have any oscillators between those frequencies, so I was unable to try it at an intermediate frequency. Since then, the owner obtained a 30MHz oscillator and was able to get reliable operation from it, with the CPU at 60MHz. He also told me he had heard of another Draco user changing his oscillator to 33MHz, giving a 66MHz Draco. I was unable to duplicate this, so I assume some other modification was additionally made to allow this to work. Almost all 68060 chips I have seen will work reliably at 66 Mhz, so it is clear the PLL in the Draco is the limiting factor. Also note that the 060 did not get noticeably hotter, due to the modest amount of overclocking and the fact it is already fitted with a heatsink & fan

Insulation tape used to insulate the oscillator

It may be wise to stick the oscillator down with some double side tape to prevent it flopping around.

How to do it - step by step
  1. Using a good quality 14 pin IC socket, remove all pins except 1, 7, 8 & 14. (For an 8 pin socket remove all pins except 1,4,5 & 8)
  2. Solder a short length of black wire to pin 7 on the socket. (For an 8 pin socket, use pin 4)
  3. Solder a short length of orange wire to pin 8 on the socket. (For an 8 pin socket, use pin 5)
  4. Solder a short length of red wire to pin 14 on the socket. (For an 8 pin socket, use pin 8)
  5. Carefully insulate all 4 pins (including the unconnected pin 1) with heatshrink tubing.
  6. Carefully cut the track leading diagonally from under the 25MHz oscillator to pin 1 of the PLL chip (as shown in pic 2 above - note that on this style chip, pin 1 is in the centre along the top. A dimple moulded in the IC package is next to it)
  7. Solder the black wire to the pint labelled "GND" as shown in pic 2 above. Note that this corresponds in position to that on the new oscillator.
  8. Solder the orange wire to pin 1 of the PLL chip. Refer to step 6 and pic 2 for it's location.
  9. Solder the red wire to the point labelled "+5V" as shown in pic 2 above. This also corresponds in position to that on the new oscillator.
  10. Plug in the appropriate oscillator. Pin 1, as marked by the squared corner and dot goes to the unconnected pin. If you intend testing the Draco at this point, make sure the metal can is not touching anything!
  11. Insulate the oscillator can with tape, & fix it in place with some double side tape.

CPU speed (using Syspeed): 66.48 (50MHz), 76.26 (57MHz)
FPU speed (using Syspeed): 26.58 (50MHz), 30.5 (57MHz)
Render a 1 second fade: 33.17 sec (50MHz), 29.6 sec (57MHz)

First Published: September 21st 2000 - Updated April 1st 2001

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