- The LaST Upgrade -


Last updated October 23, 2023


Video noise has long plagued the ST series of computers. The original STM machines were notoriously bad for video output. Things slowly improved over the STF/M range but it was still never great. There has been a lot of previous research and experiments done by myself to trying improve on it but ultimately I gave up... Until now.

What I have found is there is a lot of noise on the RGB lines. I first tried adding various capacitors and resistors to smooth it out. Where it did help. But it also made the image look a bit blurred. The shifter itself seems to be the cause of the problems. I tried simply buffering the RGB outputs with a Schmitt buffer to clean up the lines before they go into the transistor drivers. But it did not help much, if at all. I also have swapped out the 2N3904's with a generic BC548 transistor and this helped reduced the problem. I have tried a BC337 but there wasn't much different between that and the 2N3904's. There is also noise on the 5V rail as it travels from each RGB driver transistor. A 1uF capacitor on the collector of Q3 to 0V cleans the 5V line up, but does not really help much with video quality. The old style resistor ladder isn't ideal either. a lot of the problems are just inherently down to bad PCB layout.

take the layout of the motherboard below for example. When the power supply enters the motherboard just to the left of the shifter. We basically have the CPU on the far right of the motherboard. This basically means that the majority of the digital noise is passing by the analogue shift area back to the power supply. Any digital noise is then coupled into the entire shifter video circuitry creating a bit of a mess! later motherboard revisions had the CPU towards the front middle of the motherboard which greatly reduce this type of problem.


The examples below other types of video interference. It is difficult to capture on the camera unfortunately.


This video DAC board increases sharpness and clarity of the Atari ST RGB video signals. On practically all machines have some sort of interference which is practically impossible to remove because of the cheap implementation of a RGB resistor ladder. We bypass that circuit and feed it with a more modern video DAC IC instead.

The image above is of the prototype video DAC board. It plugs into the shifter socket and the shifter plugs on top of the board. There are three resistors which are linked to the video output connector which will be explained below. These are removed and fed with the new RGB signals from the DAC IC.


There are generally three resistors of 27R value which are located and removed from the motherboards video area. It is important to note that the resistor numbers change across various motherboard revisions and should not be used. Similarly. The 3 transistors also need to be removed and again, do not assume the numbering is the same across all motherboards.

The best method of locating the resistors is to place a small piece of solid strand wire ( or a resistor leg) into the RGB socket and then using a multimeter continuity check all the resistors under the RF can around the shifter until you locate where the RGB signals are. Once you find those resistors remove themand then solder the RGB wires from the DAC board into the relevant locations which connect to the RGB video connector.


The fourth wire which is located on the right-hand side of the video DAC board is the BLANK signal which comes from pin 36 of the PLCC68 GLUE IC.

In the case of my motherboard there was three diodes which all connect to the BLANK signal next to the shifter.

If all goes well you should have a very nice crisp and clear video output :)


The images are spoiled by my crappy TV and camera unfortunately.

I did a video to explain how to install the kit here https://www.youtube.com/watch?v=qYKmz6PzUjY



Version 6 holds the 16way connector which fits the 4MB MMU RAM upgrade kit. This way you can use the DAC board and the RAM upgrade without having to stack 2 separate boards.



If that capacitor is in your way, you can simply remove it.

You can fit a smaller 100uF 16V capacitor if you so desire.