Task: Modify the Video Circuitry of Your Atari 1040STF to Make it Compatible with RGB to SCART to HDMI Conversion to Connect to an LED Screen
Needed: Soldering Iron, Desoldering Iron, Solder, Several Electrical Components
Time: 1-2 Hours
Introduction
I have previously posted about purchasing an Atari 1040ST as one of the computers from the late 1980s featuring the Motorola 68000 CPU. As I mention in that post, I ordered an RGB to SCART cable and a SCART to HDMI converter to be able to send video to an LED screen. However, this setup did not work as many claimed. I was at a loss and ended up posting a request for help on an Atari ST Facebook page. Someone responded to let me know that the STF is missing important video circuitry that creates a composite sync signal required by SCART. Fortunately, this is an easy fix if you know how to solder. Note that some ST boards (especially STFM or Mega ST) already include this circuit; others leave it unpopulated as was my case.
Instructions
First, you will need to locate or purchase the necessary components. You will need two diodes with model #1N914. You will also need three resistors (100, 150, and 10K Ohms). Finally, you will need one transistor with model #2N3904. Here is the Amazon link to the diodes, resistors, and transistor I purchased.
Second, you will need to open up your Atari case and remove the keyboard, RF shield, floppy drive, and power supply. Note the steps to reverse the process to put everything back together again later. Disassembly is pretty straightforward. Note that I discarded the top RF shield as it is not needed.
Third, locate the area where some components need to be soldered. This is the empty space in the top middle of the board as shown below. You can see the white silkscreen markings where the components go. The next photo shows this area in more detail.
Fourth, you will need to remove the solder in the holes where the components go. I use a desoldering iron for quick removal without damaging the solder pads. You could also carefully use solder wick. Below is the photo of my board after the solder had been removed.
Fifth, it is time to solder in the components. The diodes go in CR10 and CR11 as shown below. Note the end with the back band matches up with the white line on the silkscreen. The 10K Ohm resistor goes in R49. The 100 Ohm goes in R50 while the 150 Ohm goes in R51. The transistor goes in with the flat end to the back as shown on the silkscreen. Note that the bottom of R51 did not have a solder pad on top for some reason. It did on the bottom. This leg does not connect to anything on the circuit board and is floating.
Sixth, that is it! Time to reassemble the computer.
Seventh, now it is time to connect the RGB to SCART cable to the computer. I bought this SCART cable from a seller on eBay for about $20. Here is the SCART to HDMI converter I used. Connect the HDMI to your LED screen.
Eighth, it is time to power up the Atari to test the video signal. Below is what mine looks like.
Comments
Who knew the 1040STF needed extra components to work properly with SCART to HDMI?
Here is an Atari Age forum post that I worked from for this mod.
Note that this mod fixes sync, but the SCART cable must still provide proper RGB wiring and switching voltage. Some passive cables will not work with HDMI converters.