4375M Thompson - MGA Multi-sync monitor with the ST


  1. You must invert the Horiz and Vert Sync (use 4069 IC)
  2. Use Practical Solutions Monitor switcher
  3. Monitor does not have audio, must use external amp and speaker
  4. Add 68 ohm resistors in series with RGB from Computer.


The DOT Pitch on the Thompson is the same as the original ST Color monitor (.31) and the convergance on this monitor is very good. The only problem I have is that it is a 14 inch monitor and you need to sit back another two feet for the scan lines to look the same (the Atari Monitor is a 12 inch). Or you can reduce the size of the screen with the Vert and Horiz size controls.

Monochrome is good (higher scan rate) but you need to change the vert size to max to get the proper ratio (Horiz/Vert size). You need to add 68 ohm resistors in series with the RGB lines from the computer as Color output has a tad too much drive for this monitor. Monochrome out from the ST needs to drive three lines (R,G,B) and loads down so that its drive is slightly lower. (resistors not needed)


This monitor has superimpose ability, multi-sync ability, switching from TTL to Analog mode with either a switch or line going high-low on the Analog plug. Composite Video, TTL, EGA , VGA, PGA and Analog modes are all supported.


This monitor should give you a very useable picture and give you the added benefit of having Mono mode available with the click of a button. Not to mention that the new Computers Atari is coming out with will need a Multi -Sync for those higher resolutions and more colors. By the way, the current price of this monitor is $399.00 mail order.


Thompson brings out a 5v supply on the Analog plug that you can use to power the sync inverter (4069 IC) this line can't power anything more than .25W. I used 1/2 of a printer cable I had lying around...the DB25 male that plugs into the ST for the Analog plug to the Thompson.

Below are the pin assignment for the ST and the Thompson.

 Thompson  DB 25 Plug                    Atari  Monitor Plug
 Pin #  Signal description           Pin #   Signal description
  2     Red video .6v +going         7       Red Video 1VPP +going      
  4     Green video .6 +going        6       Green Video 1VPP +going
  14    Blue video .6 +going         10      Blue Video 1VPP +going
  16    Horiz sync TTL Pos           9       Horiz Sync TTL Neg
  17    Vert sync  TTL Pos           12      Vert Sync TTL Neg
  21    5 Volt supply                8       12 Volt supply for pullup
  1     Ground                       13      Ground
  3     Ground                       1       Audio output
  5     Ground                       4       Monochrome Detect
  15    Ground                       11      Monochrome 1vpp +going
  25    Ground  

There are many ways that you can connect, using switches relays or solid state switching IC's, these pins together to achieve Mono and Color modes. I'll just give examples for both modes and leave the actual choice of method to you.

                ************* COLOR *************
             Thompson      Res 68 ohm           Atari
              Pin 2--------~~~~-----------------Pin 7
              Pin 4--------~~~~-----------------Pin 6
              Pin14--------~~~~-----------------Pin 10

                  4069 IC    (Sync Inverter)
   Pin 9 Atari -|1     14 |--5v Pin 21 Thompson
Pin 16 Thompson-|2     13 |-Gnd
  Pin 12 Atari -|3     12 |
Pin 17 Thompson-|4     11 |-Gnd
           Gnd -|5     10 |
                |6      9 |-Gnd 
           Gnd -|7      8 |

All grounds are carried through. Pin 14 of the 4069 is bypassed to ground with a 4.7 Ufd cap. Unused input pins on logic chips should not be allowed to float. They must be attached to ground or B+.

               ********** Monochrome **************
                 Thompson                  Atari
                 Pin 2---------------------Pin 11
                 Pin 4---------------------Pin 11
                 Pin 14--------------------Pin 11
                                      Pin 4 to Gnd (Mono Switch Line)
                                     Pin 13 Gnd

All grounds carried through....use same Sync circuit. Sync is the same for both Color and Mono mode. Audio is not provided in the Thompson Monitor and you will have to provide an external amp and speaker for the sound.

The most important thing to realize is that monochrome and color lines can not be hooked up at the same time, that's why you need a multi-pole switch like in Monitor master and the new Astra Monitor switch box. You can make your own or buy one from the above companys. The only part that might be hard to get would be the Atari Plug. One source of the Monitor Plug is Practical Solutions 602-884-9612.

There are many possibilities allowed for in design of the Atari Monitor output plug. Pin 3 of that plug is a general purpose Output line that can be used to throw an electronic latch so that no mechanical button is necessary to switch between Mono and Color mode.

                            Jeff Rigby
                     Intersect Software Corp.
                         3951 Sawyer Rd. 
                       Sarasota, Fl. 34233

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