Sega Mark III RGB cable & amp

The Sega Mark III has RGB output capabilities, and can use the same RGB cable for the Sega Master system. However the RGB signal is very weak and will display quite a dark picture. To fix this the RGB signal needs to be amplified. This can either be done with internal modification or an external amplifier in line with the RGB cable. I chose the external modification option so I could keep the console original.

Although unrelated to this RGB output mod for the Mark III, when playing SG-1000 games in a Mark III the colours will be slightly but noticeably different due to the colour palette the Mark III uses. Unfortunately my desire to play SG-1000 games in RGB will need to be achieved using an SC-3000 mod.

Most information I used for this was taken from https://shmups.system11.org/viewtopic.php?t=60233 and https://nintendosegajapan.com/2015/03/14/sega-mark-iii-external-rgb-amplifier-with-fm-audio-break-in/

There are several ways to build the amp, I went for the method using a THS7314. These are cheaply available, and if you look on eBay you can get them pre soldered onto Dip 8 adapter – making it easy to solder onto strip board.

The schematic below shows how to create the amp circuit using a THS7314. I found that the input volatge for the RGB signals needed to be reduced as the resultant image was too bright on a Sony PVM.

Schematic credit – gatsu25 on https://shmups.system11.org/viewtopic.php?t=60233 

 

On the outputs 0.7V peak to peak is needed. The output from the Mark III need to be attenuated before it hits the input of the amp. The amp amplifies the signal by 6db / times two. To achieve this a voltage divider can be used on the inputs. As the voltage needed to come down from 1.4 to 0.7 on the inputs, I tried a voltage divider with 2 x 580 ohm resistors and it worked ok.

So there are much cleaner and tidier ways to make these circuits using fabricated PCBs, but for convenience I just go with strip board.

Here is the THS7314 amp.

The Mark III video port uses a U shaped 8 pin din – soldering cable wires on and heat shrinking each connection.

Starting to prep the strip board before soldering. Planning out where components will go, I grind some holes in the strips to break the connection using a burring bit on a multi tool.

The video amp will be housed in a small project box. Marked out some holes for cables to go in and out of the amp box, then drilled out and finished up with files.

Even though I don’t own a Mark III FM unit, I am going ti install a breakout port which will allow the FM unit to be used in conjunction with the FM cable. This is needed as the FM unit does not support RGB, only composite. Essentially this way the console will provide the RGB signal, with audio provide by the FM unit via the breakout port. A 5 pin din socket that has all the pins aligned in the lower half in a curve formation is required for the FM unit. To avoid risk of shorting if anything came loose, some heat shrink tubing applied to the DIN socket.

Construction of the amp quite simple. Used a couple of traces to join up ground connections.

Starting to install into project box. to connect the cable they need to be fed through the project box holes before soldering onto the PCB. Also used rubber grommets to hold onto the cable tightly to avoid movement within the casing.

Now complete. Applied some labels.

The screenshot below doesn’t really do it justice, but its a much nicer RGB picture.

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