RGB SCART to JAMMA Adapter

This adapter is intended primarily for connecting video game consoles to arcade cabinets, but it can also be used with old microcomputers and other A/V devices. It's basically an enhanced version of my AC Coupled Video Amplifier with an audio amp attached.

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Features

* 6 Watt audio amplifier with large volume control.
* Line level stereo audio output for use with external amplifier.
* RGB Video amplifier which contains clamp (DC restoration) circuit.
* Brightness (DC level) and contrast (amplitude) of the video output are adjustable.
* Separate Horizontal and Vertical as well as Composite sync outputs available.
* Sync input can be in the form of Composite sync or Composite video.
* Both RGB SCART and crazy Japanese pinout RGB cables are supported.
* Even if you don't like like video games it still makes a fine piece of voyant objet d'art.

Instructions

Observe all these connections.

Audio/Video Input
Audio and RGB video from the game console can be connected by either a European SCART cable or a Japanese 21 pin RGB cable (i.e. one or the other, not both at the same time). The SCART cable being by far the most common in the western world. If you don't know what type of cable you have, just make a guess. They are mechanically compatible with one another and you won't damage the game console or adapter by plugging it into the wrong socket.

I highly recommend the use of official or high quality third party cables. Poor quality cables are often wired incorrectly, break easily, and cause a buzzing sound to be heard through the audio. If you wish to build your own video cables, might I suggest my page of Game Console RGB SCART Cable Diagrams as a reference? Take note that this adapter does ignores the SCART switching voltages on pins 8 and 16.

All pre made SCART/JP_RGB cables will used composite video for sync information. If for some reason you would like to connect a device which does not have composite video available (such as an Amiga or other old microcomputer) you may want to construct a non-standard cable with a TTL composite sync signal in place composite video. There are a set of jumpers located under the label plate the base of the video input connectors. Removing the two screws that hold it into place, you now have access to JP2 and JP3, the sync configuration jumpers.

JP2 on, JP3 off  = Terminated composite video. Default.
JP2 off, JP3 off = Unterminated composite video. Try this if you have any sync related problems.
JP2 off, JP3 on = TTL composite sync or TTL horizontal sync.

The video amplifier will not function without some form of sync input.

Power
The adapter requires a well regulated 12 volt supply capable of supplying at least 1 amp of current. This can be connected via the JAMMA card edge or the terminal block. Other voltage from the JAMMA power supply are not used but are passed through to the female JAMMA connector on the other side of the board.

Sometimes older switching power supplies have trouble keeping regulation without a load on the +5V line. For these situations you will need to connect a power resistor (supplied) between the +5V line and ground to act as a dummy load. This resistor will get hot so should be connected directly to the power supply, away from stray fingers.

Controls 
Control lines from the JAMMA card edge are passed through the adapter to the female JAMMA connector and are also available at the terminal block. What to do with them is left as an exercise for the user. I suggest 'hacking' a control pad and connecting with wires to the terminal blocks or to a finger board seated in the female JAMMA pass through connector. There are arcade stick -> game console encoder boards available such as the UPCB which should be possible to adapt.

Speaker Output
The speaker must have an impedance of 8 ohms or higher (two 4 ohm speakers in series, or two 16 ohm speakers in parallel are ok too) speakers of lower impedance or two 8 ohm speakers in parallel may overload the amplifier. The speaker would normally be wired to the JAMMA but may be connected to the terminal block if desired.

The onboard amplifier is monaural (i.e. has only one channel) which mixes both left and right audio channels together. The speaker must be connected between speaker -ve and speaker +ve, not between either speaker terminal and ground as is normally done with a HiFi amplifier.

There is a place on the board for JP1, a jumper to disable the on-board amplifier. It's not populated and hard wired to the 'on' position. If you would like to make use of this, cut the trace on the bottom linking the two pads together, then solder in a two pin header.

The volume control adjusts volume of the sound, as all good volume controls do.

Video Output
Red, Green, and Blue video is output from the JAMMA card edge as well as a composite sync signal. Video and C-sync are also available from the terminal block along with separate horizontal and vertical sync outputs (I hear some pre-JAMMA arcade monitors require sync signals like this).

There are two controls which affect the video signal, contrast and brightness. The contrast control adjusts the amplitude of the video signal, the brightness adjusts the black level. The brightness control is a high accuracy 4 turn trimpot which is pre adjusted to a black level of 0.3V. This is suitable for most monitors and will usually require no additional adjustment. Note that if the black level is set to less than 0.2V the picture may cut out, even with the contrast at maximum. There are a pair of test points next to the brightness trimpot. I used them to calibrate the black level but you may use them too, should you wish to do so.

Audio Output
The pair of RCA sockets above the the contrast control is a buffered line level audio output from the game console. Useful if you feel the built in audio amplifier is far too puny and would like to use something with a bit more heft.

The level is unchanged by the volume control.

If you are still feeling lost refer to this diagram:

Appendix I: Pin Listings

JAMMA Card Edge + Controls Pass-Through

Terminal Block
(anti-clockwise from the top right)

21 Pin Audio/Video Input

Appendix II: Electrical Specification

Input Voltage Ratings

Input Impedance Characteristics

Output Voltage Characteristics
Power supply = 12V / R, G, B video input = 0.7Vp-p / CV input = 1Vp-p / L, R audio input 0.5Vp-p
Horizontal, Vertical, Composite sync output load = 1.5k / R, G, B video output load = 1k

Other Characteristics

Appendix III: Crap SCART cables and buzz in the audio

To do.

Appendix IV: Resources

• Circuit Diagram - Useful to modify or repair one of my adapter boards, or even build your own.

Page created 13/1/2010.

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