Supergun ("Control Box" in Japan) is a strange name for devices that allows using arcade boards (not only MVS) as consoles. Many different models can be bought as finished products or kits. (Flawed) schematics can also be found.
Most often, the supergun doesn't supply power to the board itself but provides a standard socket to plug an off-the-shelf computer power supply.
A/V signal adaptation
Video output (RGBS) from the JAMMA edge is high-level and non-uniform impedance. Very-high impedance, video-bandwidth amplifiers/buffers are necessary to prevent distortion and provide a steady impedance match for 75 ohm TV inputs. Wiring JAMMA video outputs directly to a SCART cable will produce a very dark (attenuated RGB voltage) picture. The picture will also have non-linear distortion because the video DAC cannot source enough current into the TV's load for most colors.
Audio output is low-impedance and (relatively) high-level for direct connection to 8 ohm speakers. An attenuation circuit ("line-out converter") should be used to drive line-level TV audio inputs. Otherwise the signal's large swing will fall outside the amplifier's linear range and could potentially damage an unprotected input stage.
The Neo Geo, amongst most arcade systems, outputs a malformed sync signal without proper serration or equalization pulses. This will not noticeably affect operation most TVs as the horizontal and vertical oscillator aberrations will typically fall within blanked video. Like video, JAMMA sync outputs are high-level and high-impedance. To drive a TV or broadcast monitor's sync input, the JAMMA signal should be buffered, attenuated and impedance-matched. Because the signal is bi-level, a biased logic gate buffer is sufficient. The attenuation/impedance match circuit should have a bandwidth >500 kHz to preserve sync's edges; a typical single-resistor solution forming a voltage divider with the 75 ohm terminator may not be acceptable.
Video output conversion
RGBS can be converted to Y/C or CVBS. Conversion to HDMI is way more complex and is often done by a separate device.
PAL/NTSC CVBS burst generators ? Component output ?
Joypad signals translation
A supergun may accept different kinds of joypads (Genesis, SNES, ...) and translate the serial signals to straight parallel signals.
Special I/O for arcade cabs can be broken out to solderable pads, screw terminals... Coin-in inputs are often connected to momentary switches.
Voltmeters and ammeters can be hooked up between the PSU and board to measure voltage and current.