Power Up Basics (Neo Geo MVS MV6)
The Neo Geo MVS 6 is an MVS arcade setup with 6 cartridge slots.
The system has 2 PCBs. The stack also has a metal shield over the top.

Once you remove the metal cage it exposes the 2 PCBs stacked together.

And under the top game slot PCB is the main motherboard.

The system is powered up by 5V and 12V input at the bottom left of the bottom motherboard.

If we look in the Neo Geo MVS Operation Manual we can see the pinout.

This means we need to send 5V and 12V into the system.

However, if we follow the 12V rail, it just goes to the HA13001which is the audio amplifier for the external speakers, and then into the 7805 regulator to power the Yamaha YM2610 sound chip and several other audio related chips.
If you like, you can ignore the 12V input and test without audio, or temporarily bridge the 5V output of the 7805 to the main 5V rail, and power the system via a single 5V input.
This will allow you to test the audio out of the AV connector but not use external speakers.
Here is an example of joining the 5V main rail and 7805 output rail together, and powering from bench.

Notice the pad on the far right is joined to the 5V main input on the connector, it is just a convenient place to join the 7805 output to the main 5V rail.
Then the two cables coming off the 7805 are the input from bench (ground = black, 5V = red).
This allows you to power the system from a simple 5V bench power supply, so long as you do not expect external speakers to work.
To connect a monitor up the output is RGB on the same edge connector.
Pins 12, 13 and 14 (and the 2 left pins underneath) are the RGB and Sync pins.

The top 3 pins from left to right Red, Blue and Ground.
The bottom 2 pins from left to right are Green and Sync.
The vias connect the bottom pins to the top just above the vias for the top pins.
I usually use a CleanRGB cable with some flying wires off the AV connector, to an OSSC to get HDMI output from the MVS.

If you power up the system with no top game slot PCB just the bottom motherboard, 5V applied to the input and RGB output to monitor, you should see the checkerboard pattern on screen.
Power draw should be around 2A.
Once you connect the top game slot PCB the power draw will jump to 6A to 7A (depending on how many games are inserted.

Making sure the top game slot PCB is connected and the 2-pin 5V cable in the top center is also connected, you can now insert games while the power is off, then boot up.
Current will jump to 6-7A so you will need a power supply capable of handling that.
If the game loads correctly it should boot straight into the game. If the game load fails for any reason you will end up seeing the checkerboard screen.