The guide for how to fix 99% SNK MagicKey issue
When your SNK MagicKey have some issue, like no graphic / no sound / image display or sound chaos etc. you can try to do :
 SNK MagicKey need more DC power current to work, you should use the 9V/3A or 12V/3A output DC adapter, the original 11V/700mA or 5V old snk DC adapter is can't use with SNK MagicKey. And if your snk console is 5V version, then you should use 5.5V~5.7V and with 3A output DC adapter, to make sure SNK MagicKey get enough DC power current to run stable.
You can not use the normal 5V adapter, because if you use 5V,pass some power circuit adjust in SNK console ,the SNK cart slot will get 4.1V~4.3V finally, this voltage is too low and will make magickey working unstable even stop to work totally.
BTW, you can use some "special" 5V DC adapter still, like 5V/5A big power current adapter, this type 5V adapter can afford enough 5A DC power to make a very low voltage loss at the end, so this type DC adapter will working stable also.
 Many MVS arcade game cart is second hand and the golden finger is very dirty , you need use the pure alcohol or rubber to clean these golden finger , to make sure it can offer a good connection.
 If continued failure still,maybe it's the magickey socket connection issue ,because some MVS cart's PCB thickness is not the standard 1.6MM (specially the copy cart), if the PCB thickness less than 1.6MM will make the socket can not connect to PCB very well and stable, even just 1 pin connection not good then will cause the sound or graphic trouble , so now you need try to adjust the magickey socket pin distance ---
a. You need a suitable screw tool firstly
b. Choice the right one
c. To find the pins that stand back more than other one
d. Use the screw tool insert to the grip and press to the center , to make sure this pin can press the MVS cart PCB enough, to get a good pressure and connection.
e. Don't press to center too much at once, you have to adjust little by little slowly ,utill you can see all pins in a line. Now you can try to run it on SNK console.
f. In this photo you can see which board control the sound , and which control the graphic. For example if your kit no sound, then you just need check the sound board socket firstly and adjust these pins. But if no sound still after your adjusting, then you need to check and adjust the graphic board too, in a very FEW case the graphic board will make not sound output.
These above 3 step can fix 99% SNK MagicKey issue, if after all fixing job your kit can't work still, then you should contact your shop to ask help.