So back in October of this past year (2015), someone went digging into the code for MK1, MK2, and MK3 and found out that the rumored secret Ed Boon menus actually do exist and are somewhat easily accessible. By pressing the Player 1 and Player 2 'Block' buttons in a particular sequence, you can access each game's secret Ed Boon (EJB) menu. Here are the details regarding how to accomplish the menu access and what each game's individual button combos are. Check them out below:
https://tcrf.net/Mortal_Kombat_(Arcade)#EJB_Menu
https://tcrf.net/Mortal_Kombat_II_(Arcade)#EJB_Menu
https://tcrf.net/Mortal_Kombat_3_(Arcade)#EJB_Menu
https://tcrf.net/Ultimate_Mortal_Kombat_3_(Arcade)#EJB_Menu
And if you'd rather just use this thread as a guide instead of clicking on a bunch of links, here are the cliff note code listings...
The code is P1 Block (x times) then P2 Block (x times) then P1 Block (x times) etc...
MK1...
P1 (5), P2 (10), P1 (2), P2 (1), P1 (2), P2 (3), P1 (4)
MK2...
P1 (5), P2 (10), P1 (2), P2 (8), P1 (2)
MK3/UMK3...
P1 (5), P2 (10), P1 (3), P2 (1), P1 (2), P2 (2), P1 (3), P2 (4)
Utilizing my arcade machines, I have verified that these codes work 100%. Here is a video...
Ed Boon has tweeted about how there are still things in the original MK games that no one has found yet. It's believed that these hidden menus might be what he is talking about. Then again...they may not! We shall see. On a somewhat related note, I have a friend who knows how to manipulate and read the game code and he tells me that he is fairly sure the UMK3 Wavenet code is still buried in the Rev 1.2 UMK3 code and can be extracted to create a playable version of the game. It's not confirmed yet and I'm not trying to get anyone's hopes up, but it appears to be a possibility at this time. So stay tuned. If I hear anything, I'll let you all know.
It just amazes me that after 20+ years, new things are still being discovered about the original MK games. It's just awesome. With this all being said, feel free to try it all out for yourselves. Thanks and always remember to FINISH HIM!