Enter the title screen[1] while holding left+down OR right+down [2], then select Credits. One of two secret messages will be printed before the thanks.
[1]: For example by exiting the controller screen.
[2]: When the title screen appears you may release the buttons.
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March 13 ninja edit:
These Game Genie Codes will turn Solar Wars into a graphic-less sound test mode:in [url=http://forums.nesdev.com/viewtopic.php?p=135000#p135000]this post[/url], tepples wrote:There's also some apparently unused code related to the NerdTracker II playback code at $8006, but I have no idea what it's supposed to be doing.
UENNXYLK
OENNUNSN
TEKKLLLA
The A button advances to the next track.