#1
iv had my marshall combo for about a year and recently the input jack fell inside the app and the cover part on the outside fell off, guitar center fixed it free but why did it happen, and has anyone else had this problem? thanks
#2
Because the nut holding the input jack on came loose. It happens pretty often. Most of the time people notice the jack is loose and tighten it before it falls completely through.
#3
Yep. Get a spanner or a socket, and tighten it. If it comes coming undone, use Loctite or something similar to lock the threads of the jack.
Marshall amplifiers are the truest purveyors of rock and roll known to man.

"And give a man an amplifier and a synthesizer, and he doesn't become whoever, you know. He doesn't become us."

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#5
Totally agree with the above. Just to add if you see it's about to go but you can get a jack plug in then do it. That way you wont lose the internal bit and can tighten the nut with the jack holding it.