Hi, welcome to the madness

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Its pretty much all covered if you make use of the search function but generally all you need to convert an existing Jamma cab is a Jpac and an Arcade VGA card from Ultimarc. Plug the Jpac into your jamma harness and connect your AVGA to the Jpac.
TBH if you are going to build a MAME cab from scratch why not restore the existing Jamma cab for PCB's and build a dedicated MAME cab using an IPac?.
I recently converted a Jamma to MAME for more or less the same reason as you but if/when I build a machine from scratch I'm definitely going the dedicated route its less messing about with seperate power supplies and such.
Anyway, best of luck and get us some pics up if you can.