Take an ohm meter and test continuity between the three isolated terminals on the 1/4 jack. This will tell you what the wire is used for. The first two terminals on the 1/4 inch plug should be for channel switching and effects loop and the third would be your ground. Based on that you should be able to figure out what wire goes where. You can try contacting marshall for a wiring diagram but schematics of their newer products don't usually get let go. Good luck!
Answers & Comments
Take an ohm meter and test continuity between the three isolated terminals on the 1/4 jack. This will tell you what the wire is used for. The first two terminals on the 1/4 inch plug should be for channel switching and effects loop and the third would be your ground. Based on that you should be able to figure out what wire goes where. You can try contacting marshall for a wiring diagram but schematics of their newer products don't usually get let go. Good luck!
No problem brother