Hey DjC, the original batteries went way longer than we expected. Most MP1s are pretty much due for a change. We started getting more and more battery fail issues. There are a few posts IIRC regarding the dead batteries
. When the battery goes you loose all your presets, so if there are any you want to keep write them down. The factory presets are stored in the EPROM so you can load them again easily. The battery keeps the RAM alive and is where all the changed presets are stored.
The battery mod is reasonably straight forward. Just involves removing the old battery and installing a battery holder and new battery (CR2032 Lithium). Then future battery changes a easy.
The battery holder required for MP1 is the type that mounts flat to the PCB. You can buy them from electronics stores (MP2 batteries mount vertically). MJMP also has them available including battery and instructions (http://www.marshalljmpmodshop.net/miscellaneous_parts_and_replacement_tubes.htm).
The battery needs to be 2.7v or better, less than 2.7v you should change.