Heads up, It seems there's a spate of MP1 battery failures lately, the year is upon us :facepalm: , If you have a really early MP1 (i.e. with switch on top and line level rear jack), make sure you've backed up your patches or written them down (well good idea regardless). The ram chip that stores the patches uses the battery to maintain them when the MP1 isn't powered up. And after all these years the batteries are starting to die (way past the original time spec BTW). It takes a 3v Lithium battery, easiest to source is probably a CR2032, you don't need to spend the $s on getting an original (apparently you can get them but there's no need and they are harder to find and cost more). and MJMP has a battery mod kit (so in another 20 years you can just swap it out without soldering LoL) (http://www.marshalljmpmodshop.net/miscellaneous_parts_and_replacement_tubes.htm).
How prophetic...my MP-1 battery died today....
I found some CR2032 batteries with the "legs" attached in the electronics store a while ago. They weren't particularly expensive and are ready to just solder in. While I like the idea of a socket so you can just swap the battery out (while powered up so you don't lose patches), given they last so long and you have to solder in the socket anyway, it seems an easier option to go with the ready to go batteries.