Adrian, check for alkaline leaks from batteries. It kills everything it touches on the board.
For those who still have their mooshimeters in working order, replace your batteries with AA-lithiums now. Or actually get lithium batteries for all your toys/tools that are hard or expensive to replace. Those batteries seem expensive at first, but if your device works for a month or more with couple batteries, it is quite cheap insurance against alkaline leaks, which will get you eventually.