I bought my JM 4 used in the US, but only when I really started to use it (which was when I was back in Germany), I noticed that the Mic Trim poti doesn't work at all. I do get a signal though, and the Signal Level Settings in the display work, but I cannot give any extra gain level with a low output microphone (like my Beyer Ribbon mic)!!! Any ideas from a distance? If the poti was really broken, the mic input would probably not work at all, no?
I'm quite good in fixing obvious things with a soldering iron, but I'm not enough of an electronic freak to measure and analyze what's wrong with a cirquit.