Updating firmware in these radios is a real ordeal. Generally there are three different parts to the update process, and following each step is crucial that they are followed exactly. Below is a YouTube video which walks you through this full process. Hope this helps those whom are struggling to get their radios updated.
Saw this on another website. The Mars mod for the FTM300D appears to be a straight forward process.
Well after mating up the HRO Zum Radio/Modem to my Icom IC-121S this was the best transmitted audio quality I could get. Oddly enough the receive audio works great! I’m sure I just missed some discrete setting somewhere… Details: (Trying to get this working in C4FM) Zum Radio-MMDVM for Pi (GPIO) (firmware = MMDVM 20190130)… Read more »