OpenGD77

It should not be a mechanical problem. I even tried different electret microphones with some wires and open transceiver housing. So the GD77 seems to be the better overall radio. Although I like the additional buttons of the DM-1801. But performance on DMR seems to be limited then...unless somebody...

Well, finally I got a grab on my CDR-300UV. As a test, I've successfully programmed it via RT73 CPS - so most likely it's really the same radio. I've opened it to investigate. It's really a superhet, though I found only 1st IF filters - but it's HR_C7000-based, so I doubt it will be possible to port...

AFAIK, this radio is just a relabeled Kydera CDR-300UV.
The manufacturer claims it's a dual-conversion superhet, so an RF part is clearly different from GD77/dm1801.
Can't say a thing about DMR chip. There are no PCB photos available, or at least I couldn't find them.