I am making progress with my MTR-5B based GoBox project, and in the course of trying a few QSOs, I noticed that the MTR-5Bs sidetone volume is insanely loud, as compared to the MTR-3B.
Fortunately, I think I found a solution: Heinz, HB9BCB has found that resistor R26 seems to be the devil and must be replaced by a 100-120k variant. I have not yet tried this but will shortly.
Here is Heinz’ original post:
73 de Martin, DK3IT