The unit operates between 70 ... 72 MHz with IF (from my FT-450) between 28 ... 30 MHz.
RF output power is appr. 10 Watts.
Although, I do not see much activity on the 4 m band, I spotted some stations operating in MSK144 on 70.175 MHz. Maybe that is something to focus on coming days.
Circuit diagram 28 <=> 70 MHz transverter
Transverter board
Update May 6, 2020 :
Update May 24, 2020 :
First FT8 4m QSO with the transverter over 24 km with PA3EWP.
Update May 27, 2020:
Frequency counter and V/A meter incorporated: