Hello to the Group !
I have the same problem.I built the uBitx V3 from scratch.First I thought that I have ground loops ,but doesn't solve the problem. After many days of research I noticed that if I disconnect the BFO signal from the mainboard the oscillation stops.In this condition I can go with the vol potentiometer to 100% and no oscillation. Now comes the mistery part : with disconnected BFO if I short D5 pins 1 and 3 the audio oscillates again. Remark : I am using 1N4148's for all mixers.
By shorting as above, practically I connect one winding of T7 and T6 through R115 to the ground and to the input of the audio chain . I think that T6 or T7 a responsible for picking up some audio radiated from the TDA2822 output. I used even a screened cable to the speaker. Maybe the pick up is on the mainboard. The layout is similar to KMK version.
The only idea at this moment is to use binocular TV balun cores for T7 and maybe for T6.
I ask like Andy ANY IDEAS ??