It seems that a number of new users of uBITX are not familiar with the "zero-beat"
process, and not familiar with BFO offset relative to crystal filter bandwidth.
If doing a zero-beat and your BFO is not properly aligned then the received signal
will not be placed properly inside the filter passband. This Allows the opposite
sideband to be heard over top of the zero-beat.
Technical info..... When setting the BFO frequency relative to received frequency,
the BFO should be set approximately 12 db down one side of the crystal filter
skirt. Which skirt is determined by which sideband is going through the filter.
You have a DSB signal prior to the filter and setting the BFO 12 db down the
filter skirt further reduces the unwanted sideband. Doing this also rolls-off low
frequency audio tones to make your sideband signal more efficient.
The uBITX assembly instructions do include a block diagram that should make
this easier to understand.