Hi Mike,
The most common (and easiest to correct) reason for this is incorrect antenna selection for the SUB RX. If you hold the BSET button it will momentarily flash which antenna is being used by the SUB RX on the lower display. You probably want to be using the same antenna as your MAIN RX so you want to see MAIN on the lower display and not AUX.
73,
Mike K2MK
mike wrote
Sub receiver all of sudden does not produce signal. Audio is not dead, but cannot hear the VFO B signal in split operation. Where should I start to find what has gone wrong? ..mike