Karl, Audio, RF, image and Test ADC I/Q sweeps on the one hand and LPF sweep on the other hand? are using different ways. After reading QMX Terminal Hardware Sweeps you may have a clue where to debug. Maybe it's a broken PCM1804 (see QMX QMX_plus Audio PCM1804 ) or a failure in the section from Q508 up to IC407 (PCM1804).