The most common cause of the velocity channel missing is core in the velocity sensor falling off extension that connects it to the yoke. To check this remove the front cover, mark the velocity sensor (small round sensor) so you can put it make on in its original location in the block. Remove the sensor. If the core has unscrewed from the extension reinstall use blue lock tight and hand tighten. Do not flip the core end for end or you will reverse the signal.

If the velocity and displacement channel are missing (only show a flat line) the d-sub connector connecting the sensor wires to the electronics board may have come off. Remove the rear cover and check the connectors at the bottom of the electronics board.

If all channels show a flat line the power supply my not be plugged in or defective.