Hello, i have a Uni-T UT55 and is currently giving wrong readings i think the calibration is completely off, as it measures negative voltages in the VDC scale having the test leads in the air, this also occurs in VAC, measuring -230Vac in the air, or measuring negative frequency, no reading is correct, also measuring the resistance touching both test leads gives negative reading in some scales, is this a calibration error or could some part of the circuit be damaged, if it is a calibration error, how could i return it to normal readings. I attach the schematic i found online.