Hameg HM404-2.02 Manual

Hameg HM404-2.02 Manual Manual pdf 31 page image

31Subject to change without noticeAdjustmentsThe polarity of an unknown diode can be identified bycomparison with a known diode.Testing TransistorsThree different tests can be made to transistors: base-emitter, base-collector and emitter-collector. The resultingtest patterns are shown below. The basic equivalent circuit ofa transistor is a Z-diode between base and emitter and anormal diode with reverse polarity between base and collectorin series connection. There are three different test patterns:For a transistor the figures b-e and b-c are important. Thefigure e-c can vary; but a vertical line only shows short circuitcondition.These transistor test patterns are valid in most cases, butthere are exceptions to the rule (e.g. Darlington, FETs). Withthe COMPONENT TESTER, the distinction between a P-N-Pto an N-P-N transistor is discernible. In case of doubt,comparison with a known type is helpful. It should be notedthat the same socket connection (COMP. TESTER or ground)for the same terminal is then absolutely necessary. Aconnection inversion effects a rotation of the test pattern by180 degrees round about the center point of the scopegraticule.Pay attention to the usual caution with single MOS-components relating to static discharge or frictionalelectricity!In-Circuit TestsCaution!During in-circuit tests make sure the circuit is dead. Nopower from mains/line or battery and no signal inputsare permitted. Remove all ground connections includingSafety Earth (pull out power plug from outlet). Removeall measuring cables including probes betweenoscilloscope and circuit under test. Otherwise bothCOMPONENT TESTER leads are not isolated againstthe circuit under test.In-circuit tests are possible in many cases. However, they arenot well defined. This is caused by a shunt connection of realor complex impedance - especially if they are of relatively lowimpedance at 50Hz - to the component under test, oftenresults differ greatly when compared with single components.In case of doubt, one component terminal may be unsoldered.This terminal should then not be connected to the groundsocket avoiding hum distortion of the test pattern.Another way is a test pattern comparison to an identical circuitwhich is known to be operational (likewise without power andany external connections). Using the test prods, identical testpoints in each circuit can be checked, and a defect can bedetermined quickly and easily. Possibly the device itself undertest contains a reference circuit (e.g. a second stereo channel,push-pull amplifier, symmetrical bridge circuit), which is notdefective.AdjustmentsAs described in the “Menu” section, the instrument`ssoftware contains the submenu “CALIBRATE”. The controlsregarding the menus are described under item MENU (34) insection “Controls and Readout”. After a warm up time of 20minutes, the following items can be used by operators notequipped with precision instruments.Both items are regarding the instrument’s temperatureresponse under extreme environmental conditions. Similareffects can be caused by component failures as a result ofthe application of too high voltage at an input and thereforecannot be compensated by the automatic adjustmentprocedure. During these automatic adjustments there mustbe no signal applied to any input.