NAD M25 C Service Manual

Contents

NAD M25 C Service Manual Manual pdf 5 page image

ALIGNMENT PROCEDUREA. THERMAL SENSE ADJUSTMENTThis adjustment should be always done when the M25 is cool condition. (That means thetemperature inside the unit should be almost same as the ambient temperature.Connector P208B that connects the transformer and power board should be removed toavoid that the Idling current heats up the unit excessively) At ambient temperature of 25 Cdegrees and when the unit is just turned on (still in cool condition), adjust the potRV103(as marked as “Thermal Sense ADJ” in below drawing) for a voltage between theTHERMAL TEST POINT and AGND to be 750mV+/-10mV. If the ambient temperature isdifferent, the voltage setting should be changed by 20mV per degree difference. So at20C degrees, the voltage should be 850mV+/-10mV; and at 30C degrees, the voltageshould be 650mV+/-10mV, etc.B. DC OFFSET VOLTAGEThe DC OFFSET between OFFSET Test Point and AGND must be within 0+/-3mV. If it’sfound to be out of this range, change the TL084BCD to one selected for low offset voltageto meet the requirement.C. IDLING CURRENTThe idling current in the output stage of the poweramp should be set to 40mA+/-10mAper pnp-npn transistor pair( This means the mid point voltage is 17.6mV with down limit of13.2mV and up limit of 22mV). The alignment procedure is: At first, adjust RV102 (Markedas “Idling Current ADJ” in the drawing below) for a voltage of 17.6mV +/- 1mV betweenTest point 1 and 2. Then preheat the unit for 5 minutes, let idling current settled and thenreadjust to 17.6mV +/- 1mV.Pls note:1. The idling current must be adjusted when all modules are assembled into the unit.2. Though the idling current is set as 17.6mV +/-1mV during the adjustment, due to theinfluence of varied ambient temperature and mains voltage etc., the idling current will driftafter some time. During the normal checking of idling current in the service field, it’sacceptable as long as the voltage is in the range of 13.2mV—22.0mV.D. ISC Current sense zero-set.The ISC current-sensing voltage at no signal should be 0mV+/-50mV. During factoryalignment, it is set to 0mV+/-10mV . That is to adjust Pot RV104 (as marked as “ISCSENSE ADJ” in the below drawing) to get a voltage between the “ISC SENSE TESTPOINT” and AGND to be 0mV+/-10mV.( Pls note the ISC should be adjusted when all modules are plugged in. It’s not possible to get0mV +/-10mV if any of the modules are removed.)E. Fan Cut Off Point AdjustmentCheck the Fan sense voltage first. Turn Pot RV201 to the end clockwise and confirm TP2of J201 is in negative voltage. Run the unit to heat it up and drive the fan to start turning,- 5 -