NAD T 975 C/AH, Service Manual, Page: 5 / 40

ALIGNMENT PROCEDURE (T975)
A. THERMAL SENSE ADJUSTMENT
This adjustment should be always done when the T975 is in a cool condition. (That means the
temperature inside the unit should be almost same as the ambient temperature.)
Connector P208B that connects the transformer and power board should be removed to
avoid the Idling current heating up the unit excessively) At ambient temperature of 25 C
degrees and when the unit is just turned on (still in cool condition), adjust the pot
RV103 (as marked as “ Thermal Sense ADJ ” in below drawing) for a voltage between the
THERMAL TEST POINT and AGND to be 750mV+/-10mV. If the ambient temperature is
different, the voltage setting should be changed by 20mV per degree C difference. So at
20C degrees, the voltage should be 850mV+/-10mV; and at 30C degrees, the voltage
should be 650mV+/-10mV, etc.
B. DC OFFSET VOLTAGE
The DC OFFSET between OFFSET Test Point and AGND must be within 0+/-3mV. If it ’ s
found to be out of this range, change the TL084BCD to one selected for low offset voltage
to meet the requirement.
C. IDLING CURRENT
The idling current in the output stage of the poweramp should be set to 40mA+/-10mA
per pnp-npn transistor pair (This means the mid point voltage is 17.6mV with down limit of
13.2mV and up limit of 22mV). The alignment procedure is: At first, adjust RV102 (Marked
as “ Idling Current ADJ ” in the drawing below) for a voltage of 17.6mV +/- 1mV between
Test point 1 and 2. Then preheat the unit for 5 minutes, let idling current settled and then
readjust 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 the
influence of varied ambient temperature and mains voltage etc., the idling current will drift
after some time. During the normal checking of idling current in the service field, it ’ s
acceptable as long as the voltage is in the range of 13.2mV — 22.0mV.
D. ISC SENSITIVITY
The ISC sensing voltage r a n g e i s 0mV+/-50mV. But, for the alignment process during
production, we tighten the limit to 0mV+/-10mV. That is to adjust Pot RV104 (as marked as
“ ISC SENSE ADJ ” in the below drawing) to get a voltage between the “ ISC SENSE TEST
POINT ” and AGND to be 0mV+/-10mV.
(Please note the ISC should be adjusted when all modules are plugged in. It
’ s not possible to get
0mV +/-10mV if any of the modules are removed.)
E. Fan Cut Off Point Adjustment
Check the Fan sense voltage first. Turn Pot RV201 (on the power supply board) to the end clockwise
and confirm TP
1
of J201 is negative voltage. Run the unit to heat it up and to start turning the fan.
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