Nad S-300 Service Information

Service Information S300

Circuit Change

PROBLEM: You may receive a complaint that the unit goes into protection
mode.

CAUSE: The transistors in the gain stage maybe overheating causing a dc offset
in the output.

SOLUTION: Increase the value of resistors R311/313 and R312/314 from 93
ohms1/4w to 180 ohms 1/4w. The dissipation of transistors Q307/311 and
Q309/313 drops significantly.

Please pay attention to any discolouring of the pc board as it may indicate the
transistors need replacing (prolonged heat may cause transistor to drift or
breakdown).

After performing the above change, check the idle current. A value of 30 -35mV
is normal. Excessive idle current can blow the amplifier channel.

Also if the idle current cannot be adjusted up enough then change the value of
R501/502 on each amplifier board. If these resistors are 4k on the board (the
service manual may indicate 1.5K) increase slightly. A 4.7 k gives good results.

Please note: A variac is definitely required for setting the idle current. Bring the
AC voltage up slowly while monitoring the idle current.

Contact:

NAD Electronics International
633 Granite Court
Pickering, ON Canada L1K 3K1
Voice: 905-831-0799 FAX [phone]

www.nadelectronics.com 10/6/03