Lets take a look inside:
This amplifier seems to be newer and more sophisticated than NAD 2140. The power transistors are the nice and juicy Sanken 2SA1215 and 2SC2921 with a 15 A, 150 W rating. These could easily power 100W+ amplifiers. But the 50 in 2150 means that the amplifier is regarded a 50 W unit. We will see that this is a typical NAD understatement.
The protection IC TA7317P monitors the health state of the amp and controls a relay. This system also implements a soft on function, waiting a couple of seconds before turning on. The 2140 did not have this relay feature.
Problem: Protection LED never goes off, relay never clicks. Measurements showed that the power transistors in both channels had insufficient VBE, they were "off". The output stages had insufficient bias voltage. Explanation was that a certain function of the powersupply was blown, propably a current source. Q802 (a BC556) was changed and everything was OK! As this transistor gets very hot, a heatsink was attached to it.
Q802 was the faulty transistor
The bias current was rather low, about 10 mA and was adjusted to about 40 mA, as a matter of personal preference.
Another problem showed up. After power on, one channel seemed to have a series resistance in the output. Turning op the volume for a moment removed the resistance. The relay was the culprit. Fortunately it could be disassembled and the contact surfaces were cleaned and roughened, which fixed the problem.
These contact surfaces clearly have seen better daysBy Lars Ole Pontoppidan, Dec. 2005