NAD 7150 Trail of Tears

NAD Front & Back

This 1983 vintage unit was acquired in non-operative condtion as a "gift" from a friend.

The original fault was traced to an open resistor in one of the power amplifer stages which caused an output offset thus triggering the protection circuitry.

The one remaining anomaly was an interaction between channel memories 3, 4 & 5. Setting channel 5 after setting 3 & 4 would result in channel 4 being set equal to 5. And setting 3 after setting 5 & 4 would set 4 equal to 3. A minor annoyance.

It worked for a couple of years of occasional use and then failed again. This time an open resistor was found and replaced in the protection circuitry.

Again it worked for a couple of years and tiring of repairing it sans schematic it was stuck in the closet.

I then obtained a schematic and ventured deeper into the swamp.

Checking over the protection circuit, I discovered R670 and R668 to be open. These were replaced with the proper values and the unit stumbled back to life.

One of the channel outputs was still intermittent and if the power was cycled within a couple seconds after power off, the protection circuit would not engage the speaker relay.

The intermittent output was found to be a dirty relay contact. And the cycling problem was found to be a leaky C646 in the protection circuit. The relay contact was cleaned, the capacitor replaced and voila! A fully functioning amplifier.

I left the unit on for 24 hours to "burn in" ..., or out, and at the end of that time it had developed a slight hum in one channel.

This was found to be caused by the preamp supplies being out of regulation and up against the 60V rails due to, no surprise, another open resistor: R807.

After fixing the positive preamp supply, the negative tracking supply was still at 60V negative, and again an open resistor, R803 was at fault.

With the negative supply repaired, the amplifier was once again fully functional.

Flushed with success, I decided to investigate the tuner preset problem previously noted. I observed the tuner control IC, a TC9137BP, had been changed circa 1989 and the soldering flux had not been removed. I cleaned this to no effect.

I then checked the voltages around this part and noticed that it was running on 4 volts. That seemed a bit odd so, I investigated further and found that L301, which was supposed to be a 2.2uH inductor per the schematic and silk screen, had been factory installed with a 220 ohm resistor. Correcting this original error brought the supply voltage up to 5V on this part and the prescaler but, still "no joy" regarding the tuner preset anomaly.

In conclusion, if one ever has to service one of these units, look for open resistors even if they're employed at a fraction of their power rating, especially in the power supplies and output protection circuitry!