NAD 2600 repair and a valuable lesson learned
A friend of mine picked up an NAD 2600 power amp a few days ago. It would not come out of protect.
I took the cover off and probed the resistors that feed the protect circuit. One side looked good (it wasn't), the other showed about 24 volts. So I started on that channel.
All the voltages were off, but I couldn't find anything wrong. I finally decided to look at the other channel to compare voltages, but now it looked like the other channel was funky too.
Since both channels looked bad I now focused on the power supply. The supplies that feed the output boards were all good, but the +/- 18 volt supplies that feed the input board weren't right; the +18 was only about 1.5 volts.
Replacement of the 7818 3 terminal positive regulator restored the amp to service.
My initial misdiagnosis was caused by dull (as in not sharp) test prods. They are the original test leads that came with my Fluke 177. When I used a sharper prod I got more consistent readings. Don't know what the issue is, maybe flux or tarnish/corrosion. All I know for sure is I spent several hours chasing my tail.
So, lesson learned or at least reinforced. Make sure you're making good contact when taking measurements. Good sharp test prods are key, and as a bonus less likely to slip and short something out.
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