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Old 4th March 2011, 04:11 PM   #1
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Join Date: Mar 2011
Default NAD 3020 with low freq. hum and bimetallic clicking sound

Hello,

My old (well I guess they're all old at this point) NAD 3020 has developed the following symptoms: When turned on, after several seconds a click is heard that sounds just like a bimetallic switch element turning on or off, and a low (about 60 Hz) hum is heard that does not change regardless of the volume setting. The switch apparently continues to cycle back and forth every several seconds, with the hum coming and going.

I have seen the other threads dealing with the hum symptom such as THIS ONE but am not sure if my symptoms represent two distinct problems with the amp (the switching/clicking and the hum) or if they are both caused by the same problem. Will probably try to locate the rectifiers per the suggestion in the thread I linked above and replace them. I suppose I can also pull the amp out of its cabinet and plug it in to locate the switch or fuse that is making the clicking noise, but I imagine if it is a thermal safety switch it is cutting in and out for some protective reason, and I will need to figure out why it is doing this.

Any guidance or suggestions appreciated!
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Old 4th March 2011, 09:02 PM   #2
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Join Date: Apr 2010
Location: Coffs Harbour, on the east coast
There certainly are bimetallic protection fuses in the 3020. You have found them doing what they are supposed to do when there is a DC fault or short at the output stage.

Indeed, there are several NAD 3020 and very similar model threads to be found here by using the search button. Every week there are 3020 type problems, so you will understand what is going on, knowing that this sound is effectively a fuse blowing repeatedly. It is likely that with this sustained fault condition, that more parts than just the output transistors are blowing or stressing.

In the first instance, remove all speaker loads and signal inputs, turn vol. down and meter across eachpair of output terminals on both channels in as brief a time as possible to minimise further destruction. You are looking for low, 5- 25mV readings but the opening switch will interrupt this, and read zero when open. Let's see what you measure as best you can.
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