NAD 3020A Constant Hum

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A few months back I recovered an old NAD 3020A from my father's garage. Hooked it up and all works well aside from a fairly loud humming noise coming from both the loudspeaker and the headphone connections.

The hum in question is not affected by the tone controls or the volume knob (unless the phono channel is selected). It's also substantially quieter when the amp is initially powered on thanks to the muting circuit. The only thing removing the pre-amp jumpers does is stop the muting part.

After doing some searching I came to the conclusion that the problem was with the four big caps in the middle of the board and possibly the two at the back right. So I bought and soldered in replacements, powered it up, and there was absolutely no difference. This has left me out of ideas short of replacing more of the caps (which I plan on doing regardless).

Any bright ideas? I'll attach a picture of the amp in question. The black caps are the ones I put in this evening.


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Download a schematic. I understand from various forum comments that model A is almost the same as B, and since free downloads for model A seem scarce, you may have to improvise to identify the relevant parts.

Make sure you have replaced all the electrolytics for the main and preamp power supplies and any others in the power amplifiers. Check the rail voltages are correct in both 8 & 4R modes and that all rectifier bridge diodes are functional. There have been comments in a few forums that the muting JFETS become very noisy and cause bad hum but they are at the output of the preamp and I haven't heard that fault.

Don't forget to search the forum for the other threads discussing the many antique 3020 models with their very common hum problems.
I'll take another look at the B schematic, it didn't seen to match up too well with my A though.

The A doesn't have individual 8 and 4R modes like the B does. How would I go about checking the rail voltages and the rectifiers? (My electronics knowledge is far from great, but I can fill in most of the gaps with some googling.)

I have read other threads on the issue, but have been unable to find an answer as of yet. The mentioning of rectifiers does ring a bell though.

I've also found another big leaky cap underneath the ribbon cable to the right. That and its neighbour are most likely next in line for replacement.
Two other places worth looking. the bridge recifier may have gone - or be only working half n half. You really need to check what's on the rails for this. Also, there could be a small value resistor, but wirewound usually that goes between signal ground and case ground. Abtualy, it may be worth seeing that the transformer centre tap is actually grounded in the first place.
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