After connecting all the gear, everything sounded good (so, so good), but occasionally the right stereo channel would drop out. A sharp tap on the amplifier would bring it back. Apparently this is symptomatic of failed or failing capacitors, because electrolytic capacitors are generally not designed to last 30 years or more.
I found the service manual for the NAD 3020 online and was amazed to find a full design circuit diagram, parts list, and labelled circuit board wiring diagram. Replacing the caps was going to be easy.
I put together a quick parts list of all the capacitors I had to replace - 42 in total. Only the electrolytic capacitors required replacement because the conductive electrolyte inside them can (and did) leak over time. The ceramic and mylar capacitors inside all passed a visual inspection.
I placed a Digikey order (~$60 for the bare minimum parts but I ordered spares and ended up around ~$90) and warmed up my soldering iron. Sure enough, a good number of the caps were trashed:
One daugherboard with caps removed. You can see the gunk where the electrolyte leaked in at least two places.
The order arrived and I started installing new caps Here's that daugherboard back in place. Anything blue or gold in this pic is brand new:
These 2200 μF caps were the biggest, and replaced 4 completely trashed caps on the input power filtering (I think). All the new parts were different brands and dimensions than the old ones, but that's okay. For cap replacements, as long as the capacitance is the same and the voltage rating matches or lightly exceeds the original, you should be good. Or so I read.
Powered back up!
The first thing I noticed was that everything sounded a bit sharper, but the amp still has a beautiful warm tone. When I listen to music on this setup, I am amazed to hear new details in songs I've heard hundreds of times - instrumental trills, soft vocals, basslines, and more.