Measuring A Capacitor

My feeling, unfortunately, is that you'd be wasting your time trying. To properly test a capacitor you would need a capacitor tester. And you would probably have to remove at least one end of each capacitor from the circuit it is connected to, so that the readings wouldn't be confused by the surrounding circuits. And there are undoubtedly dozens or hundreds of capacitors in the unit, and without a schematic diagram you would probably have no way of knowing which ones to focus on.

The likeliest bet, though, would be that the problem is an electrolytic capacitor in the power supply section. Those are probably relatively large "can" type capacitors. If you wanted to go to the trouble, you could simply go ahead and replace those, and that could very conceivably resolve the problem.

Otherwise, I'd suggest connecting shorting plugs to the phono inputs, and disconnecting all other inputs. If you still hear the hum, try reversing the polarity of the ac power plug, if it has a two-prong plug, and if that doesn't help or if it has a 3-prong plug, try floating it and/or the power amp, using a 3-prong to 2-prong adapter, with the safety ground left unconnected. If you still hear the hum, and it is only when phono is selected, the preamp probably needs to be either professionally serviced or replaced.

Regards,
-- Al

DISCHARGE caps 1st.They can zap you across the room !!Techs just replace caps as pulling to test and resoldering is time/labor consuming.Cheaper and quicker to just replace them.Check for leaking,cracks or deformed looking caps 1st.Usually electrolytic's.Almarg above has good advice except for replacing unit imho.Should be able to fix.JD

I think you got bad advice. It is very common to get hum from a phono section with a ground loop, missing ground, bad connection, etc. That section has very high gain compared to the other inputs so unless things are just right it will hum. Even leaving them with no input can cause hum.

The first thing to do is insert shorting plugs into the phono inputs. If hum goes away it is external. If it still hums your NAD has a problem.

Need shorting plugs? Go to Radio Shack and buy some cheap phono plugs and somehow connect the two lugs (solder together, short with piece of bare wire, etc.)

Can I just make shorting plugs with an RCA cable I don't mind sacrificing?

I would first try taking a single interconnect and connect one input to the other. That might be enough to shield the input and if the hum stops you know it is coming from outside. If it still hums no need to sacrifice anything. Plug an interconnect into each input and on the other end gently jam something between the center pin and the outside ring to short them, piece of aluminum foil would work.