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Measuring A Capacitor

I have a preamp (NAD 1155) that has a hum problem. It started when I got a new turntable, so I thought it was a TT grounding problem. But no... The preamp's phono section hums every time it is selected. Even if there is no phono present. All the other inputs are dead quiet.

Anyway, a fellow 'goner thought it is a capacitor that's going 'round the bend. I'm willing to take my volt/ohm meter, and find the offending component, and replace it. All I know about capacitors is that they are measured in picofarads, and they discharge their energy in bursts, when it's requested.

Any help in this regard is much appreciated. I have a mountain of vinyl waiting to be played.

TIA

Lee
licoricepizza
jdlepera's avatar
jdlepera
234 posts
 
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
herman
2,252 posts
 
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.)
licoricepizza OP
48 posts
 
Can I just make shorting plugs with an RCA cable I don't mind sacrificing?
herman
2,252 posts
 
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.
licoricepizza OP
48 posts
 
The IC to every input didn't work, so now I'm on to shorting the ICs. We'll see... Do I need to turn anything off while I'm doing this? I'd hate to damage my Vandersteens while performing this. Also, I have a question. Someone thought a cap is going bad, hence the buzzing. If a component like that is bad, shouldn't the buzzing be present constantly? For me, there are times when the buzzing is totally absent.
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