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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
licoricepizza OP
48 posts
 
Well, I tried the suggestion clarified by Herman. Buzzing still present. Now, I am going to try shorting the interconnect, and see where that gets me...
herman
2,252 posts
 
What kind of hum/buzz are we talking about here? The kind that you have to put your ear to the speaker to hear or the kind that you walk into the room and say "whats that noise?"
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almarg
9,670 posts
 
I can't imagine a scenario where shorting an input would cause noise on the output but I suppose it won't hurt to turn it off first.

Here's one possible scenario: As he is inserting the foil or whatever he uses to create the short, it comes into contact with the center pin an instant before coming into contact with the ground. Meanwhile, his body is in contact with the foil. The static electricity on his body (perhaps he's even just been walking on a carpet) discharges into the center pin, then is amplified by the 80db or thereabouts that the system gain is likely to be, from phono input to speaker output. That's a voltage gain of 10,000 times.

Not something I would want to risk doing on my system!

Regards,
-- Al
licoricepizza OP
48 posts
 
The buzzing noise is at 60Hz (my guess), and is pretty loud (volume dependent). I know it's there, so I get bothered by it's presence.
licoricepizza OP
48 posts
 
OK, I did the shorted IC exercise, and there was no hum. What does that tell me?
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