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
You can buy an instrument that measures capacitance - I think I paid about $70 for mine.

Otherwise, you need a bit of AC theory - send a signal of a known amperage and frequency and measure the voltage drop. You can easily find the formula on the Web.

http://www.pupman.com/listarchives/1998/April/msg00625.html

Now, where to find a signal generator - actually I found one in a dumpster at work. ;-)

Regards,
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.
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.
Do I need to turn anything off while I'm doing this?

Yes, definitely, although it might be less critical if a different input were selected while you applied or removed the short. But that would be bad practice anyway.

If a component like that is bad, shouldn't the buzzing be present constantly?

Probably, but not necessarily. The component could be marginal or intermittent.

Regards,
-- Al
In order to be perfectly clear, I:

Turn the stereo off
Put the shorted IC in place
Make sure Phono is selected as the input
Turn the stereo back on
Turn my house into a smoldering pile ashes (just kidding)
Listen for the dreaded hum on the phono input

Thanks for all your help.

Lee
Yes!

-- Al
Sorry I wasn't clear. I meant take one interconnect and plug it into the right channel input of the phono and the other end of that same wire into the left input.

If it were mine I would short the input with the system powered up. 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.

Good luck
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...
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?"
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
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.
OK, I did the shorted IC exercise, and there was no hum. What does that tell me?
It says that the problem is not the preamp! It is outputting hum because hum is going into it. There are many threads at Audiogon on phono hum problems that you can search through. A few possible causes that come to mind:

-- Your power amplifier is too close to either the preamp or the turntable or the turntable cable.

-- Lack of good grounding between the turntable and the preamp (as you mentioned in your first post).

-- The phono cable being in close proximity to ac power cords, or to the power amp.

-- Ground loop issues between the preamp and other components, which become more apparent in phono mode than when other sources are selected because the signal levels are much lower for phono. Try disconnecting everything from the system except for the turntable, preamp, power amp, and speakers.

-- Try reversing the polarity of the ac plugs of each of those components, and/or floating them (using a cheater plug).

-- When the turntable is connected, is the hum present both when it is turning and when it is not? What kind of cartridge are you using? Grado's in particular often have a problem picking up hum from the turntable motor (when it is on), because they are unshielded. (It could be that the hum you hear when the turntable is connected is not the same hum that you heard when nothing was connected to the preamp inputs).

HTH,
-- Al

Here's what I did: I used shorted RCA cables on the phono input - no discernible hum. Then I used tin foil (as a shield) between the phono cable and the power amp power cable - hum still present. Then I went wild with the tin foil, and put a few 2" squares between the phono input, and the adjacent input (tuner - unused) on the preamp - buzz still present. Then I put a sheet of it between the amp, and the preamp - buzz still present. So, either tin foil is a bad shield material, or the problem is in the preamp. Here's why I say that: the hum can be reduced, or eliminated by my touching the faceplate of the preamp. It's possible that when I did the shorted RCA plug test, the hum was gone anyway.

FYI, the TT I am now using is an MMF 2.2 LE, with an MMF Tracker cartridge (Goldring Elektra) moving magnet type. It has a 2 pronged plug. The same can e said of the NAD preamp. The amp, which is a very recent addition, is a B&K Sonata ST-140, with a detachable IEC, 3-pronged plug/cable. My old Carver M-500T was put back in place as a test - still buzzed. I used to have an old Denon DP37F in the place of the MMF. It had a Sumiko Blue Point MC on it's tonearm (and it never buzzed).

What do you think my next move should be, given all that?

Yours, in vinyl,

Lee
Most problems of this type are not caused by pickup of electromagnetic fields through the air, which would be potentially helped by shielding (if it is properly grounded).

They are more often caused by differing ground potentials between the chassis of the components that are involved, which causes extraneous currents to flow in the shields of the interconnects between them (which are connected to chassis, and which serve as the return path for the signals the interconnects are carrying, resulting in the extraneous currents being seen as signal by the load device). That will only be helped by foil if it provides a path between the components that is both complete and of lower impedance than the shields of the interconnects.

With both the TT and the preamp having 2-prong plugs, and ac safety ground therefore not being connected to the chassis, they both have chassis that are pretty much floating with respect to ground. If they happen to float to different levels, it would result in the extraneous current flow that I spoke of. When you touch the faceplate, you affect the level to which the preamp chassis floats.

So I would again suggest trying all possible combinations of ac plug polarities (which affects the levels to which the components float). Also, try connecting a wire between the preamp chassis and ac safety ground (the screw at the center of the ac outlet cover). Also, the other things I mentioned in my last post that have not yet been addressed. And seeing if anything can be done to improve or shorten the connection between preamp and turntable ground.

HTH,
-- Al
Yet another reason why I gave up on LPs: phono pre-amps :)

Lee, the tin foil must be grounded to be an effective shield. Simplest way to do that is to use an alligator clip from the a known ground on the (pre/integrated)amp (chassis ground lug, chassis, outside of a RCA jack,etc) on one end and then clip the tin foil on the other. That will create an effective EMI/RFI shield... which still may not be your problem.
You are chasing the wrong problem with the aluminum foil. You have a grounding issue. They can be very frustrating and confusing as evidenced by the fact you touch it and it goes down. I was once trying to track down a hum problem by using a clip lead to connect various points on various pieces (preamp, power amp, phono stage, turntable) and after having no luck I clipped one end on the preamp with the other end hanging in space just as place to put it while I thought about it and the hum went way down. This was very repeatable and I called in others to make sure I wasn't insane.

Almarg gives good advice on things to try but his paranoia about static being amplified 10,000 times is just that, paranoia. In order for static to jump you need quite a few volts so you aren't going to get quite a few times 10,000 volts out of your system. At worst you will get a very, very, very brief pop equivalent to a very, very, very, brief clipping of the amp. However, it could be a problem because that static discharge into the input could damage the transistors it hits, so his advice is once again good even if the reasoning why is flawed.
Almarg gives good advice on things to try but his paranoia about static being amplified 10,000 times is just that, paranoia. In order for static to jump you need quite a few volts so you aren't going to get quite a few times 10,000 volts out of your system. At worst you will get a very, very, very brief pop equivalent to a very, very, very, brief clipping of the amp. However, it could be a problem because that static discharge into the input could damage the transistors it hits, so his advice is once again good even if the reasoning why is flawed.

Please re-read my post. I said nothing about static jumping, in the sense of a spark such as might occur if a static-charged person puts his finger near a grounded light switch. I also said nothing about 10,000 volts.

I referred to the static charge being directly conducted into the phono input, through a conductor (the foil). And obviously any significant discharge (greater than the few millivolts that the cartridge would normally put out) would clip the system before the full 80db gain (10,000x) were realized, but that is beside the point. The point is that there would be a very large transient put into the system, albeit a very brief one (as you stated). And its very brevity could very conceivably exacerbate the problem, because the consequent high frequency spectral components would cause the transient to be routed by the speakers' crossovers to the tweeters, which are much more vulnerable than the lower frequency elements.

I certainly can't prove that damage would occur, or even that it would be likely, but it would certainly seem prudent to turn the system off.

Regards,
-- Al
Please reread your own post. You did say it would be amplified 10,000 times. And it would not be "a very large transient." it would be a slight pop that clipped it for a minuscule amount of time. Look, it happens all of the time in the winter. I walk across the room to change the record and when I touch the tonearm I feel a discharge which creates a pop in the speakers. Over and over and over and over again. The tweeters still work just fine.

please reread my post. You evidently don't understand electrostatic discharge. You don't have to have a spark like the kind that jumps from your hand to the light switch for a discharge to occur, Most are never felt. For it to occur it would have to at some point jump the gap. When it did, however small it was, it would drive the system to clip for a instant. There is no way in the world that a very brief pop from the system would take out a healthy tweeter. It is however not unlikely that a static discharge through the input devices could damage one of them and that is the point that we both missed earlier. As a matter of fact, turning off the system would do nothing to protect those input devices. The static discharge through them would be the same no matter if the system was on or off. The only protection in that case would be to follow safe ESD practices.

You seem to be one of these people who loves to give advice but can't stand it when they are offered it. I stood corrected and admitted that my advice was bad because a static discharge into the input could damage the system by damaging the input devices. Suck it up and admit that this same pop would never take out a tweeter.

.
You did say it would be amplified 10,000 times.
I neglected to say the obvious, which is that it would be less if the system were clipped, which will most likely happen.

And it would not be "a very large transient." it would be a slight pop that clipped it for a minuscule amount of time.

It would be very brief, that has nothing to do with its amplitude, which is what the phrase "very large" refers to.

I walk across the room to change the record and when I touch the tonearm I feel a discharge which creates a pop in the speakers.

The static is not going directly into the signal path, as it would be in the scenario I hypothesized. Big difference!

There is no way in the world that a very brief pop from the system would take out a healthy tweeter.

Let's say that it was so brief that it lasted only 50 microseconds (a single period at 20kHz). We are both agreed that its amplitude could be full power, to the clipping level of the amplifier. Say that is 100 or 200 watts. Will a single cycle of 20kHz at 100 or 200 watts damage a tweeter? I don't know, but it certainly sounds like a cause for concern. It certainly will not damage it via thermal effects, but the concern would be the over-excursion it would result in. If you are certain that it will not harm the tweeter, please provide a link to some documentation or technical rationale that is more persuasive than your simply saying it will not.

You seem to be one of these people who loves to give advice but can't stand it when they are offered it.
If you knew me better you wouldn't say that.

Basically, this thread has become unpleasant because you chose to describe my "err on the side of caution" advice by using the word "paranoid," twice. The fact that you said what could easily have been said in a constructive manner, in an inflammatory and disrespectful manner instead, would seem to say something about what kind of person your are.

You'll have the last word, because I don't intend to devote any further time to this utterly pointless and unnecessary argument.

-- Al
The hum is finally gone! The ultimate fix was a simple one. All I had to do was flip the preamp plug 180 degrees. I thought I had already done that. Live, and learn I guess. Now I have about 5,000 LPs that are waiting to get played... You all know what LP stands for, don't you? Most people think it mean Long Playing, but it really means "Licorice Pizza".

Thanks to all who contributed to my affliction with vinyl.

Lee
Outstanding! Phono hum is a very common problem, as others have noted above, so I'm sure the information brought out in this thread on the various possible approaches will prove useful to others in the future. Glad it worked out.

Regards,
-- Al
An update - This morning, the hum had returned, so I retraced my doings of the day. It turns out that the cause of the hum wasn't plug orientation, like I first thought. But rather, it was airborne RF interference caused by my amp. As long as there is a piece of foil between the amp and the cables going into my preamp, everything is fine.
Excellent! Make sure the foil is grounded and it will make an excellent EMI/RFI shield (or even a Farady cage if you're so inclined).