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

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

Yes!

-- Al

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

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

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

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

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

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