Are film capacitors directional?

Good info, Rodman.

Bryon, if you have a good digital multimeter, which can read AC voltage down into the millivolt region, you could probably use that instead of an oscilloscope. Note in the jimmyauw writeup that the amplitude scale on the scope displays is apparently 5 mv per major division (there being 8 major divisions on the entire vertical axis of the display).

Best,
-- Al

Also, Bryon, if you use a battery powered DMM instead of an AC powered oscilloscope, it may be that instead of using your hand as the "signal source" you might have to drape an AC power cord (that is conducting current) over the capacitor. The 50 Hz waveform that is shown in the scope photos (he's undoubtedly in a 50 Hz rather than a 60 Hz country!) is probably contributed to by leakage between the scope's AC input and the ground of the probe, that is in turn connected to one end of the capacitor.

Best,
-- Al

Please forgive my ignorance, but once I have the meter, what exactly do I do?

There's no ignorance to be forgiven, aside perhaps from my own :-)

Thinking about it a little further, I believe that to get meaningful readings with a battery powered DMM you MIGHT have to provide it with an AC ground reference. That could be done by connecting a piece of ordinary hookup wire between the screw on the wall plate of an AC outlet, and the capacitor lead to which the DMM's "common" terminal is connected. I'll describe the procedure as if that wire is necessary, although I'm not completely certain that it will be necessary.

What you would do is as follows:

Set the meter to read AC voltage, on a fairly sensitive scale that would resolve a millivolt or so. If the meter has a "LoZ" (low input impedance) provision, disable it. Connect the meter's com (common) input lead (or whatever the equivalent nomenclature may be on the particular meter) to one end of the capacitor. Connect the wire from the wall plate screw to that same point, perhaps holding it in place with an alligator clip that may be provided with the meter, or that may be attached to the end of its lead. Connect the other lead from the meter to the other end of the capacitor.

Put your hand on the capacitor, and see if you get a reading. If not, drape the AC power cord of some powered up electrical device across the capacitor, and see if that gives you a reading. Note the value of the reading.

Remove the three connections from the capacitor. Connect the com lead of the meter and the wire from the wall plate screw to the opposite end of the capacitor than the end they were originally connected to. Connect the other lead of the meter to the opposite end of the meter than it was originally connected to. Repeat the voltage measurement.

The end of the capacitor that was connected to the com lead and the ground wire when the lower of the two measurements occurred is connected to the capacitor's outer foil.

Best,
-- Al

Hi Pehare,

Thanks! I don't know the answer to your question, though, as I haven't ever used a RS DMM. FWIW I did use a RS FET-input (high input impedance) analog multimeter many years ago, and it seemed to work well, and lasted a long time.

If the opportunity arises, perhaps you could borrow a Fluke or other good meter from someone, and compare readings.

Believe it or not, the only meter I've had here in recent years is a not particularly accurate Triplett analog multimeter, supporting both my audio and antique radio endeavors. I do have a low end oscilloscope, though. I'll have to get myself a good Fluke DMM at some point, probably one that also measures capacitance and perhaps inductance as well.

Best regards,
-- Al

Hi Bryon,

Are you using a setting for AC (not DC) volts? Not sure how you could sometimes be getting a negative number for AC volts.

Also, charge that is stored in a capacitor can sometimes "bounce back" to some extent after a connection that has been used to discharge it has been removed. That can occur as a result of what is known as dielectric absorption. So it would be a good idea after discharging the cap to use the meter to verify that no DC voltage exists across it.

Beyond that, it may be a good idea to tape the power cord in place on the capacitor, to be sure that its exact position relative to the capacitor doesn't change as the leads are being removed and applied. And also try to keep the test leads as far away as possible from the power cord.

Also, let us know the make and model of the meter, and I'll try to take a look at its description, which might give me some further ideas.

Best,
-- Al

At the defense electronics company where I spent my career, in the forms that were used to report the results of investigations into test failures or other equipment problems there was a failure category entitled "operator error." That category was frequently applicable, despite the strong qualifications of those performing the tests :-)

I can't get through to the Gardner Bender site either, but it seems evident looking at the photo that for AC volts only 500V and 200V scales are provided. I don't know how many digits are provided, but yes, it seems a safe bet that the meter isn't sensitive enough.

Being a dedicated audiophile isn't easy ...

Best,
-- Al

Hi Bryon,

I looked through the Fluke 115 literature. The one notable thing I found is that I think you should perform the measurement using the "AC Volts" mode, not the "AC Millivolts" mode. The meter's input is AC-coupled in the "AC Volts" mode, meaning that any DC that may be present (which might otherwise confuse the measurement) will be blocked in that mode. "AC Millivolts" is DC-coupled, which is undesirable for what you are trying to do. The "AC Volts" mode can resolve down to 1 mv, which should be more than adequate.

On the Model 115 the "AC Volts" mode corresponds to the large rotary mode select switch being at the position that is immediately to the right of the "Off" position.

Also, given my uncertainty about the need for connecting the ground wire to an outlet screw, try it both with and without that wire, if necessary and if you already haven't.

Hope that helps. Best,
-- Al

Hi Bryon,

Yes, the Mundorf's are probably oriented correctly. The two wordings are consistent, although confusing. The lowest impedance in this context is the output impedance of the amplifier. So by orienting the outer foil of the capacitor towards the input of the speaker, which is the same as orienting it towards the output of the amplifier, you are orienting it towards the lowest impedance point.

The reason I say "probably" is that if the values of R2 and R4 are particularly high, conceivably the lowest impedance path to the amplifier's ground could be through the corresponding driver and its interconnected crossover components. What are the values of R2 and R4?

Best,
-- Al

3.3 and 3.9 ohms seem low enough to be unlikely to change the stated conclusions.

Rodman, I'll make it a beer rather than a Coke. Next time you're in Connecticut!

-- Al