AR3 capacitor drift

[redpackman]

Thought you might find this interesting. This capacitor, removed from a very early 60's AR3 (Serial numbers in the 9xxx range). One can see how far a cap can "drift" in 58 years.  The 24 uf cap is now over 60 and the 6 uf cap is around 10 uf.  Now, i know my cap tester is hardly a tol model, but it's in the neighborhood. 

[DavidR]

What frequency is the meter testing them at?

It should be 1kHz to be accurate for NPE's. A lower signal will give you a higher reading.

[ra.ra]

16 minutes ago, DavidR said:

What frequency is the meter testing them at?

Looks like red's meter is testing them at 80Hz and 8Hz. How does that information translate to or adjust his readings for 1kHz accuracy?

 

[DavidR]

I had a meter with those same parameters. A Parts Express purchase. It will be fairly accurate for film caps but not electrolytic. There is no conversion. I went and bought an expensive handheld Agilent meter so I could accurately measure NPEs. Roy and Carl (RIP) always said NPE's need to be measured at 1kHz to be accurate. NPEs swing more to frequency than film (very little swing):

[Pete B]

Check for leakage on the Meg ohm scale, what do you find?

Isn't that a paper cap and not NPE?

[ra.ra]

3 hours ago, DavidR said:

It will be fairly accurate for film caps but not electrolytic.

This is helpful, I suppose, but also very discouraging to hear that red's meter might be useless for his intended purpose. If this is true, short of purchasing an expensive measurement tool to evaluate 30 or 50-year-old aged NPE caps from old salvaged speakers pleading for a new lease on life, what reliable and inexpensive options does the weekend hobbyist have for measuring these old components to provide useful data to answer the age old question?

"To re-cap, or not to re-cap, that is the question." (With apologies to Shakespeare.)    

[Pete B]

There are even meters that charge the cap with a constant current and measure the time constant,

which would be a very low frequency measurement, not even putting AC on the cap.

[Pete B]

I have posted this before, note his comments on measuring leakage, and important 

comments toward the end:

 

[Pete B]

My measurement system plots impedance vs. frequency  10 Hz to 20 KHz and the 

cursor can be placed at any point on the graph providing capacitance and ESR.

The free software Arta should be able to do the same thing with a laptop and some

custom cables with clip ends.  Just looked this up, you'd use the associated LIMP program:

http://www.artalabs.hr/

It tests at low voltage and it is important to know if there is leakage at the rated voltage

for an old cap, simplest thing is to just replace them at least to the fragile dome drivers.

[DavidR]

I believe those old dual capacitors are NPE

 

[Pete B]

Tom Tyson wrote back in 2004:

"You raise an interesting point regarding the capacitor changes in the AR-3a from the early (1967-1972) to the later versions (1973-1975). The first-generation AR-3as did indeed use the Industrial Condenser Corp paper capacitors up to about 1972; the later-generation AR-3as used Callins or Sprague Compulytic electrolytic capacitors (50 to 60 V). The second generation AR-3a (not including, in this instance, the AR-3a "Improved" version which was a European model) was indeed changed in several important respects: (1) the crossover frequencies"

From:  http://www.classicspeakerpages.net/IP.Board/index.php?/topic/953-speaker-voicing-and-capacitor-types-used/

 

I search this forum from google rather than using the internal search feature.

[DavidR]

On 5/24/2018 at 10:56 AM, ra.ra said:

This is helpful, I suppose, but also very discouraging to hear that red's meter might be useless for his intended purpose. If this is true, short of purchasing an expensive measurement tool to evaluate 30 or 50-year-old aged NPE caps from old salvaged speakers pleading for a new lease on life, what reliable and inexpensive options does the weekend hobbyist have for measuring these old components to provide useful data to answer the age old question?

If he's only going to do that one set of speakers he might want to find someone who can measure them for him. There are several handheld models that will measure at 1kHz but they cost at least $300 IIRC.

 

https://youtu.be/H4TP0i917Ro

 

https://youtu.be/Mp0n-Gu0aKk