Bad condenser? How to test?

[Sargeguy]

I just received thermoplastic 6H dial a 302 from eBay dated from 1948 (#10 but whose counting).  It is from Atlanta and has the unpainted inside of the baseplate as was seen on another phone a while back.  The phone has wool feet in really good condition.   The dial plate and fingerwheel are trashed but otherwise the phone is remarkably well-preserved.  The ringer was detached, and someone left a note listing the proper connection.  The wiring has not been tampered with, although the dial is later.  The condenser has a glob of brown goop on it however, which looks like grease.  Is that a sign of a bad condenser, or has someone put conductive grease on it?  The handset cord was trashed so I didn't test it before beginning disassembling it for cleaning.  How can I test a condenser with a multimeter?

[benhutcherson]

Some multimeters have a setting to test capacitance. This is the easiest and most straightforward way to test.

Here are a couple of relatively inexpensive one that do

http://www.radioshack.com/product/index.jsp?productId=2103175&tab=summary

http://www.radioshack.com/product/index.jsp?productId=2223843&tab=features

There are more indirect ways of measuring capacitance, but off hand I'm not entirely sure how to go about doing it.

[Sargeguy]

Here is a scan of the feet, my camera isn't cooperating so I can't upload pictures at the moment:

[benhutcherson]

Very nice-those are far better than the wool feet I have.

I've never seen or heard anything of these before, and it's seems as though a bunch of them have been cropping up lately.

[Sargeguy]

The material is a little different from the stuff on the base of my 202.  It isn't "felted".  My wife said it reminds her of upholstery fabric.

[Dan/Panther]

Those feet look like new.
Thoses are the wool feet aren't they?
D/P

[benhutcherson]

Okay, here's how I think that you could check a capacitor.

Capacitance is defined as being the charge on the plates over the potential difference between the places

C=Q/V

So, what you can do is apply a certain charge to the capacitor, then use a voltmeter to find the voltage  across it.

Current is charge per unit time, so by connecting a known current to the capacitor and applying it for a known time, you can know the amount of charge on the capacitor.

Capacitance, of course, is measured in farads, voltage in volts, and charge in coulombs. 1 farad is 1 coulomb per volt.

1 ampere is defined as 1 coulomb per second.

Thus, you could hook a capacitor up to a battery, with an ammeter in series, and leave everything connected for a certain time-say 1 second.

The current divided by the time would be the charge. And, as you can see above, by measuring the voltage after charging, you can then find the capacitance.

[Dennis Markham]

Ben that sounds complicated to a pea brain like mine, although I could follow your instructions.  But don't we still need to know what "good" capacitor (195A) values should be or if you get a reading at all is it considered good?

It may be easier to clean off the goop and see if any more leaks out.  Meanwhile connect it up and see if the phone works  OK.  If not there are replacement 195A's out there.  In fact I saw a lot of like five of them on ebay currently.  But then the question is what to do with the remaining four?  Here is the eBay listing:

http://tinyurl.com/cf842r ( dead link 05-13-21 )

[Dan/Panther]

In the thread I posted about restoring my 302, I show the meter I used to check the condenser. It's a must have tool oif you do any electronics work, and i only paid $10.00 at Harbor freight.

D/P
Here is the link.

http://tinyurl.com/akamdp

[Dennis Markham]

I'm going to have to get one of those meters.  Thank you.  What is it called (the meter)?  What is the 20% rule?  As long as the value is withing 20% either way of specifications it's all good??  Also, do you recall where you found the information on the 195A condenser?

[Sargeguy]

I bought mine at Sears for $9.99

[Dennis Markham]

Is that called a "Multi Meter"??

[benhutcherson]

The wiring diagram here

http://www.telephonecollectors.org/library/weco/we302.pdf

has capacitance value on it.

The ringer capacitor is 1/2 μF and the other one is 2 μF.

Please note that the modern symbol for micro is the lower case greek letter mu(μ), while the historical symbol is a lower case m. Please don't confuse the "mF" in the above schematic for millifarads, which are significantly larger than microfarads.

[Dan/Panther]

Dennis;
It's a Capacitance meter. A multimeter checks voltage resistance, amps etc. this is just for capacitace.
The 20% rule is for replacement capacitors basically. If the new capacitor is within 20% + or  - of the original, it's a usable substitute.
I mentioned it in my post just to show the cap is withing the original specs.
I still don't understand why radio caps fail regularily, and phone caps don't ?
D/P

[bingster]

Is that called a "Multi Meter"??

Yep, exactly.