The AM Forum

The following information may be of interest to anyone attempting to restore pre-WW2 equipment, such as homebrew amateur transmitters and other old gear.

Over the years I have accumulated a fairly large collection of Sangamo Type A mica condensers to use in restoration projects and reproductions of old gear.  These caps were very commonly used in equipment from the 1920's up to WW2, and can be found in many, if not most, vintage homebrew transmitters of that era, and were often used in receivers as well.  Using this type of capacitor adds tremendously to the authenticity of a restoration or reproduction project.

Whenever I have needed one of these caps, I usually just grabbed one with the appropriate nominal value out of the pile and assumed it to be good, and have rarely had a piece of equipment fail to work because of a bad mica cap.  But the other day when I needed a cap, I decided to check it out with my ECG digital capacitance meter before placing it in the circuit.  I was surprised and disappointed when I picked out several capacitors from the collection and they were all way off the nominal value.  I double checked the accuracy of the ECG meter by measuring several variable capacitors of known maximum capacitance, and the meter indicated spot-on.

So I decided to test all the capacitors in the collection.  I made three piles: "good" (+/- 20% nominal value), "questionable" (50-79% or 121-150% nominal) and "bad" (< 50% or > 150% nominal value).  I would actually use only those falling into the "good" category in a circuit, although a "questionable" one might serve in a pinch.

These caps are mostly of unknown origin, acquired from a wide variety of sources such as hamfests, other hams' junkboxes, etc., acquired mostly as  loose capacitors, but some were removed by me from equipment.  Considering the large number of capacitors tested, this probably represents a good cross-section sample of this specific style of component, and likely indicates what condition can be expected from a similar item found at random.

Total capacitors checked...................................................325
Good (within +/- 20% of nominal capacitance).......................219 = 67% of total
Questionable (50-79% or 121-150% nominal)......................... 39 = 12%  "    "
Duds (<50%, >150% nominal)............................................. 67 = 21%  "    "

In summary, exactly two thirds of the capacitors in the accumulation checked within the range of values I arbitrarily chose to be "usable" (within 20% of nominal value), while one third were more than 20% off the nominal value.

Interestingly, nearly all the larger values (.005 mfd or more) checked good.  So did higher voltage caps (rated at 2500 vdc or more).  The largest number of duds were of lower capacitance (50 pf to .002 mfd) with normal (unmarked) voltage rating.  I cannot be more specific because I did not attempt to keep a numerical tally of the nominal values of each capacitor as I checked them.

I have not yet checked the caps for leakage, but as soon as I rig up a test setup, I  plan to check the leakage resistance of all the "good" ones.

One reason you may see an apparent change in value is the type of meter yuo're using, Don.  Most of those digital cap meters use the cap as part of an oscillator, and they measure the resultant freuqncy.  If the cap has leakage, the reading will be way off.

Better is to use a bridge like a HP 4290 or the digital 4291; this will give an accurate reading of the capacitance as well as the Q (and hence the relative leakage).

I went thru a pile of these same caps with my 4291 digital bridge and over half were way too leaky to consider using.

Don, I have noticed similar results measuring these capacitors for possible use in various circuits.  Jeff W9GY

I suppose they are like the infamous silver micas used in the 75A-4 and other Collins gear.

The vast majority of the crapped out ones I measured showed less than nominal capacitance, but a few showed more.  I'm thinking that the internal connections to the screw terminals corroded and lost continuity between some of the capacitor plates and the terminals.  The ones that show unusually  high capacitance may actually be indicating leakage current through the meter, not above-nominal capacitance.

My next test will be to measure leakage current with a few hundred volts DC across the capacitor.  I don't have a meter for the purpose, so I'll probably use a  a power supply and measure the actual leakage current with a meter (and pray the cap doesn't short out while the microammeter is connected - an open invitation to Murphy).

Don,
       I would expect this to be caused by 1 of 2 possible culprits.

1. "tin disease" as we have discussed here several tomes in the past.

OR

2. The same culprit that causes the demise of the imfamous "black beauties" or "bumblebees".  the Phenolic material that they are encapsulated in was not completely perfected yet and not dimensionally stable. It would continue to shrink with time until it compresses (or possibly crushes) the material encapsulated in it.

OR

the 2 combined surely be a deadly 1-2 punch for a capacitor no matter what it is made of.

                                                        The Slab Bacon

You are probably right.  Since most of the out-of-tolerance capacitors show a marked decrease in capacitance, that would indicate that one or more of the metallic plates in the stack forming the capacitor has lost contact with the terminal connection.

That old phenolic is not immune to moisture.  It could be just plain old corrosion.

There is a similar problem with old AF transformers, like the ones in 1920's era TRF broadcast receivers.  The fine hair-size copper wire used in the windings eventually becomes eaten up by the solder, and the winding opens.  This is the same effect as what causes the copper soldering iron tip to disappear.  Also, moisture impregnation inside the transformer may corrode the fine  copper wire and open the winding.

This would suggest that if you have any of those  capacitors in old equipment, or you plan to use some in a building or restoration project, it might not be a bad idea to carefully check them for capacitance and leakage.

You are probably right.  Since most of the out-of-tolerance capacitors show a marked decrease in capacitance, that would indicate that one or more of the metallic plates in the stack forming the capacitor has lost contact with the terminal connection.

That old phenolic is not immune to moisture.  It could be just plain old corrosion.

There is a similar problem with old AF transformers, like the ones in 1920's era TRF broadcast receivers.  The fine hair-size copper wire used in the windings eventually becomes eaten up by the solder, and the winding opens.  This is the same effect as what causes the copper soldering iron tip to disappear.  Also, moisture impregnation inside the transformer may corrode the fine  copper wire and open the winding.

This would suggest that if you have any of those  capacitors in old equipment, or you plan to use some in a building or restoration project, it might not be a bad idea to carefully check them for capacitance and leakage.

And even if they check good, periodic rechecking for deterioration should be a part of routine maintenance.

I also used to feel that the "postage stamp" micas were just about bulletproof untill I had one cause a dramatic failure in my Valiant. It went short and applied the full LV B+ from the plate of the buffer to the grid of driver tube. it was a prety dramatic failure that launched the 5763 off to the happy hunting ground.

I have previously had then change value, but that was the first one I had seen go dead shorted. (that hasen't been fried by excessive amounts of RF current) It fried the 5763 to the proverbial crisp. I have since been suspicious of them as well.

When you mess with this old stuff, Murphy's law is always the alligator waiting to bite you on the butt!!

                                                          the Slab Bacon

Very true. 

One of the worst culprits is the little red silver micas used in Collins equipment.  I must  have had a dozen of them to fail in my 75A-4's.  When used as coupling caps, they tend to become leaky and put positive voltage on the grids of stages, causing distortion and lack of sensitivity.  It the tuned plate circuit, they have been the cause of all my converter xtals not oscillating - never bad xtals, resulting in my now having a couple of "spare" xtals.

I've seen the same failures here, Don.

Hello Don,Frank, and others,
                                     I just happened to stumble across this post and find it very interesting! It reminds me of what is knows as the "Dreaded silver-mica'disease"! it seems on some mica's and silver mica caps, the material actually etches its way out of the cap on to the body of it. Consequently, you have a part that is either way out of spec, or just plain "NFG".

I was doing a electrical resto to a National NC-33, and came across this for the first time about a year ago. The NC-33 used quite a few silver micas in it, and much to my chagrin, most of them had the leaching problem. I pulled them out, and checked them with my trusty - hi! $18 Chinese digital LCR meter, and they were way out of spec or just plain NG.

Not to start another topic or get off the main topic, but I used to think the old "bathtub" type oil filled caps used in National SW-3's and otherrigs of the early to mid 1930's were indestructable. Upon checking failed ones I now have a different opinion of them. I used to never bother replacing them since they were oil filled, but after a period of amny years, time has taken it's toll. I now check them and replaced questionable bathtub caps with regular non polarized poly caps; which seem to work very well.

I have a EH. Scott "RBO" WWII Shipboard hi-fidelity entertainment receiver that has a whole compliment of leaky and out of spec oil impregnated paper "bathtub" caps that need replacement; and are on my "roundtuit" list!

Many "oil filled" capacitors are really just plain old paper capacitors, whose paper dielectric has been impregnated with oil. The internal contents are not flooded with liquid oil.