Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Silver mica cap's  (Read 24960 times)
KD0ACY
Member

Posts: 93




Ignore
« on: November 09, 2011, 05:12:41 PM »

In re-caping these older rigs, I run into a loot of silver mica caps and have read, "they don't need to be changed" and others say they have "migration" of the  silver.
My question is which is valid, to change or not to change? Another question ----- why did they use that type of cap as apposed to the regular caps?
Mike   
Logged
AC5UP
Member

Posts: 4546




Ignore
« Reply #1 on: November 09, 2011, 05:24:23 PM »

Mica is a very stable mineral that doesn't change size very readily due to stress or temperature and has excellent dielectric properties. The same can be said of a ceramic substrate. When condensers are made the two primary concerns are deviation from the intended value (tolerance %) and how well the condenser will hold that value long term.

Silver mica's can go bad, and it's typically the older postage stamp variety that go soft and change value. As for migration of the silver, I haven't heard of that... Which doesn't mean it can't happen. My experience has been a dipped SM condenser is one of the last parts I'll suspect of going bad.

SM condensers are still found in new equipment despite the more modern materials available and that tells me something.........  Wink
Logged

N4NYY
Member

Posts: 5224




Ignore
« Reply #2 on: November 09, 2011, 05:53:59 PM »

Based on recommendations here, I do not change them. However, I have run across a couple radios that had 2, and the cost was minimal, so I changed them.

There is a thing called "silver mica" disease. If you here popping, then it may be that and you have to replace them. But those are usually found within the cans.
Logged
WB2WIK
Member

Posts: 21836




Ignore
« Reply #3 on: November 09, 2011, 05:55:58 PM »

Silver micas are generally good with RF current and quite stable which is why they're used.

Some ceramic caps are, but some aren't.  Depends on the mix of the barium titanate.

NPO ceramic caps are usually very good and quite stable.  X7R might be reasonably good.  Z5U and Y5V and other such cheap ceramics can be terrible, although they're still ceramic.  They're just not stable.

I have a 56 year-old 75A-4 with lots of silver mica caps in it, and they're all original and the receiver still works the same today as when I bought it second hand in 1973.  That might say something about life and stability of the components.
Logged
WB6DGN
Member

Posts: 618




Ignore
« Reply #4 on: November 10, 2011, 03:08:56 AM »

Quote
X7R might be reasonably good.  Z5U and Y5V and other such cheap ceramics can be terrible, although they're still ceramic.  They're just not stable.

Steve,
I'm curious; how does the temperature coefficient affect the quality of a capacitor?  Is the "instability" you're noticing a small change in capacitance with a change in temperature?  If so, they're supposed to do that; that's what the temperature coefficient defines.  These capacitors are used to maintain fairly constant circuit operation with wide variations in ambient temperature.  And, yes, NPO capacitors would appear to be the most stable; the NPO stands for negative, positive, zero.  Translated, with positive or negative excursions in temperature, the rated capacity remains constant within the specifications defined for the capacitor.  The other coefficients need to be looked up on a chart or data book; I never was able to memorize them.

Tom

A handy chart I found on the internet that might be useful:

www.niccomp.com/Products/TC_Ceramics.pdf
« Last Edit: November 10, 2011, 04:56:24 AM by WB6DGN » Logged
G3RZP
Member

Posts: 1278




Ignore
« Reply #5 on: November 10, 2011, 03:31:27 AM »

The postage stamp ones aren't always silver mica. Some of the real oldies are stacked foil and mica, even down at values like 25pF. The true silver mica has a tempco of around 30 to 50, sometimes as high as 70 ppm/degree C. They used to be made by stacking sheets of silvered mica, riveting the leads on which held the sheets together, and either wax dipping or encapsulating in a plastic molding. These days, most are sintered silver mica: these are more stable over time: the silvered sheets of mica are soldered together at the edges under pressure with a silver loaded solder paste. Leads are added and then most are epoxy dipped.

One problem with them is 'scintillation', where the silver particles don't make a good contact. This leads to random minute frequency changes when used in an oscillator: the old UK DEF STAN had a limit on this.

The old wax dipped style definitely drifted if used in a humid atmosphere, unless the wax was particularly thick - usually, at least two dippings  followed by a flash dip - and the humidity could lead to silver migration or even corrosion. Running silver micas with too much RF current can lead to long term reliability problems.

Generally speaking, I only change them if they need changing.
Logged
KA5N
Member

Posts: 4380




Ignore
« Reply #6 on: November 10, 2011, 03:38:01 AM »

Silver mica?  Silvering is where a thin deposit of metal is sputtered on a substrate (in this case mica which is a naturally occurring mineral)  just like the silvering on mirrors.  Most of the time the metal is aluminum and I doubt that silver mica capacitors have ANY silver in them.  Also the coating is very thin.  I suggest that the greatest cause of failure in a silver mica is invasion of moisture etc. into the innards of the capacitors which will occurr when the casing material is cracked or imbedded leads come loose.
So if silver micas are good in old boat anchors, leave them alone as they may last forever.  If on the other hand they are bad, replace them.
Like snivets (look it up) silver migration is not something most of us have to worry about.
The PRICE of silver is another thing.
Allen
Logged
G3RZP
Member

Posts: 1278




Ignore
« Reply #7 on: November 10, 2011, 03:41:28 AM »

Older silver micas are definitely that - unless aluminium goes black in air. Modern ones may use aluminium.
Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #8 on: November 10, 2011, 05:11:22 AM »

Older silver micas are definitely that - unless aluminium goes black in air. Modern ones may use aluminium.

The modern caps I've taken apart are still silver. It's tough to solder a tinned copper or steel wire to aluminum. :-)

I've replaced a few SM caps in old gear, mostly because of leakage current at high voltage, but some because of noise or erratic capacitance.

I have never heard of silver migration, although geese and crows do fly past my antennas.
Logged
W1BR
Member

Posts: 4189




Ignore
« Reply #9 on: November 10, 2011, 08:12:20 AM »

Silver Migration is a very common problem that develops in IF transformers that use
fixed mica caps molded in the transformer base.

Pete
Logged
N4NYY
Member

Posts: 5224




Ignore
« Reply #10 on: November 10, 2011, 08:44:17 AM »

Quote
Silver Migration is a very common problem that develops in IF transformers that use
fixed mica caps molded in the transformer base.

Are you talking of Silver Mica disease? Maybe that is the same term for it. I have heard of Silver Mica disease.
Logged
WB2WIK
Member

Posts: 21836




Ignore
« Reply #11 on: November 10, 2011, 08:59:41 AM »

Quote
X7R might be reasonably good.  Z5U and Y5V and other such cheap ceramics can be terrible, although they're still ceramic.  They're just not stable.

Steve,
I'm curious; how does the temperature coefficient affect the quality of a capacitor?  Is the "instability" you're noticing a small change in capacitance with a change in temperature?  If so, they're supposed to do that; that's what the temperature coefficient defines.  These capacitors are used to maintain fairly constant circuit operation with wide variations in ambient temperature.  And, yes, NPO capacitors would appear to be the most stable; the NPO stands for negative, positive, zero.  Translated, with positive or negative excursions in temperature, the rated capacity remains constant within the specifications defined for the capacitor.  The other coefficients need to be looked up on a chart or data book; I never was able to memorize them.

Tom

A handy chart I found on the internet that might be useful:

www.niccomp.com/Products/TC_Ceramics.pdf

It's not just the TC, although that is one element of "stability."  Capacitors have a temperature coefficient and also a voltage coefficient, where the actual capacitance changes with applied bias.  NPOs are very stable for VC, and so are silver micas.  The higher density mixes aren't and some can change value by 90% of their stated capacitance (which is usually measured at 1 Volt, or possibly 10 Volts) when you bias them.  Most of those high density mixes, besides being unstable, are also lossy at radio frequencies as their dielectrics are actually dissipative at higher frequencies.  Many capacitors that are great at 60 Hz, or even 1 kHz or 100 kHz (where most would be measured by automatic bridges) are terrible at 10 MHz and worse still at 100 MHz.

Logged
G3RZP
Member

Posts: 1278




Ignore
« Reply #12 on: November 10, 2011, 11:21:41 AM »

As I remember it, silver migration can happen with the effects of DC and a humid atmosphere, in a somewhat analagous way to the effects of current density and temperature on the aluminium in an IC.

Not very common in my experience, although I've seen it many years ago in ceramic caps being pushed on RF current and operating at high DC simultaneously.
Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #13 on: November 10, 2011, 04:35:10 PM »

As I remember it, silver migration can happen with the effects of DC and a humid atmosphere, in a somewhat analagous way to the effects of current density and temperature on the aluminium in an IC.

Not very common in my experience, although I've seen it many years ago in ceramic caps being pushed on RF current and operating at high DC simultaneously.


I did some searches and several references say it is a moisture and strong electric field problem. Electromigration.



On a less productive note, I found a website that claims it is aggravated by high energy photon bombardment, and results in bent filament helices if Rp is too high.
Logged
KD0ACY
Member

Posts: 93




Ignore
« Reply #14 on: November 10, 2011, 04:45:22 PM »

I am enjoying the education and have another question! 
while replacing a resistor, I overheated the ground connection that had a SMC on it also. Do I need to be concerned about over heating and if so, where do you obtain replacements, or what could be used as a substitute. 
Mike
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!