[Boatanchors] Micamold Recovery

[Richard Knoppow]

-----Original Message-----
>From: Rob Atkinson <ranchorobbo at gmail.com>
>Sent: Jun 4, 2014 10:37 AM
>To: David Hallam <dhallam at knology.net>
>Cc: Boat Anchors List <boatanchors at mailman.qth.net>
>Subject: Re: [Boatanchors] Micamold Recovery
>
>That has never been my understanding--micas are real mica and called
>mica capacitors.  micamolds are bogus with a paper dielectric.  By the
>way, some old micas get leaky due to silver migrating in the
>dielectric.
>
>73
>
>Rob
>K5UJ
>
>On Wed, Jun 4, 2014 at 12:17 PM, David C. Hallam <dhallam at knology.net> wrote:
>> I think some of what we call "micamold" capacitors can be either mica or
>> paper.  If they have a 6 dot color code and the upper left hand dot is
>> either black or silver, it probably marked with the old AWS code.  A black
>> dot meant a mica capacitor and a silver dot meant a paper capacitor.
>>
>> David
>> KW4DH
      
     Micamold was the name of the company.  They made both mica and paper caps. As noted above the JAN code for value also indicated the type of dielectric.  Micamold made both.  I don't know of a catalog on the web that shows the line but I suspect there is one.  
     There were two types of mica caps, one is the familiar silvered mica the other used unplated mica with foil electrodes. The later were the earlier types and were also used for high voltage applications.  Both can be of very high quality. General Radio made secondary standard caps using the stacked mica and foil type.  
     Mica was in short supply during WW-2, considered a strategic material. The old RCA AR-88 handbooks indicate certain caps were mica that in my receiver were Micamold and Solar flat paper caps. I think the reason was that the paper caps would do and mica was not available.  For the more critical positions silver-mica caps were used.  The RCA caps were lozenge shaped and of an odd violet-pink color.  I have never dissected one so can't be sure they were mica but RCA held a patent on a method of making stacked mica caps so I think they were plus the notation in the parts list.  Both the Micamold and Solar caps have similar construction (I have dissected both). This is a flat winding of impregnated paper and foil. Unlike the BB caps the windings in the Micamold are neat and not distorted.  I do not know what the impregnant was but the appearance is similar to the paper in Black Beauty caps which were advertised as using plastic impregnated paper.  I think the reason was to reduce the degradation of the paper with time and voltage. There are probably patents covering the construction but I have not found them. 
     I measured the removed caps on a good impedance bridge (General Radio 1650A) and measured leakage using both an Eico cap tester and a General Radio megohm bridge. Both gave similar results:  the capacitance values were not too far off and ESR or dissipation factor was acceptable although a bit high but the leakage was rather high.  New plastic film caps do not show any leakage that I can even detect using what I have. Also, the dissipation factor is so close to zero I can't tell it from the bridge residual.  As far as I can tell the inductance is very low.   I also checked some of the oil-filled paper caps used for by-passing.  These had all leaked considerable oil.  Curiously, the values were not too far off and they were still within acceptable specs.  Nonetheless I replaced them all. I did not want to deal with the possible PCBs inside to I mounted the new film caps on terminal strips.  I have no idea of whether this re-capping improved performance.  I never measured sensitivity and noise with the old caps.  After recapping the performance is very good slightly better than the handbook numbers. 
     While I can understand the desire to keep old gear authentic I also am interested in performance and reliability so I prefer to use new parts rather than trying to salvage old ones.  Many others disagree with this philosophy and I do not fault them.