[Boatanchors] Micamold Capacitors

[Richard Knoppow]

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----- Original Message ----- 
From: "Michael D. Harmon" <mharmon at att.net>
To: <boatanchors at mailman.qth.net>
Sent: Saturday, July 06, 2013 6:06 PM
Subject: [Boatanchors] Micamold Capacitors


> Was it this group that had the thread earlier in the week 
> about the Micamold "mica" caps that were really paper 
> caps?
> I did some digging in my junk box and found a bunch of 
> Micamold caps I was keeping, thinking they were micas.
>
> When I looked at the color code chart, it said that micas 
> have a black first dot and molded papers have a silver 
> dot.
> However all the square-cornered green Micamold caps I have 
> show a white first dot.
> Most of the dozen or so round-corner dark-brown Micamolds 
> didn't have any color dots at all, so I pitched them.
>
> I kept the Aerovox, El-Menco, Cornell-Dubilier and Solar 
> caps.
> Anyone know about them?  Do they have the same issues as 
> the Micamolds?
>
> Thanks!
> Mike, WB0LDJ
> mharmon at att dot net
    I am pretty sure these are mica caps. Micamold made both 
types.  There are at least three systems of marking postage 
stamp type capacitors.  The Micamold paper caps I removed 
have a single row of color dots indicating the value.  There 
is a molded in arrow showing the direction they should be 
read. When holding the cap with the arrow pointing to the 
right there is a separate dot in the upper _right_ hand 
corner. On my caps the dot looks white rather than silver. 
These are definitely paper caps because I have dissected two 
of them.
    Now, when I went to look just now I made an interesting 
discovery: not all the caps I pulled out were Micamold, a 
few were made by Solar.  These have NO dots but rather have 
the value stamped or engraved in the case.  They also had 
RCA part numbers hand written in ink. I don't know what type 
of caps these are but may dissect one because they are just 
as bad as the Micamold caps.
     Most military handbooks will describe the various 
marking methods but briefly there are three main ones:
3 dots with small arrow heads indicating the direction they 
should be read and two systems of six dots, one the JAN 
system the other the RMA system The only version of the RMA 
system I can find is for mica caps.  The JAN system has a 
black dot in the upper left position for mica and the lower 
center is the tolerance.  For paper caps the JAN system has 
a silver dot in the upper left end and in the lower center 
position. I think your white dots may actually be silver. It 
would appear that Micamold had their own system of marking.
    Of course the final arbiter is to test them.  My 
Micamold paper caps show rather low parallel resistance when 
measured on a Hewlett-Packard 410B. In comparison modern 
film caps or good mica caps show nearly at the resistance 
limit.  I also measured the leakage using a small high 
voltage power supply and micro-amp meter. This showed 
considerable leakage current.  The other test, which I just 
repeated was using the Eico 950B capacitor checker. This is 
a magic eye indicator which closes down when there is 
leakage but does not give a value for either leakage 
resistance or current. The Solar caps are rated at 600 volts 
and close the eye at about 200. Leaving the caps on the 
checker makes no difference.  The Micamold caps are probably 
rated at 400 or 450 volts, they are not marked. These also 
close the eye completely at around 200 volts and higher 
voltage causes it to overlap. Modern film caps or mica caps 
do not cause the eye to close at all.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com