[ARC5] Mica Capacitor

[Richard Knoppow]

     Micamold was a brand name. The company made several types of caps 
including real mica caps but is best known for the flat paper ones.
     I had not heard that early polystyrene caps had problems but can 
believe the cases got leaky.  Cracking at the lead entry is a problem 
with dipped caps even now. One must be very careful not to stress the 
leads. This applies to all caps including dipped micas and ceramic caps 
where the coating material can crumble and expose the element.
     A note on resistance. There is a confusion between series 
resistance and parallel resistance. Series resistance can be stated as 
ESR or dissipation factor. DF is the inverse of Q so it varies with 
frequency  while ESR is constant.  Leakage is from parallel resistance. 
High leakage, i.e., low parallel resistance allows DC to pass and also 
ruins Q.  It is mostly the DC that causes problems when a capacitor is 
used for coupling since it can upset the bias on the following stage. 
Electrolytic caps have inherently poor leakage characteristics but it 
gets worse as the cap ages. Measurements like bridges that indicate DF 
or ESR will often not show excessive leakage but most capacitor checkers 
do show it. Leakage sometimes varies with voltage so its necessary to 
test caps for it using a voltage near its working voltage.
     Modern film caps are made differently than the old plastic film 
ones are are extremely reliable. Polypropylene caps also have excellent 
RF qualities.  It is sometimes difficult to get reliable information 
about capacitors but the larger manufacturers do have it available.

On 9/18/2016 1:43 PM, Ron Barlow via ARC5 wrote:
> Way back in my TV service tech days, I encountered many problems with
> polystyrene caps, that suffered from the corrosion problems, where the
> foil "plates" were bonded to the wire leads. These things were very
> unreliable, in my experience, as the plastic case would crack at the
> wire entry point, allowing moisture entry, and the resultant corrosion.
> As for the "Micamold" caps being discussed, the Micamold brand name is
> misleading, in this case. As others have previously pointed out, these
> things actually were of paper dielectric construction, not mica.
>
>
>
> On Sunday, September 18, 2016 12:02 PM, Scott Robinson
> <spr at earthlink.net> wrote:
>
>
> HI J,
>
> Well, not quite; a corroded internal connection can case a series
> resistive loss without making the cap leaky. It might have little or no
> leakage current but still mess up a resonant circuit it was part of.
>
> Regards,
>
> Scott
>
> On 9/18/16 8:35 AM, J Mcvey wrote:
>> lossy and leaky are about the same thing - high ESR .
>> I just pulled a few micas (250pf) out of a WWII radio where the leakage
>> resistance was so bad
>> that it could be measured with a DMM. I think it was around 1.5 meg,
>> which totally messed with
>> the circuit bias.
>>
>>
>>
>> On Sunday, September 18, 2016 10:56 AM, Scott Robinson
>> <spr at earthlink.net> wrote:
>>
>>
>> Folks,
>>
>> In  my  experience, micas don't get leaky,,,BUT they occasionally go
>> open or get lossy (low Q). I 've had to change out 5 or 10 of them in my
>> R390A.
>>
>> Regads,
>>
>> Scott Robinson
>>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL