[ARC5] Receiver current drain / output impedance / C31

[Dennis Monticelli]

Les,

Your silver migration theory is believeable.  Silver migration is a well
known problem that crops up in a lot in silver plating applications.  The
ingredients for the phenomena are just silver, DC voltage, and moisture.
I'm pretty sure elevated temps hasten the process.  I'd say the old
fashioned silver mica caps are good candidates for these unwanted stray
paths.

Dennis AE6C

On Fri, Jan 7, 2011 at 8:23 AM, Leslie Smith <vk2bcu at operamail.com> wrote:

> Henry (and group),
> A common failure, well known to many members of this group, occurs in
> silver mica capacitors as they age.  The silver migrates "around" the mica
> insulators - or at least this is my understanding of why sm caps fail.
>
> I have seen this several times, including a failure of the capacitor across
> the output transformer, probably C31, in the plate circuit of the 12A6
> output "tube".  In my case the DC resistance of the failed capacitor was
> less than 300 ohms.  This had the effect of "shorting" the signal at the
> primary of the output transformer, giving low AF output (or in my case, very
> little output at all).
>
> The fault isn't simply diagnosed because the DC resistance of the
> transformer masks (to some extent) the low resistance of the capacitor.
>
> I learned a great deal from the various posting given in answer to the
> simple question.  "How much current should a "command" set draw?"  Great
> question!
>
> BW & 73s
>
> Les
>
>
>
> > ----- Original Message -----
> > From: "Henry Mei'l's" <meils at get2net.dk>
> > To: "Dennis Monticelli" <dennis.monticelli at gmail.com>
> > Cc: arc5 at mailman.qth.net
> > Subject: Re: [ARC5] Receiver current drain / output impedance
> > Date: Fri, 7 Jan 2011 07:59:08 +0100
> >
> >
> > Dennis,
> >
> > I've also observed improved oscillator frequency stability in my WS19
> MkII
> > using reduced  B+ voltages.
> > Also use the then-standard plate to 2-8 ohm trafo to drive speakers on my
> > Command Set receivers.
> > Happen to a have a couple of 600 ohm speakers and several 600 to low-ohm
> trafos.
> > Philips produced 600 ohm speakers at one time.
> >
> > Henry
> > oz1uf  n2nr
> >
>
>
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