[ARC5] Receiver current drain / output impedance / C31

Leslie Smith vk2bcu at operamail.com
Fri Jan 7 11:23:27 EST 2011


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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