[R-390] More C709 info

[JMILLER1706 at cfl.rr.com]

It does not surprise me that a mica capacitor could fail in the 390a.  
I have had 3-4 fail in a SW model.  Several have failed in the IF and 
Mixer cans, causing poor performance on some bands and inability to 
peak the coils.  Whether the failure is an open or a leak I don't care, 
they just fail.  Replacing them with modern mmica caps is the cure (I 
get mine from Mouser, 5% is just fine).  They tend to be more prone to 
failure if they are across an inductor that in in a plate circuit 
carrying B+ to a tube.  Maybe the constant B+ surges during power on 
over the radio's lifetime gradually tear down the caps.  So when I find 
a filure, I replace it with one that has a next higher voltage rating 
than the original.

Ditto for numerous .005 bypass caps, especially those on screen or 
plate lines to tubes (again the breakdowns caused by constant HV).  
When in doubt replace them. 

And while I'm at it, ANY and ALL plate or screen resistors are suspect -
 they will drift way off with age (again, the constant exposure to HV 
and current may be a factor).  Most are 2200 ohm half watt.  If I find 
a suspect, I replace it with a 2200 ohm ONE watt unit (the modern ones 
are actually the same size).

Nothing is sacred in a radio.  Replacement is a good thing.  It's all 
good.

Hope this helps.

----- Original Message -----
From: Barry <n4buq at knology.net>
Date: Tuesday, May 2, 2006 11:46 pm
Subject: [R-390] More C709 info
To: R-390-List <R-390 at mailman.qth.net>

> I was able to carefully lift one side of C709 and watch the output 
> of the
> PTO with and without it connected.  With it disconnected, the 
> output nearly
> doubles in amplitude.  Is this expected behavior or is the 
> capacitor most
> likely leaky?  I'm just not sure of its intended function.  I 
> assume it and
> R704 limit/shape the output (but I may be waaay off on that).
> 
> Any input or other info?
> 
> Thanks,
> 
> Barry - N4BUQ
> 
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