[R-390] More C709 info

[n4buq at knology.net]

On Wed, 03 May 2006 10:07:52 -0400, JMILLER1706 at cfl.rr.com wrote :

> 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.
> 

The 2.2k plate resistor in this PTO checks about 2.8k.  I put a 10k 
resistor in parallel with it just to see what effect it would have 
(effective resistance should have dropped to just under 2.2k).  It only 
made a difference of a couple of volts at best on the plate and very little 
(if any) difference in the PTO output.

While it's technically out of tolerance, I don't think this is far enough 
out to warrant replacement (yet).  I would replace it now, but the gain-to-
damage ratio to replace it in those tight quarters just isn't worth it at 
this point.

Thanks for the reply,

Barry - N4BUQ