[R-390] More C709 info

[n4buq at knology.net]

I bought a pack of 510pF, 5%, 500V capacitors today and will try one 
tonight.  I'm curious to see what effect (if any) I see with a new 
capacitor in there.  I gave up on finding new 2% capacitors for this 
without having to pay a large minimum order.

I just checked the calculated frequency of the secondary circuit of the 
output transformer in the PTO.  5.75uH and 510pF yield 2,933,264Hz.  Pretty 
much in the middle of the PTO range.  The Q, however, is quite low (0.01?) 
due to the 150-ohm resistor across the circuit.

I'm curious if I'll see much increase (if any) signal level after 
installing the new capacitor.  I assume the output level would peak nicely 
at some frequency near 3MHz without the resistor in place, though.  It 
would be interesting to watch but unless the resistor is easily unsoldered, 
I don't want to do that just to see the resulting effects.

Barry - N4BUQ

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.
> 
> ----- 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