[R-390] More C709 info

[n4buq at knology.net]

On Tue, 9 May 2006 11:41:51 EDT, Flowertime01 at wmconnect.com wrote :

> Barry, Tim,
> 
> I have not been keeping up with my mail.
> 
> Who wins in rock-scissors-pancake?
> 
> Stop and look what the receiver design is.
> 
> The VFO varies across it range. That happens to be 2.455 to 3.455.
> 
> The idea is to drive a mixer with the VFO output across that range.
> Coming out of the mixer we would like every thing to have the same 
> performance level no matter which point the VFO is set to.
> 
> The objective of C709 and the transformer is to compensate the loss of 
> oscillator output as the oscillator frequency goes up.
> 
> If the VFO has more than enough output to make the mix, then the exact 
output 
> level is a so what case. It is over the line, and more over the line is 
noise.
> 
> If some where across the range, the output is lower than minimum mix 
level 
> then we loose some of the incoming signal into the noise floor. No so 
good. 
> 
> Barry, why do you want a peak in the middle of your mixer range?
> 
> Why should a tuned circuit not be flat across a span of 1MHZ?
> 
> I think pancakes fall to rock and scissors.
> 
> Other than you have played with the C and T and would like to get them 
back 
> to their optimal value, I do not think you have a problem with the VFO 
output 
> circuit.
> 
> Barry, every thing you have posted is right in line with how I find the 
VFO 
> circuit to behave.
> 
> Roger.

I'm really not trying to get it to peak at a given frequency.  The purpose 
of "sweeping" the transformer was to ensure that the LC[R] circuit wasn't 
so far out of tune (due to a shorted winding, bad/leaking capacitor, 
whatever) that it was causing the the output to be substantially low.  
Since it did show a peak value somewhere near the center frequency of the 
PTO's range, I'm satisfied the coils are okay (at least the primary and 
most likely the secondary).

Sure, the receiver works with the output value what it is; however, it 
makes me suspicious that something in the PTO is not where it should be and 
may be on the verge of going so bad that it stops working.  I'd like to 
circumvent that if I can, that's all.

I checked with Fair Radio for another PTO in hopes of finding one more for 
spare parts than anything.  A dented case with no connector is $22.  I 
don't think I'd get enough good out of it for that price so I'm going to 
contiue with this one.

I tried a NOS Phillips (from a sleeve of NOS) 5749W last night.  The output 
dropped slightly from the Sylvania that's in there so I don't think tubes 
are an issue here.

I'm determined...

What I'd like to know is the expected value of the oscillating voltage on 
the grid.  I'm wondering if that's not of sufficient value to get me a 
good "swing" on the plate?

Thanks for all the input, guys!

Barry - N4BUQ