[TenTec] 238 Tuner Mod

[Dr. Gerald N. Johnson]

On Tue, 2008-03-04 at 14:37 -0700, pfizenmayer wrote:
> Guess there must be 238's, 238A's  and 238B's - I have one I bought new 
> circa Feb 2005 called 238B SN 09C10104 where the padders on the Schematic 
> are shown as 2 each 220 pfd in parallel in each location.
> 
> The parts list says they are 180 pfd and my unit has 150 pfd. The variable 
> cap is shown as 40 - 500 pfd.  I suspect the reason they kept sneaking the 
> value of the paralleled caps lower and lower is that the stray inductance 
> and the little network inserted to tune out the residual SWR makes the 
> paralleled values look larger than the actual capacitance.

I don't think so. Its probably that the variable capacitor didn't have
the delta range planned for as its supplier changed and its plate
spacing was  increased to handle the voltage better. And you don't want
the switched capacitors to have a bigger jump than the delta range of
the variable.

>  And even at 150 
> pfd paralleled in each section I have many cases where for instance LOZ-1 
> will not quite match at max variable capacitance even on 80 meters and LOZ-2 
> is too much capacitance.(var cap at minimum is too much )  No huge deal as 
> the final SWR is less than 1.3 :1 but often thats as close as you can get.
> 
> Anyhow - I also suspect the paralleled caps instead of single caps helps the 
> "drift" substantially.

For sure the paralleled capacitors divides the current between them and
so the heating. And NPO capacitors are easier to find in the smaller
values.

>  I have never noticed any drift with the settings I 
> have to use with my dipoles at a kw. Usually in LOZ1 or 2.  On 160 the 
> configuration I have makes me have to add another  CM45 2000 pfd  externally 
> and using that I still notice no drift at a kw.

But the CM45 is mica, a whole lot more stable than the general purpose
ceramic dielectric.
> 
> 73 de Hank K7HP
> 
73, Jerry, K0CQ