[Milsurplus] Re: Tuned circuit design question

[J Forster]

David Stinson wrote:

> ----- Original Message -----
> From: "J Forster" <jfor at quik.com>
> Subject: Re: [Milsurplus] Tuned circuit design question
>
> >.... The person doing the
> > work has no test gear.
>
> That's about like trying to fix your car engine without any hand tools.
> Why would he think he can "reverse engineer" with none of the
> assets needed to do the job??

Yup. I agree, but am trying to help a bit, if possible.

> Anyway- To get a rough approximation from which to start,
> if he knows the value of the tuning cap at a specific freq,
> he can use that to calculate the value of the inductor,
> then use the solenoid formuli to figure the turns.
> Then figure the length/diameter of the FB coil as a percentage
> of the size of the main coil, which should give you the inductance of
> the FB coil, assuming it's also a straight solenoid.

Yeah, but the trick is what % for the FB coil. W/o knowing the stage gain
at RF the best I can guess is cut and try.  The stage is a 1T4 with a 2M
grid resistor and an unkinown RFC and AFC in the plate crcuit. The screen
is connected to 22.5 VDC B+ via a 50K resistor. The frequency range is 1.8
to 10 MHz in 4 bands.

The tuning cap is 140 pF fully meshed in the original, apparently. Here are
the bands:

1     5.5 to 10 mc
2     3.5 to 6. mc
3     2.5 to 4. mc
4     1.8 to 2.9 mc

I don't know if the band ends are all at the same dial positions. The
ranges are a bit under 2:1, so the Cmin is likely roughly 35 pF, which
seems high for an air variable. There must be significant stay capacitance.

Anyway, if the shorted turns don't alter the coil L, I can easily figure a
rough design.

Other suggestions welcome.

Best,
-John


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