[ARC5] BC-453B dial calibration saga.

[J Mcvey]

Many of the old service manuals for radios of that era use variants of the 1/3 2/3 tuning points to obtain dial calibration.It was a combo of using the trimmer on the high end ( 2/3) and the slug on the low (1/3) of the range.You go back and forth until the correct balance of L and C are obtained for the dial tracking.
I never had an ARC that was so out of cal that a tweak of a trimmer didn't fix it , though
 

    On Friday, July 28, 2017 1:24 AM, Leslie Smith <vk2bcu at operamail.com> wrote:
 

 Hello Skip,

I think there is a swap in your thinking between "standard" and "dial".
This is because the problem is counter-intuitive.
Your "standard" (a signal on 210kHz) is LOW compared with your dial.

Now I will illustrate what is going on with a "demo" calculation.

Assume, for the discussion that your total tuning "C" is 155pF at
210kHz.
You HEAR the signal at 210 (it's coming from your signal generator, and
you know that IS the frequency because you measured it with a counter.) 
 But the dial reads 215kHz (say).  The signal is your STANDARD.  The
dial is only an indicator.

Now I calculate:
The inductance (L) of your coil is therefore 25330.3/(0.210*0.210*155)
uH
This is:  3705.97uH.

Say you increase the inductance of the coil by 2% - that by 74.12uH 
(say you used ferrite).

You haven't touched the dial (the capacitor) but now your set tunes 
sqrt(25330.3/4789.09 * 155)  = 184.7kHz

You ADDED inductance and the error increased!!  You need to REMOVE
inductance to move your receiver toward the correct point on the dial. 
You must think very hard to understand why this works
counter-intuitively.

This is why a trial-and-error maths calculation works.

Les

You probably wonder where 25330.29 comes from!

Taking uH and pF into account, 25330.29 equals 1000,000/(4 pi squared).
And the 4 pi squared comes because resonance happens when X(l) equals
X(c)
and 2 x PI x F x L equals 1000,000/(2 x PI x F x C).
So - solve for L or C or F and you have the answer.  - Les





                                          o-o-o  end  o-o-o

On Fri, Jul 28, 2017, at 14:10, Waldo Magnuson wrote:
> Hi, 
>    I'm still trying to get the dial reading to be a little more accurate.
>    The closest I can get on the 210 kHz alignment is about 216 with C-9
>    fully unmeshed.  So I thought I would add a little more inductance to
>    L-5 so I could align to 210 kHz.  Now the closest I can get is 230 or
>    so.  I increased L-5 by adding a small ferrite core above the slug.  
>    Am I thinking wrong?  Comments?  Thanks.
> Skip Magnuson
> Spokane, WA
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