[R-390] Theory Question - RF Transformer Alignment

[Miles B. Anderson, K2CBY]

Actually, Mike, you answered your own question.

The coil slug does have a greater effect at low frequencies and
the trimmer capacitor does have a greater effect at high frequencies.

First, let's consider an "ordinary" receiver -- one that is tuned
with a variable capacitor rather than moving coil slugs the way
am R-390 is tuned.

At the low end of the band the tuning capacitor is fully meshed
-- maximum capacitance. At the high end of the bank the tuning
capacitor is fully unmeshed -- minimum capacitance.

The trimmer capacitor is in parallel with the tuning capacitor,
(say somewhere between 15 pF and 360 pF for AM broadcast band)
depending on whether iot is set to the low end or the high end,
so the total capacitance in the circuit equals the total capacitance
of the tuning capacitor plus the capacitance of the trimmer
(say 15 pF) plus stray capacitance from the tube, wiring,
bandswitch, etc. (say another 10 pF or so).

When the tuning capacitor is at maximum value (low end frequency)
the total capacitance would be 360 plus 15 plus 10 or 385 pF. The
trimmer represents 15/385 or aboiut 3.9% of the total capacitance.
When the tuning capacitor is at minimum value (high end frequency)
the total capacitance would be 15 plus 15 plus 10 or 40 pF. The
trimmer would then be 15/40 or 37.5% of the total capacitance. Thus,
the trimmer capacitance is a much greater percentage of the total
capacitance and has a much greater effect on the frequency at the
high end than it does at the low end.

One other point. In the "ordinary" receiver, the value of the
inductance remains the same regardless of where the tuning
capacitor is set. Thus, adjustment of the coil slug had just
about the same effect on the resonant frequency at the low end
(maximum tuning capacitance) as at the high end (minimum
capacitance) of the band.

The net result during alignment is as follows:

Adjusting the trimmer capacitance at the low end of the band has
very little effect on the frequency. The result of adjusting the
trimmer capacitance at the high end of the band has a huge effect
on the resonant frequency. Adjusting the coil slug has about the same
effect on the resonant frequency at both ends.

The trimmer capacitor therefore has the effect of stretching out or
compressing the dial calibration by moving the high end frequency higher
(less trimmer capacitance) or lower (more trimmer capacitance) with
very little effect on the frequency at the low end of the band.

The coil slug in effect sets the "starting point" at the low end of the
band.

What all this adds up to is the old maxim: "Set the coil at the low end
of the band. Set the trimmer at the high end of the band."

How does this work for the R-390A where the tuning is by adjusting
the inductance instead of by a big mechanical variable capacitor?

Just take the foregoing discussion and substitute the words "variable
inductance" for the words "tuning capacitor" leaving everything else the
same.


Miles B. Anderson, K2CBY
16 Round Pond Ln.
Sag Harbor, NY 11963
Tel.: (631) 725-4400
Fax.: (631) 725-2223
e-mail k2cby at optonline.net