[ARC5] R-25 dial nonlinearities

[Kenneth G. Gordon]

To put another possible technique in "the pot", I found it very interesting that the Heathkit 
LMOs, used in their SB rigs, all of which exhibit very linear tuning, use a very small portion 
of the main tuning capacitor to cover the required range of 500 kHz.

I would guess it to be less than 1/4 of the total available capacitance range. The capacitors 
are very, very well-made, but the plates don't look to have an unusual shape either.

I have noted this as something very interesting, but have not yet investigated it to see if I 
could learn why they did this, and how the very nice linearity occurs.

The LMOs were never made by Heathkit, who bought them from two companies, one of 
which was TRW, and I cannot remember the other one. Perhaps Glenn Zook can tell us.

Most of those LMOs were tube-based, but the last one, that in the SB-102, was solid-state.

I also found it interesting that the article posted by Clare includes a capacitor from an 
ARC-5 transmitter's VFO.

Which reminds me: do we yet know for certain that those capacitors were made with 
Invar, which is inherently temperature-compensating?

We DO know that the ARC-5 transmitters' VFO capacitor is very, very much different from 
the other two capacitors in the transmitter.

I was told once many years ago that they were made with Invar, but have never seen 
anything definitive written on this subject.

Ken W7EKB


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