[ARC5] The Bandspread BC-475A Transmitter

[Bruce Long]

Roy:There are two general ways to bandspread an air variable capacitor tuned vfo.
One is to remove tuning capacitor plates to reduce the capacitor max-min spread and then add fixed capacitance to move the reduced tuning range to the desired band.  This method will affect tuning linearity assuming the vfo uses a straight line frequency tuning capacitor.  I am pretty sure the ARC% transmitter used straight line capacitance - semi-circular capacitor plates so is not relevant in this case

Or you can reduce tuning range by adding a fixed capacitor in series with air variable tuning capacitor and then add another capacitor in parallel with the series fix- air variable capacitor combination to place the reduced tuning range as desired.  This method has terrible effects on tuning linearity, having very slow (kHz per degree rotation) at the max capacitance end of the air variable tuning range,   and extremely fast tuning at the minimum capacitance end of the tuning capacitor range.
However if you restrict the modified vfo tuning range to the middle 1/3 or 1/2 of the dial and hand calibrate the result should be entirely acceptable.

You ought to be able to tell which of these two methods was used by visual inspection.
If you can determine how the bandspreading was done you might not need the original bandspread modification article.


      From: Roy Morgan <k1lky68 at gmail.com>
 To: ARC-5 List <arc5 at mailman.qth.net> 
 Sent: Friday, March 10, 2017 3:59 PM
 Subject: [ARC5] The Bandspread BC-475A Transmitter
   

Arc-5 folks,

I have here the following transmitter and would like to find the article that I assume was published telling how to do the band spread modification:

BC-457A  Unfinished aluminum.  This one has been modified to spread the band out to the 80 meter ham band. The dial has been re-calibrated by painting over and scratching in the new frequencies.  One of the rear tube sockets is marked for “crystal” so it might be re-worked for use with crystals only.  This one has a flower pot type cap inside - taken from a receiver, perhaps?  The key jack and switch are wired to both final tube and the 1626 also.  I think that some plates have been removed from the main tuning capacitor (s).

This transmuter, and a 40 meter one, were given to me in the late 50’s or early 60’s by a nearby ham.  I don’t remember ever having them on the air.  It’s about time!

Any leads to articles gratefully received.

Roy

Roy Morgan
k1lky68 at gmail.com
K1LKY Since 1958

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