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Re: [TenTec] Corsair II DDS VFO

To: tentec@contesting.com
Subject: Re: [TenTec] Corsair II DDS VFO
From: Don Rasmussen <wb8yqj@yahoo.com>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Tue, 22 Mar 2011 08:53:29 -0700 (PDT)
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
Just to add to what Steve has properly suggested, the most practical problem 
with the PTO precision is not with the display, rather it is with the slop 
within the worm gear, pistol, grease and spring.

Even with the smoothest PTO if you are trying for (example 14.250), there is 
some backlash and so you end up wherever the pistol settles with the worm gear. 
The most subtle twist of the main tuning knob and that spring is only building 
up pressure inside, a little more twist and then it pushes the pistol too far, 
past your intended target. Repeat coming back from 14.250.1 ;-)

Ignoring drift, you could never get to 14.250 precisely to start with, and if 
you did, you would drift away from it anytime you transmitted, the finals 
heating up the PTO coil when you transmit and then it cools when you are off 
the key.

The N4YG DDS is not affected by PA heat so your drift is limited to the Corsair 
base frequency crystal drift plus some negligable amount from the DDS board 

In practical terms for CW and SSB it's a night and day improvement.

For repeatability, I will tune to (example 14.249.9 just changing to 14.250.0) 
with the fine precision of the DDS then a very small rotation in "fine" mode 
using the main tuning knob to push the display over to 14.250 solid.

While it is true that repeatability is ultimately determined by the accuracy of 
that counter board and some drift in the band crystal, it's still a world 
better than before and quite acceptable for CW/SSB net buddies who otherwise 
would have howled uncontrollably about having to adjust their RIT control for 
you - ONLY you - end EVERY time it is your turn to transmit!!! ;-)

Steve's design improvements have been incorporated into the new N4YG boards 
with the added benefit of the removal of a daughterboard circuit.   



The DDS modification still uses the Corsair's "native" frequency readout 
circuitry, so setting accuracy is no better or worse than with the PTO.

Steve G3TXQ

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