[HBR] HBR2K -- Chapter 13 -- The VFO Revisited and Futher Work

[hbr@mailman.qth.net]

In a message dated 2/9/03 4:46:03 PM Eastern Standard Time, 
waltah@earthlink.net writes:

>  Replacing the 6DZ4 with a 12AT7 with the second 
>  triode serving as a cathode follower cleaned up the slight roughness 
>  in SSB signals.   

Glad to hear it!
>  
>  I also tweaked the temperature compensation slightly.   Now here's 
>  no drift at all, after after the first 15 minutes.

COOL!
>  
>  The VFO's something of a paradox.   On one hand it's the part of the 
>  design where there's the greatest payoff for being able to just copy a 
>  working design.   But on the other hand, it's the part which is the 
>  hardest to build so that it *can* be duplicated.  If the dial calibration 
>  is to be accurate the coil has to be exactly right, the 'thermal design' 
>  is not 100% obvious, temperature compensation is another issue.   I 
>  have a second FT-101 VFO unit now, so before I finish the 
>  documentation I'll start from scratch on that one, writing it all down 
>  from the beginning.

In situation where the actual VFO freqs must match a predetermined scale or 
dial, or be linear, the design can be complicated. I use a different method 
for homebrew projects - the old "make the scale/dial match the VFO". The 
trick I use is to  get the VFO working just the way I want it (drift and freq 
coverage), then make up a hand drawn scale out of cardboard using the LM freq 
meter. 

Then I use CAD software to make up a nice dial scale, print it out, and use 
it for the actual dial. 
>  
>  Next is another alignment and then doing the two-tone dynamic 
>  range measurement.  Also need to check the skirt selectivity -- may 
>  need a shield across the filters, or ???  

Good idea, but also check decoupling caps everywhere. Do not be afraid to add 
more bypasses, add chokes, rewire with shielded wire, etc. Sometimes it takes 
a combination of capacitors of different values to get a low Z bypass at all 
important freqs. Do not overlook the heater circuit as a feedback or bypass 
path. 

>  I hope someone else will build this thing.   It works fine -- actually, a 
>  lot better than that -- but there's going to be plenty of room for 
>  improvements.   Maybe 50% on the notes and schematic.   

What is really neat about this project is the concept of recycling an old 
unwanted rig that is not worth restoring into something completely new and 
unique. 

73 de Jim, N2EY