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

[hbr@mailman.qth.net]

A while back, Jim, N2EY, said:

> You might consider some sort of buffer to prevent osc pulling.

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

I also tweaked the temperature compensation slightly.   Now here's 
no drift at all, after after the first 15 minutes.

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.

Does anyone have a good source for NPO and neg. temp. coefficient 
ceramic caps?   I have been buying two of every value at fleamarkets 
for twenty years but would like to put a source in the notes.

Picture with the case on:

http://www.timbreblue.com/hbr6.jpg

(The other pictures are at successively lower numbers.)

I did replace the dual concentric control for RF/AF gain with 
independent controls.   Controls now (left to right along the top line):  
STBY, PWR ON, (spare) 100kcs/off/25kcs, tuning, PRESELECTOR, 
RF gain.   Lower line: MODE (LSB/USB/CW/AM2kcs AM4kcs),  AF 
gain, BAND.

It needs paint but that will have to wait for warmer weather.   The 
parts set case is rough; I may sandblast it before painting.

The STANDBY switch is wired, ditto the headphone jack and the 100 
kcs/25 kcs calibrator switch; I haven't done the 25 kcs multivibrator 
yet.   Yeah, it has to be 25 kcs because the 2nd IF (3180 kcs) is a 
multiple of 20 kcs.   

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 ???   I still have to switch the RF 
and 1st oscillator over to +90 regulated HV.   The volume is greater 
on SSB than on AM; I need to look into that.   

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.   

Walt Hutchens
KJ4KV