[Boatanchors] [Swan] SWAN 250C six meter rig question

[N4ch at aol.com]

The VFOs in the Swan 250 and 250C are essentially the same;  the injection 
frequency was moved 200 KHz lower (because the IF moved up, from  10.7 to 
10.9 MHz) when the "C" version happened.   There were also  some mechanical 
changes made, but I think electrical changes were  minimal.   No matter how 
much money U want to spend, it's not going to  be easy to make a free-running 
L/C VFO running at 40 MHz stable.    Unfortunately, ALL the Swan tube-type 
rigs suffered a couple of VFO  stability problems........firstly, the VFO was 
ALL L/C; there was no  heterodyning......mixing a low frequency VFO (such 
as a 5.0 to 5.5 MHz PTO) with  a much more stable crystal-controlled signal 
(like what was done in the Heathkit  "SB" series, and all the Drakes, 
Collins, etc. ).  If a typical 5  MHz PTO drifted a little, it didn't impact the 
final operating frequency as  much, since percentage-wise, most of the 
frequency determination quantity was  crystal-controlled, and was inherently MUCH 
more stable.   All of the  Swans had the unstable VFO characteristics 
compounded by the second  issue........... that of band-changing.   When U changed  
bands, one of the sections of the bandswitch switched  L/C components in  
the VFO.   Even if the switch contacts were relatively clean, this  switch 
could contribute significantly to unstable frequency control (if U don't  
believe this, just listen to the radio's calibrator or an on-the-air  signal 
while gently rocking the bandswitch back and forth a little.    Listen to the 
wobble and drastic frequency shift).   There were  attempts to "fix" this 
problem (one was using a higher quality ceramic instead  of phenolic material 
for this switch section).   Some solid-state  radios also had this issue; the 
popular Atlas 210/215/X series of radios were  basically solid-state 
versions of the same single-conversion platform as used in  the Swan 350, 500, 
etc., with all the quirks that came with it.   Herb  Johnson (who founded both 
Swan and Atlas) never did much with using heterodyning  to enhance stability 
in his designs; the one notable exception was the high-end  (for its time) 
Atlas 350-XL.   Its VFO covered the same 0.5  MHz frequency range on each 
band, and was mixed with a crystal-controlled  signal.........the same 
frequency generation scheme most others had been doing  for years by then.   
Another "good" by-product of doing a VFO  this way is being able to use the same 
dial scale on each band, versus having to  have several scales on the dial.   
The downside of using heterodyning  (other than the expense of another 
oscillator, more crystals, etc.) is that  additional filtering is needed to drop 
undesired mixing products to acceptable  levels.
 
73, Herman, N4CH.