[Elecraft] VCO voltages for 160m band??

[G3VVT at aol.com]

K2 serial no. 4168 for which the tests were reported does not have the  K60XV 
installed. The use of 5MHz frequencies in the UK are only available by  issue 
of a special licence to a restricted number amateurs for experimental  
purposes on USB only.
 
4000kHz had the highest of the VCO control voltage measured  when completing 
alignment at 6.85V with my particular K2. When meeting the  minimum 
recommended VCO control voltage of 1.50V at 7000kHz, a watch  on the readings at 4000kHz 
had to be considered at the same time. Bringing the  7000kHz VCO voltage much 
above 1.50V resulted in the voltage at 4000kHz  exceeding the Elecraft K2 
manual recommended maximum of 7.50V.
 
The AGC threshold adjustment was done after noting postings on the  reflector 
on the subject a few months back pointing out the method of testing. I  found 
my K2 exhibited the rise in background when switching the AGC off  compared 
to that with AGC on (when testing under quiet band conditions).
 
Don, W3FPR in his reply has apparently misread the reason for testing at  the 
various AGC threshold voltages.
 
The original new setting tried was 3.60V as on first observation this  
appeared to fulfil the AGC threshold requirement. However when tested with  the 
bench signal generator at 7100kHz, I found that compared with the initial  AGC 
threshold setting of 3.80V the level needed to give S9 on the K2 with the  preamp 
off had dropped from 63uV PD to 25uV. This was after careful adjustment  of 
the S LO and S HI required as per the K2 Alignment Instructions posted  on 
Don's web site. (Very well worth keeping a copy of this with your K2 manual)  This 
three fold increase in S meter sensitivity was not really welcome as I had  
tried to keep all my receivers at 50uV for S9 where practical.
 
A new start was made to find the optimum setting for the AGC threshold  
setting.
With this I adopted the following when testing:
1. The antenna input was terminated to prevent any external noise  
influencing the results.
2. The K2 preamp was switched on.
3. The receiver audio O/P was monitored by headphones to more easily detect  
the slight change of audio level AGC on to off. (probably a sensitive scope  
would have done the same test)
 
With these conditions in place it appeared with K2 #4168 that the optimum  
setting for the AGC threshold worked out at 3.70V. Again the S LO and S HI were  
reset to compensate for the changed AGC threshold voltage and a new test was  
done for S9. With the preamp off it came out at 40uV PD  (-75dBm) which much 
closer at -2dB compared to the standard I use  of 50uV PD (-73dBm) for S9.
 
The changes to the readings noted for S9 are only given as an observation  to 
the changes seen with the three AGC threshold voltages tested at, though  
could be significant where a disparity of S meter readings between various K2  
are noted. The absolute accuracy of my bench generator is unknown, though does  
compare favourably with a Marconi 2955 Communications Monitor which had  a 
much later calibration date. Sending my test equipment for calibration is  a 
luxury beyond my means these days now after joining the band of the  retired.
 
Bob, G3VVT
K2 #4168