[Elecraft] L34 tuning and Spectrogram

[Ron ZL1TW]

Hi,
      I can almost hear the groans "not again" with that heading, but 
having just built another K2 for a friend, I thought these two points might 
be pertinent for new builders.
When tuning L34, it is most important to follow the book and make sure you 
tune for strongest signal strength and lowest noise, but please follow the 
book where it says: "SELECT  CW NORMAL and FL2 ( 700hz nominal bandwidth) 
"....please note "CW Mode", not LSB or USB.
This will apply even if you are using the K2 for SSB only ( :-0 ) ....the 
reason being that when SSB is selected, there is a 100pf cap switched in 
via Q25 to "pad" the L34 tuning and flatten the perceived SSB filter 
response for better received audio.
If you try and tune L34 with this cap switched in, you will peak the tuning 
signal at the top of L34, or most likely not record a peak at all.
I include this as I have seen messages on here where builders haven't been 
able to tune L34. This is possibly why.  It is easy to miss a specific 
instruction in the manual.
Now the old favorite Spectrogram......If the plots you are getting look 
decidedly lop-sided, plug the computer audio lead into the "Speaker" jack 
on the radio, rather than the "Headphone' jack.
The reason being, when you are plugged into the headphone jack, there is a 
2.2uf cap attached to each leg of the output. When the "speaker jack" is 
used, the cap in the speaker leg is shorted out by J2 ( see Appendix B K2 
RF board sht 1 of 4 bottom lh corner)  Your plots will look appreciably better!
It is easy for builders with the KPA100 to leave the amp unplugged while 
tuning the filters with CAL FIL, and use the headphone jack, then wonder 
why the plot looks rotten. (believe me!)
Nothing new in this posting for a lot, but could possibly be of some help 
to our newer constructors? ( or forgetful ones...like someone I know !)
Cheers.......Ron ZL1TW


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