[Elecraft] Re: K2 BFO test after adding a capacitor

[Leigh L. Klotz, Jr.]

I finally got around to applying Gary's fix (Thanks!) for the problem 
where I couldn't center the narrow filters at 1000Hz in one of the RTTY 
modes (reverse R for low bands, normal R for high bands).  My K2 #3121 
apparently had a stiff set of X3/X4 crystals.

I used Elecraft's 2.7pF NP0 capacitor and put it between the junction of 
the crystals and L33 to ground on the bottom of the RF board.   I found 
which xtal pin it was by tracing which side of the xtals was not 
connected to C174. That lowered the bottom end of the BFO from 
4913.22KHz to 4912.36KHz, without affecting the top end. That was enough 
to let me re-run CAL FIL to get the BFO reach the 1000Hz center 
frequency for all of the R and R-reverse filters.

I have a picture at http://wa5znu.org/log/2005/08/k2-bfo-range.html -- I 
think I got it in the right place because it works!

Leigh / WA5ZNU

Rolf Moberg, OH6KXL wrote:

> Hello!
>
> After few weeks building incativity due to hot summer I'm back in my 
> shack. My K2 #4759 has been on the air several months. However, BFO 
> lower limit frequency didn't meet the required 4912.7 kHz
>
> Leigh, WA5ZNU sent me a mail which he had got from Gary from Elecraft. 
> Gary adviced to add 1-4pF NP0-capacitor to junction of X3 and X4. I 
> mean the lower junction in schematics. That is connected to L33. I 
> added 1.5pF NP0 capacitor on the bottom of the RF board. Other pin of 
> capacitor was soldered to ground pad of X4.
>
> BFO is now working OK. You can find the new frequencies below. 
> Original frequencies are shown in the brackets.
>
> BFO High Freq. 4917.17 kHz (4917.23 kHz) must be >= 4916.3 kHz
> BFO Low Freq.  4912.50 kHz (4912.95 kHz) must be <= 4912.7 kHz
>
> I'd be interested in the technical report why adding a little cap to 
> the paralleel of L33 and series with parallel connected crystals 
> lowered the oscillator frequency. I'm not an engineer. But I'd like to 
> understand.
>
> Should I make CAL FIL (and CAL PLL) again after this mod? I assume 
> they are not needed.
>
> Rolf Moberg
> oh6kxl