[Elecraft] K2 BFO test after adding a capacitor

[Rolf Moberg, OH6KXL]

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