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
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[Don Wilhelm-3]

Rolf,

Yes, re-do CAL FIL to properly reposition the filter passbands - the BFO
controls that positioning, so any change in BFO frequency will cause a
shift.

You should not need to run CAL PLL since the PLL reference oscillator has
not been changed.

Why does adding a capacitor change the frequency?  The BFO is actually a
VXO, and the frequency is controlled by changing the capacitance in the
circuit - normally that is done by changing the voltage on the varicaps, but
since your total capacitance range was slightly out of normal a small padder
capacitor needed to be added.

73,
Don W3FPR

> -----Original Message-----
>
> 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
>

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[Leigh L. Klotz Jr WA5ZNU]

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

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