[Elecraft] K2 BFO drift problem

[Don Wilhelm]

Some component (or components) in your BFO (or the generation of the 
VBFO signal) is changing characteristics.
It could be the crystals (X3 and X4) or the capacitors (C173 and C174) 
or the varactors (D37 and D38), or L33.
It also could be soldering on the R-Pak RP6 or any of the above components.
Since my first guess is that there is a solder connection that is slowly 
gaining oxidation, I would re-solder all the components in question 
before doing anything else - use a hot (750 degF) iron and add just a 
tad of flux from a bit of new solder.

There is also the possibility that the VBFO signal coming from the 
Control Board is varying.  This is a DC level that tells the varactors 
how much to change capacity.
To check that, leave one filter set the same at all times.  With that 
filter selected, measure and record the VBFO value to as many 
significant digits as your DMM will show.  Control Board U10 pin 14 is 
an easy place to measure it.  Then when the BFO shifts frequency, 
measure it again (do not change that filter setting).  If the voltage 
changes, then the problem may be with U10 on the control board.  
Re-solder U10 and RP2 and RP3 to see if that stops the change.
On 4/13/2014 1:24 PM, TheMG . wrote:
> Well, a few weeks ago I finished assembling and testing my K2 S/N 7520 and
> the KPA100, KSB2, K60XV, and K160RX options. All went without any problems
> at and have had several successful QSO with it.
>
> However, since then I've been noticing quite severe BFO drift. I have to
> re-align the filters using CAL FIL almost daily. The low range of the BFO
> keeps drifting upward, about 50-100Hz every day, and it hasn't stopped yet
> even after two weeks!
>
> Initially (first built and tested), the BFO low range (as measured with CAL
> FCTR) was at 4912.6kHz. Today it's at 4913.35kHz.
>
> What I've ruled out so far:
>
> -Temperature. Room temperature is quite stable around 20-22C. Even after
> long periods of transmitting with the KPA100 and the entire K2 chassis
> getting quite a lot warmer than room temperature, BFO temp-related drift
> was only about 30Hz and in the opposite direction of the daily BFO drift of
> 50-100Hz.
>
> -At first I though maybe I hadn't properly secured L33 and that maybe
> mechanical vibrations and stress from moving the K2 around was causing the
> inductor to move around, but for the last week I had it sitting on a desk
> without moving it at all and the BFO is still drifting.
>
> Also, from my observations, the BFO range is actually constricting, rather
> than simply shifting upward. The high range of the BFO has gone from
> 4917.47 to 4917.15.
>
> Gently tapping or pressing on the RF board doesn't change anything.
>
> Anyone experienced anything like this or having any idea what the problem
> might be?
>