[Elecraft] K1 VFO drift

[michael]

Folks :

A little while back I mentioned on this list that I felt that I was 
having some frequency
drift issues with my K1 since I had changed the VFO tuning range from 
wide to
narrow. I decided today that it was time to see if I could do something 
about it,
but I figured that I needed to quantify the magnitude of the drifting 
first otherwise
I wouldn't  know whether I have made things better or worse.

I hooked my K1 up to a well regulated 13.8 V supply, turned it on, tuned
it to 7.040 MHZ, started a timer and let it be. Note that the rig was 
inside the
air-conditioned house at a temperature of 24c and not subject to any 
vibration.

Here is what I observed

time: 5  min   total drift: -200hz  (freq 7039.8 mhz)
time: 30 min  total drift: -300hz  (freq 7039.7 mhz)
time: 60 min  total drift: -300hz  (freq 7039.7 mhz)

So it appears that I am getting approximately 300hz drift down in 
frequency
in the first 30 minutes and then things stabilize.

I checked the Specs in the front of the manual and it says < 200 hz / 
hour after a 5
minute warm up at 25 degrees C so I guess that technically things are 
within spec
as I am seeing only about an additional 100hz drift within the first 
hour, after the initial
5 minute warm up.

It is likely that the drift is greater in a less controlled 
environment, especially outside with the
brutal heat we have been having here this summer.

I'm still wondering if replacing C2 (68pf disc cap) with a polystyrene 
or even
an NP0 cap might make things a bit more stable when using the rig in 
the field.
Does anyone think that this is worth doing? Comments?

Michael VE3WMB

P.S. Someday I would love to see an upgraded K1 (lets call it K1.5) 
with a
DDS vfo like the KX1 and a continuously variable IF filter bandwidth 
like the KX1
or the ABX on the Sierra.