[Elecraft] re: K2 anomaly

[Ron D'Eau Claire]

Dave, G3YMC wrote:

The volume pot track is well known to become noisy over time, but the 
same applies to all four pots. My CW speed pot (K2 2498) is now quite 
touchy with only a very small change causing a significant change in 
speed, and the RF gain pot (normally left at the clockwise endstop) 
is also showing signs of noisiness. Any noise on the speed and power 
pots will be interpreted by the DACs and cause the transient display 
change we are all seeing.

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In general the audio pots on  the K2 don't have a noise problem Dave. The
original design of the K2 audio gain circuit put demands on the audio pots
they weren't designed for which gave the impression that the pots were going
bad in early K2's. Wayne redesigned the circuit and I haven't heard about
any problems with noisy controls since. 

In early K2's, the audio gain control acted as a shunt resistor across a
balanced audio line. That required that the pot have essentially zero
resistance at minimum. Most pots are not designed to provide that reliably,
including the one used in the K2. After a short period of use, the pot would
start showing a small amount of resistance near minimum, which made it sound
"noisy" and sometimes made it impossible to shut the audio off at minimum. 

I modified my K2 with Wayne's new circuit four years ago and I've not had a
noisy pot problem since.

The change was incorporated into the basic K2 some time after the mod was
developed because it required changes to the PC boards, so there were a lot
of K2's built with the old circuit. I think the change came about around S/N
3000 somewhere. From the comments I've seen, many owners never got around to
modifying their units once they had gotten used to the control sounding
noisy near minimum volume.  

If yours hasn't been modified, there are instructions for doing that under
"builder's resources" on the Elecraft web site. Look for "K2 AF Pot
Application Note". 

Ron AC7AC