[Elecraft] K2: AF and RF volume controls

[Ron D'Eau Claire]

Alan SP9BM wrote:

I have just acquired a used K2 (includes noise blanker, SSB adapter,
internal tuner) which seems to work fine (hears well, and I have made some
4w SSB contacts across the Caribbean islands), except for two little issues:

first, the AF gain control, when turned fully anti-clockwise (7 o'clock
position), does not silence the rig, although the control does work in
reducing/increasing audio volume over the rest of its range.  Volume at the
minimum position is about where it would be if a normal rig were at the 9
o'clock position.  I pulled the knob off and tried the bare control: no
change.

second, if the RF gain control is turned or touched at all, the rig
immediately goes silent and all the bars in the bar-graph meter are
illuminated.  Over a period of about three seconds the bars are
extinguished, from right to left, and the sound comes back up.  The control
itself works in limiting RF gain normally, depending on the position to
which it is turned.

I have another K2 (no noise blanker, no internal tuner) which works entirely
normally, so I know this behaviour is unusual.

Just wondered whether anyone had any previous experience with these
phenomena.  My impression is that the rig worked normally in the past, but
may not have been used for the last couple of years.  Serial No. is 2086.  I
got it from the original owner; although it had not been built by him, but
by one of the for-hire builders.

Would replacing the pots help?

Grateful for any assistance.

---------------------------------

That sounds like a bad RF gain pot or a missed solder connection in the RF
Gain or AGC circuits that has become 'noisy'. It could be a bad pot.
Touching the pot causing it to act up is a very good clue. I'd look very
closely at the connections to the pot first.

The audio gain control action you're seeing is also normal for that vintage
K2, but there is help available. The original AF gain control design used
the pot to "ground" the audio, rather than control the gain. To be more
precise, the K2 uses a "push-pull" balanced audio line between stages so the
traces on the pc boards won't pick up digital noise (such noise will be
'common mode' on both of the audio circuit traces and is easily suppressed).
The pot was across the balanced lines so it "shorted out" the audio when
turned fully down. Unfortunately, the pots develop a little bit of
resistance at the "minimum' setting over time and so won't turn the gain all
the way down any longer! 

The gain control circuit was redesigned to provide better control that does
not require such low resistance at the minimum setting. It also changes the
audio control to be logarithmic in nature, like most audio controls. That
happened in 2001 and was implemented at about S/N 3000, if I recall
correctly. However, Elecraft, as usual, provided instructions for owners of
older K2s to upgrade their rigs with the new circuit. The mod requires
changing a few traces and adding a couple of wires for the new circuit, but
no other parts are needed. Your present AF gain control is probably fine.
You'll need a little bit of  wire. I used some stranded wire scavenged from
a bit of old telephone extension cable. 

The mod is on the Elecraft web site at:

http://www.elecraft.com/Apps/K2_AF_pot/K2_AF_gain_app_note.pdf

For an Adobe Acrobat PDF file or 

http://www.elecraft.com/Apps/K2_AF_pot/K2_AF_gain_app_note.htm

For the same thing as a web page. 

I incorporated that mod in my K2 S/N 1289 and it works like a champ! 

Ron AC7AC