[Elecraft] possible K2 T/R Problem?

[Brian Wruble]

Hi Guys:  

I am in the Alignment and Test Part II Section of my K2 construction.
Everything has gone beautifully to this point.  I now have my first
problem. 

I am in the section for the BFO alignment.  When I first turn the K2 on,
there is no audio.  I can then get the audio to appear if I briefly key
the rig.  If I give it a "dit", I get audio.  HOWEVER, if I move the VFO
knob or move the RIT, or push any of the buttons, the audio disappears,
and the S meter goes to full scale.  Another "dit" brings it back.

Doing voltage checks suggests a possible T/R switching problem.  The 8R
voltage on the RF board (at D6 anode) is only 0.6v when the rig is first
turned on.  When I give it a 'dit", I get audio, and that voltage goes
to 8.3V.

All of the RF Board switching diodes, D1-D7 have low voltages (0-0.6v)
on turn-on, jumping to the correct voltages, per the RF Board voltage
chart, after I give it a "dit".

On the Control Board, Q1 and Q3 have the correct voltages, per the
Control Board voltage chart.  But, Q2 and Q4 do not.  Q4 has voltages of
S=0 (vs. 0 in chart), G=0 (vs. 5.0 in chart) and  D=8 (vs. 0 on chart)
on turn-on, and S=0, G=1.3 and D=0 after a "dit".  Q2 voltages are E=8.5
(vs. 8 in chart)  B=8.2 (vs. 7.3 in chart) and C = 0.8 (vs. 7.5 in
chart) upon turn-on. After a "dit", these are 8.5, 7.6 and 8.4.

Though I am not exactly sure what the "mute" function is, I think it may
be the way the K2 tells the audio to go mute when various controls are
pushed.  Since this problem occurs when buttons are pushed, or the VFO
moved, my natural assumption was that the mute circuit could be the
problem.  However, the /Mute (Mute=not ?) voltage is 3.85v. and does not
change with the comings and goings of my audio.  That voltage compares
with 2.7* in the chart, with the asterisk indicating "approximate".  Is
3.85V close enough to 2.7v?

Q23 on the RF board, according to the manual, "is used to guarantee that
8R goes to 0V on receive to maintain proper reverse voltage on T-R
switch diodes." (Found in "Theory of Operation", under "Control Board".)
Can this possibly be a typo? I would assume 8R would be at 8V on
receive, and would be pulled to 0v. on transmit.  In any event, Q23 only
has the correct voltages, per the voltage chart, when I have the audio.
When first turned on (no audio), these voltages are S=0, G=0.37, D=0.82.
These become 0,0, and 8 after a "dit", nominal readings based on the RF
Board voltage chart.


I have visually inspected, looked for cold-solder joints or shorts,
reheated any suspicious joints, etc.  Nothing seems to work.  I am
beginning to suspect component failure, possibly Q2, or Q4 on the
Control Board, or Q23 on the RF Board.  I would really appreciate any
suggestions, ideas, etc.


This is a basic K2, S/N 2528, with no upgrades as yet.



Tnx for any help.  73 de Brian W3BW