[Elecraft] Elecraft K1 / Intermittent Frequency instability solved

[Joseph Lemoine]

Hi All
I just finished assembling my K1 transmitter, and managed to make a
first QSO with it (France -> Bulgaria). Everything was OK, except that
the frequency was sometimes very unstable. I suspected some cold solder
or missing component, since I noticed that the RIT was also not working
(even if the LEDs were showing RIT and XIT modes).
I opened the covers, connected an oscilloscope to the OFFSET signal, and
found it always at 0 volts. Then I searched for eventual short circuit,
or cut wire.
The thing is that when I touched the RIT potentiometer axis, the OFFSET
voltage suddenly raised to 2 to 3 Volts. I suspected a faulty
potentiometer, with poor wiper pressure, but the resistance measurement
was correct.
Finally, the reason was an intermittent short circuit between the
Potentiometer metal body (4 x sharp legs) sitting right on PCB traces
underneath (one of them being the OFFSET signal...) the green solder
mask not offering a sufficient insulation.
Immediately, I de-soldered the RIT potentiometer, cleaned the flux
residues, and pasted a 1 square centimeter piece of scotch tape on the
PCB surface, before replacing the part. Since then, the K1 does not
suffer anymore of intermittent instability!
If this message could be of some help for other builders, or for a new
PCB design version...
Vy 73 from Joseph / F6ICS
QTH near PARIS
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.qth.net/pipermail/elecraft/attachments/20090120/d08ba99e/attachment.html