[Elecraft] [K3] Sudden Hi-SWR on low end of 40M

[Augie Hansen]

On 2/18/2013 9:54 AM, Terry Schieler wrote:
> Yesterday, during the CW contest, I noticed that my K3-100 (#474)
suddenly began to "stutter" during CW transmissions on the low end of 40
meters. I hadn't noticed this before, including Saturday, the first day
of the contest. My CW transmission had a "chattering" sound something
like the braaaaap of a machine gun (and no, my fist isn't *that* good).
My first thought was RF getting into the radio, but, I've never had RF
issues before and, again, nothing in the system had changed:
>
> K3 > LP-100A SWR/Power meter > 4 position Ameritron RCS-4 antenna
switch > dipole.

I've seen two versions of the RCS-4. An early version had three DPDT 
relays and a later version had only two 4PDT relays. My comments here 
apply to the two-relay version.

Which ANT position on the RCS-4 connects to the 40m dipole? If position 
4, your remote switch may be failing. In that position the control 
voltage imposed onto the coax center conductor along with the RF is 
alternating current, which activates both relays in the remote switch. 
If the coax connections are not solid for any reason, or if either of 
the filter capacitors across the relay coils is bad, one or both relays 
may be chattering.

You can check this by selecting antenna 4 and listening to the remote 
switch box. If a relay is chattering you'll hear it. There's no need to 
apply any RF for this initial test, but if there's no audible response 
apply a steady carrier in the troublesome frequency range and listen 
again. There could be an unwanted resonance due to changes in the RFC 
(~7-100uH) or the capacitors that triggers the event.

Good luck,
Gus Hansen
KB0YH