[Elecraft] KX3 Tuner Operation at Low Voltages

[Wayne Burdick]

Hi Dean,

The latching relays in the tuner should work just fine at low supply voltages; they run off a voltage regulator, so their coils should see 5 V even when the rig is running from as low as 8 V.

I think what you’re seeing is not enough forward power to qualify the SWR readings. If the load impedance you’re working into is low (on a given band), the instantaneous key-down TX current drain will go up and the forward power might drop below the minimum required level for accurate readings. The ATU will skip LC combinations that don’t meet the measurement criteria.

The effect could be made worse by batteries that have higher ESR, which varies with state of charge, number of charge cycles, and temperature. Any increase in ESR reduces key-down max power output.

To see if the bridge is working correctly, connect the rig to a dummy load, run it from 10 V, turn the ATU off, and check the key-down SWR reading (TUNE) at 0.5 W, 1.0 W, and 2.0 W. Let me know what you find.

73,
Wayne
N6KR




> On Feb 26, 2018, at 4:36 PM, AL7CR <al7cr at mooseaviation.com> wrote:
> 
> My KX3 internal tuner works perfectly at transmit voltages above 10v.  Below that value it will initiate tuning but finish at an SWR of between 3 and 6.  When this happens if I connect my station 13.6v supply or an external 12v battery supply it will immediately tune to the expected low SWR.
> 
> Elecraft tech support was a bit non committal on this issue.  They said that 10v was close to the minimum value for the relay drivers to operate properly.  This is a bit strange as one does not have to operate long with internal NiMh batteries before the transmit voltage drops below 10v.
> 
> An easy work around is to perform my portable tuning when I first start operating.  Still this seems strange behavior for a rig designed to work on the internal battery pack.
> 
> Is this normal operation for the internal tuner?  If not, then what should I check?  I thought that tuning at a lower power might help but the manual states the tuner operates at either 2w or 3w and the value is not under user control.