[Elecraft] KX3 temporary high SWR on PTT with external Amplifier

[Don Wilhelm]

Ignacio,

The amplifier is not switching to the transmit state within the time 
between when the KX3 pulls PTT low and the time RF appears. I believe 
that time is about 8 milliseconds.
Yes, your KX3 is working into an open load when that happens - hence the 
high SWR.
That condition is known as "hot switching", and even if it does not 
eventually damage the KX3 PA, the relay contacts in your amplifier T/R 
changover will become pitted due to the arcing.

One solution is to modify your amplifier with a QSK T/R switch or 
substitute fast vacuum relays.
Another solution is not to use QSK or VOX - connect a footswitch to the 
KX3 PTT *input* (ACC2 jack's TIP connection) and set the ACC2 IO menu to 
LO=PTT if the footswitch grounds the connection.
Then use the footswitch to put the KX3 into transmit before you operate 
the paddles or speak into the microphone.  In other words, do not turn 
on VOX.
Another way to put the KX3 into transmit mode without the footswitch is 
to use the XMIT button on the KX3.

73,
Don W3FPR

On 9/29/2016 12:06 PM, EA2BD via Elecraft wrote:
> Hello,
> I want to check if anyone experiences the same behaviour when connecting the
> KX3 to an external amplifier.
>
> - I receintly added an external amplifier to my KX3.
> - I connected the PTT cable to the ACC2 port and to the amplifier.
> - I set the Menu ACC2 on the KX3 to Active low, as specified.
> - I verified if the cable works and it closes contact all right every time I
> key down.
>
> The TROUBLE is:
> Everytime I send a dit (CW) or start talking (Voice) the SWR indicator on
> the KX3 shows full reading (Hi SWR!) just briefly and it quickly relaxes
> down to normal reading (SWR 1:1) during the rest of transmission.
>
> I believe there should be any kind of delay between the TX on the KX3 and
> the TX on the ampli, that causes the momentarily Hi SWR reading... But this
> is extrange because the PTT cable is in!
>
>     Does anyone have experienced similar phaenomena?
>     Any idea to overcome this?
>     Do you consider there will be a real overload on the finals due to this
> small transient time?
>
> I drive with 2 watt out from the KX3 to get around 90W out from my
> Amplifier.
>
> Wait for your feedback, many thanks!
> 73 de Ignacio EA2BD
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/KX3-temporary-high-SWR-on-PTT-with-external-Amplifier-tp7622916.html
> Sent from the Elecraft mailing list archive at Nabble.com.
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