[Elecraft] K2/100 Problem with Hi Reflected Power driving amp

[Stephen W. Kercel]

John:

As another poster noted, why do you use a tuner between the K2 and the 
amp unless the amp has an untuned input?

Are you sure that the tuner is actually adjusted to present a 50 ohm 
load to the K2?

Echoing some of Don's points:

Is there something wrong with the timing of the keying sequence? 
Specifically, is the amp being switched in slightly after the K2 comes 
on? One theory of your problem is that when you key the K2 it comes up 
to full power, and then the relay that cuts the amp into the circuit 
closes a moment later, and while the relay is moving from one position 
to another the fully powered K2 is looking into an open circuit.

Assuming that timing is not the problem, as Don has noted, perhaps your 
amplifier input impedance is changing as it moves from no power to full 
power. The protective circuitry of the K2 can be very touchy; mine gives 
the Hi Refl warning and starts to roll back power if my SWR exceeds 1.7.

Assuming that the amplifier does not require the full 100 Watts drive to 
operate properly (most do fine with 65-80 Watts drive) one way to deal 
with varying load impedance (provided it does not vary too much) is to 
put a 1 dB 50 ohm 25 Watt fixed attenuator between the K2 and the 
amplifier input. That would attenuate your 100 Watt output to only 80 
Watts drive, however, it would significantly dampen the variation in 
impedance that the K2 sees.

Fixed high power attenuators are not that common in ham radio, but are 
very common in professional situations. Brand new ones can be quite 
expensive, but you can get pretty good deals on them on eBay. There are 
various brands; NARDA is one of the better choices.

I am currently in the process of refurbishing an old Dentron GLA-1000B 
(tuned input) to use with my K2/100. For that reason, I am very 
interested in hearing your progress in solving your problem. I am just 
about to start building the QSK switch that AD5X (a regular poster on 
this list) published in the February 2008 QST.

I really hope you get this solved.

73,

Steve Kercel
AA4AK


John wrote:
> Hi all. I have a K2 with the KPA100 installed. I'm using the K2/100 to drive
> a tube amplifier through a tuner. The K2-100 keeps tripping off with the
> HiRefl message showing not every time, but often enough that a QSO is
> impossible. The K2/100 doesn't 'reset' (has no output) when rekeyed unless I
> rotate the power setting knob ever so slightly. 
>
> What am I doing wrong? Any way to increase the delay before the High
> Reflected power takes effect as I suspect it's transient? I also have the 
> t-r rel
>
> Any assistance will be greatly appreciated.
> 73, John  WA6YSY
>
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