[Elecraft] Amplitude variation on K3S with JT65B

[Lou Mecseri]

My K3 doing the same variation. Now I am running my K3, at 70% bellow 
100 watts so it will not push above the limit like I did with my KPA500. 
My KPA500 is back at the Elecraft hospital.

KE1F

On 10/21/2020 10:48 AM, Dr. William J. Schmidt wrote:
> I have a K3 and a K3S at my Caribbean station... I see the same behavior on
> my K3S but not on the K3.  I've calibrated it several times but have not
> diagnosed it past that.  I've seen the power vary by as much as 50% at
> times.
>
>
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> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On
> Behalf Of Conrad PA5Y
> Sent: Wednesday, October 21, 2020 6:50 AM
> To: elecraft at mailman.qth.net
> Subject: [Elecraft] Amplitude variation on K3S with JT65B
>
> Hello while I wait for feedback from the (no doubt overwhelmed) Elecraft
> support I thought that I would ask the group about a problem that I have
> with my K3S.
>
> When I run JT65B and even to a lesser extent FT8 I see unacceptable
> amplitude variation on both the 6m output and the transverter output (on
> 28MHz). If I put my E4406 Spectrum analyser on peak hold with a narrow span
> and 10Hz RBW I see amplitude variations of a little over 1dB on the
> modulation tones across the passband.
>
> To save time.
>
>
>    1.  On the ALC meter I have 4 bars lit up with the 5th occasionally
> flickering - the audio drive level is correct; it is not an ALC issue
>    2.  I calibrated the TX drive - the effect is observed at 1mw, 5W, 50W and
> 100W on 6m.
>    3.  I am using DATA A so no TX EQ or compression
>    4.  The audio has from WSJT-X been measured on a UPV audio analyser and
> there is less than 0.1dB variation when using a UMC404HD sound card
>    5.  The effect is observed with both Line input drive and the UMC404HD
> soundcard and the internal K3S codec
>    6.  My old TS2000X does not exhibit more than 0.1dB amplitude variation
> with the same sound card providing audio and the same test setup
>    7.  I have verified the amplitude variations at RF with a diode detector
> and an oscilloscope
>    8.  The effect is clearly visible on several power meters
>    9.  This AM causes some spectral re-growth, although this is tolerable
>
> So given all that I am beginning to wonder if the problem is due to filter
> ripple in the standard K3S 2.7kHz filter? I cannot think of anything else. I
> was considering buying an Inrad #718 filter.
>
> This is very critical for EME when signals are weak at the RX side, if you
> have amplitude variations and you are at the threshold of detection at the
> receiving station this can cause decodes to fail. This is more important on
> 6m EME than 2m where I have more margin.
>
> I think that this will be quite common, has anybody else got a K3S and the
> equipment to check and verify my observations?
>
> 73
>
> Conrad PA5Y
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