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K3-S RF output pulses while TX in FT8

McClain Richard
 

When I transmit FT8, I can see the P3 (with TX monitor) display shows RF output is modulated or "Pulses". Pattern reminds me of watching AM modulation of a sine wave viewed on oscilloscope.

It does not occur all the time but does occur on any band using either antenna or dummy load. I have done FT8 extensively for 2-3 years, but this issue started a couple of weeks ago. I have carefully checked Owner's manual as well as book by Fred Cady, KE7X. Can't find root cause. Any clues are appreciated. I can document more thoroughly, but want to post a brief comment in case there is an obvious solution to someone else. Thank you. 
Dick, KD4S

Equipment: K3-S S/N 10905 (built in 2018), P3 with TX monitor, WSJT-X version 2.1.2

Mike Flowers
 

My soundcard will produce a pulsing output tone at certain frequencies and under certain RF power out and WSJT-X Pwr drive levels, into an antenna or a dummy load.   A change to any of these parameters seems to clear it up.

 

- 73 and good DX de Mike, K6MKF, NCDXC Secretary

 

From: Elecraft-K3@groups.io <Elecraft-K3@groups.io> On Behalf Of McClain Richard
Sent: Friday, May 15, 2020 14:52
To: Elecraft-K3@groups.io
Subject: [Elecraft-K3] K3-S RF output pulses while TX in FT8

 

When I transmit FT8, I can see the P3 (with TX monitor) display shows RF output is modulated or "Pulses". Pattern reminds me of watching AM modulation of a sine wave viewed on oscilloscope.

It does not occur all the time but does occur on any band using either antenna or dummy load. I have done FT8 extensively for 2-3 years, but this issue started a couple of weeks ago. I have carefully checked Owner's manual as well as book by Fred Cady, KE7X. Can't find root cause. Any clues are appreciated. I can document more thoroughly, but want to post a brief comment in case there is an obvious solution to someone else. Thank you. 
Dick, KD4S

Equipment: K3-S S/N 10905 (built in 2018), P3 with TX monitor, WSJT-X version 2.1.2

McClain Richard
 

In the original post, I should have said I am passing audio via CODEC, not a external sound card. 
I have the KPA500 amplifier and KAT500 tuner, but the problem exists with amplifier off and tuner in Bypass. 
So, problem must be in WSJT-X software settings, or K3-S hardware or settings (which I have carefully triple-checked per manuals).
73,
Dick, KD4S

Bob McGraw - K4TAX
 

While I do see some slight variation in the WSJT signal, it is very minor.   Using TUNE mode, my K3S via USB control, pulses between 110 and 113 watts output, as best as my watt meter resolution provides.  Likewise when transmitting FT-8 data.   I figure this is the result of the processor in my computer doing some other housekeeping chores when running WSJT-X. 

As to the indication of ALC values on the K3S, if the tone is near 1500 Hz, I do see ALC pulsations indicated.    I attribute this to the interaction between the scan rate or display and the 1500 Hz tone.  If one changes to 1300 or 1700 Hz then the issue does not appear.  In either case, I see this as a display item and actual power output is not affected. 

73
Bob, K4TAX

McClain Richard
 

Bob, thanks for the report. My observation is very different. With K3-S PWR set to 100 watts, I see erratic display on my P3 with TX monitor while the K3-S RF output on display bounces between full scale and 50% full scale. 

I tried setting the tone at 500, 1500 and 2500 Hertz and all behave the same. 

Interestingly, when I key transmitter using the WSJT-X TUNE button, the K3-S RF output is steady. But when  I send CQ or any other message, the RF output is erratic. So I opened the PC Volume Mixer to observe the indicators of volume level and WSJY-X.
Using TUNE, the CODEC speaker is steady.
When sending a CQ message, the volume (green indicator) bounces while K3-S RF display bounces

Wonder if my problem is in the PC audio oscillator? Don't think so because earlier I ran a cable from PC earphones jack to K3-S LINE IN jack and found audio was stable.

Bob McGraw - K4TAX
 

Those frequencies are all multiples of 500/1000/1500/2000 thus related.  Try something different like 1396 Hz. or such.   Sounds like you may have RF getting into the line between the radio and computer or into the computer.  To test, use lower power and/or a dummy load.   Add clamp-on ferrite to each end of the cable, especially the computer end.   I use the size for RG-8 which is large enough to get 2 or 3 turns through the core. 

73
Bob, K4TAX

McClain Richard
 

Bob, 
Thanks for your comments. Regarding my earlier test of various audio frequencies, those numbers were rounded estimates with no attempt to be on 500 Hz or multiples. I just wanted to try to vary the audio frequency to eliminate that as a part of the puzzle.

I have clamp-on chokes on both ends of the USB cable between PC and K3-S -- each with two turns in ferrite. 

Regarding DC current drain, I have an ASTRON RS-35M with meters. In CW mode at 100 watts, the meter shows about 18 Amps (not an accurate meter). When transmitting a CQ message in WSJT-X, the current varies between 18 and 14 amps, approximately.

All measurements are with a dummy load. Also, I removed unnecessary cables -- Key, Paddle, earphones, Mic.

Also, reducing the power to 25 watts or so still has significant fluctuation of DC current and RF out as viewed on P3 with TX monitor. Further reduction to 12 watts (hear relay click for 100 watt amplifier) is difficult to judge because current drain deflection of meter is too small and P3 display is also a small variance in amplitude. 

Thanks again for your suggestions.