KX1: new build & transmit problem on 20m ...

[VK7JB]

 Hello Group,

I've just completed building KX1 # 2644, including the KXB3080 module,
and have discovered a problem.

All progress testing and alignment proceeded smoothly to the end of
both manuals.  But I've noticed that when listening to the KX1's
transmitted signal (using my K3 as receiver) on 20m only is very
raspy.  It sounds distorted, with what sounds like another signal
lower in frequency.  In fact, it sounds a little like that low
frequency buzzing you hear when you grab the audio plug connected to
the input of an audio amplifier.

Here's what else I've found:  this is present on 20m only.  The tx
signal on all other bands sounds very clean.
When switching between the bands, I can hear relays clicking for all
band transitions EXCEPT switching between 30 and 20m. No clicking
there.
RF gain control is much less effective on 20m, compared with other
bands.
20m receive is basically OK, but maybe a little less sensitive
(subjectively) compared with other bands.

Not sure where to start with this, so any advice much appreciated.

73,
John
VK7JB


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[Don Wilhelm-4]

John,

You will nost likely find the problem in the Low Pass Filter board.  My
suspicion is that the LPF is not working and letting 2nd and 3rd
harmonic energy through - and the net effect is modulation of the KX1
output waveform.

73,
Don W3FPR

On 6/5/2012 8:25 AM, [hidden email] wrote:

>   Hello Group,
>
> I've just completed building KX1 # 2644, including the KXB3080 module,
> and have discovered a problem.
>
> All progress testing and alignment proceeded smoothly to the end of
> both manuals.  But I've noticed that when listening to the KX1's
> transmitted signal (using my K3 as receiver) on 20m only is very
> raspy.  It sounds distorted, with what sounds like another signal
> lower in frequency.  In fact, it sounds a little like that low
> frequency buzzing you hear when you grab the audio plug connected to
> the input of an audio amplifier.
>

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[Stan Gibbs, KR7C]

VK7JB wrote

But I've noticed that when listening to the KX1's transmitted signal (using my K3 as receiver) on 20m only is very raspy.

John,

I had a similar experience to you (though without the 30/80 module).  My P3 showed strong 60Hz modulation of the output signal on 20m and a much smaller modulation on 40m (not audible).  The audio on 20m was definitely "raspy".

The KX1 was connected to a dummy load via 25 ft. of RadioShack RG58 and I had no antenna on the K3.  I thought the modulation was an oscillation at first, but after monitoring the KX1 output signal on my scope, I realized that the modulation had to be due to some other cause, and I suspected RFI in the power supply or cables because of the 60Hz sideband spacing.

I changed my coax to a 3 ft piece of RG213.  The 20m signal immediately became clean on my K3.  The longer the coax, the more apparent modulation of the received signal.

All signal reports that I received from QSOs gave me a clean tone report and I asked several ops whether there was any hum or buzz on my 20m signal.  Nobody reported any modulation.

Do you have a scope to look at the KX1 output?  If not, you might want to get some on-air reports to determine if there is a real problem in the transmitted signal.

[VK7JB]

Hello Stan,

A very interesting observation.  I don't have a scope, but I'll re-work the dummy load and receiver setup and see if that changes the raspy hum/buzz on 20m.  If it doesn't resolve, then I'll avail myself of Don's repair service, as I won't have the expertise to troubleshoot this kind of problem myself.  

Thanks for your interest.

73,
John
VK7JB