[K3] Hum JT65/JT9 and Vox. Revisited.

[Larry Lopez]

This is actually covered already but I need to mention it.

When the WSJTX waterfall started losing signals into the noise
I noticed that there was a neat periodic set of signals that
lined up with the frequency ticks.  Eventually it dawned on me
that they were 120 Hz apart and eventually I found a 60 Hz
fundamental.

What I found is that the the line out was picking up the
linear power supply that was behind it.

What I also found out was that as the K3 got closer the
the transmitter came on.

So the magnetic field from the current spikes
from the diodes in the power supply were being picked
up line transformers in the K3.

So I moved the power supply.

Since the transformers get rid of ground loops I wouldn't complain too much.

End of Story.

PS it was really unnerving to move the K3 and have it turn on.

[Larry Lopez]

I was wildly optimistic in this.
A second source of "hum" is the switching supply
of the laptop.

It's similar to the power supply hum.
Even though it's clearly a switching supply.

Curiosly the laptop need 12 volts.
So perhaps I can pick that off of the linear
power supply.

Or maybe run everything off of a battery.

[Larry Lopez]

I switched to a Creative Sound Blaster X-Fi Go!
USB dongle and that got rid of the hum from
the laptop charger.

I got this originally because the desktop
I use has no sound support at all.
It was an extremely cheap server box
that runs Linux.

[Larry Lopez]

So I decide to give up and set the audio level right.
So I get the free for private use sound card oscilloscope
http://www.zeitnitz.de/Christian/scope_en.

I picked this one because it was first when I goggled "sound card oscilloscope".

Very nice.

But gee, there are 2 channels for the mike and a balance control.
One channel has "the signal" one channel has "chaff".

So I set the balance control to kill the "chaff".
I then raise the volume in the K3 line out until it flat tops
and I back it off by 2.

next step, get it to sample at 48000.

obsessive, I am.

[Rick WA6NHC]

The gain of the sound card  (the volume) can be changed by the driver so even if the K3 output doesn't change, the levels can still change.

I leave the software (driver) maxed out and the K3 Lin Out at 1 (almost off), but this risks swamping the card AGC so I use the K3 filters to limit what the sound card gets.

73,
Rick wa6nhc

Tiny iPhone 5 keypad, typos are inevitable

> On May 18, 2014, at 12:42 PM, Larry Lopez <[hidden email]> wrote:
>
> So I decide to give up and set the audio level right.
> So I get the free for private use sound card oscilloscope
> http://www.zeitnitz.de/Christian/scope_en.
>
> I picked this one because it was first when I goggled "sound card
> oscilloscope".
>
> Very nice.
>
> But gee, there are 2 channels for the mike and a balance control.
> One channel has "the signal" one channel has "chaff".
>
> So I set the balance control to kill the "chaff".
> I then raise the volume in the K3 line out until it flat tops
> and I back it off by 2.
>
> next step, get it to sample at 48000.
>
> obsessive, I am.
>
>
>
>
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/K3-Hum-JT65-JT9-and-Vox-Revisited-tp7588936p7589252.html
> Sent from the Elecraft mailing list archive at Nabble.com.
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[Jim Brown-10]

On 5/18/2014 12:42 PM, Larry Lopez wrote:
> So I set the balance control to kill the "chaff".
> I then raise the volume in the K3 line out until it flat tops
> and I back it off by 2.

Good method, BUT -- better to drop the level by half the voltage from
the "flat top" point. That's because the distortion of many (most?)
computer sound cards drops by 10 dB or more at half their maximum output
voltage.

73, Jim K9YC
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[Larry Lopez]

What's interesting is that all of 60 Hum and it's harmonics are gone.
The line out is 2.

I have the Mike volume:

Balance is all the way over to one channel.
Volume in full

I adjusted line out to flat top and halved  it.

The JT65/JT9 display looks good now.

[Larry Lopez]

Thanks Jim & Rick for your help.