[Elecraft] K3 - Power supply question

[Bill W5WVO]

Guess you stumped everybody -- no responses in the list!  :-)

I'm not an expert in switchers, but it seems to me that the problem might be a 
combination of the imperfect sine wave output from your inverter coupled with 
the fact that you're putting a very low load on the switcher in receive mode. 
(Assuming the switcher doesn't produce this hum when plugged into the mains.) 
I think I would try an analog supply in place of the switcher and see if you 
still get it. I think you wouldn't.

Second, I wouldn't necessarily trust on-air reports about your transmitted 
audio where the signal strengths aren't extremely strong. You might have a hum 
on there that is quite a few dB down from peak audio, and it isn't detectable 
under the ambient received noise. If a local guy who is copying you S9+20 says 
you have no hum, then I'd believe it (if you trust his integrity and technical 
competence to make such a judgment).

Or it could be some kind of ground loop problem, but it sounds like you have 
that covered. Ground current analysis in a complex home electrical/radio 
system is still black magic to me.  :-)

Bill W5WVO


Matt Zilmer wrote:
> Hi to all,
>
> This is not a critical issue, but it might interest some.  It's also
> not necessarily specific to K3's.  Keywords might be "switching power
> supply", "inverter", and "grounding".
>
> The problem I describe below was never experienced with the K2, only
> with the K3.  I exchanged the two radios to confirm this
> experimentally.
>
> The power supply scenario is as follows.
>
> Qty 2 - 50W Siemens solar panels on the roof above the shack supply a
> 12V 100A-Hr SLA battery.  The battery is charged through a Micro-X
> charge controller, and this arrangement has been used for a few years
> with my K2.  It seems to work well under all conditions.  The K2 is
> the low power QRP version, and generally draws less than 2.5 A from
> the battery at power levels I run.
>
> The battery is connected to a Xantrex RS-400 sine wave inverter.  THD
> on the 60 Hz output is supposed to be < 3%.  Looking at its output via
> a Tek 2465A scope shows a very clean sine wave, even under the max
> 400W load.  The inverter is rated 400W, with surge to 800W.
>
> The inverter feeds an Astron SL-11A switching power supply that powers
> the K3/10.  Its output is 13.6V, and under max load at the K3 we're
> talking about 2.8A, or 39W.
>
> All units described above are bond-grounded to each other using a 1/2"
> braided strap.  The strap drops 10 feet or so to an 8 foot ground rod.
> The ground rod is also bonded to the house ground nearby.  The house
> ground is used for all main and backup loads for a much larger Solar
> PV system that we use to power the house here.
>
> When using the Battery => Inverter => SL-11A power supply => K3 power
> supply path to power the radio, the switching supply hums quite a lot
> in weird non-periodic ways.  I can sometimes hear the same hum on the
> K3 RX audio, but have had no reports of it showing up on TX audio when
> I asked the other party.  "Very clean audio", they would always say.
>
> Given the long-winded narrative above, does anyone have an idea about
> the rx noise I'm hearing?  Seems like it's not getting in through the
> DC, since it doesn't show on SSB TX.  So maybe I'm hearing it over the
> air?
>
> I thought this might be common mode noise, but the TX reports I've got
> with this power supply setup won't support that.  Grounding can affect
> that, but since low impedance paths are connecting all the inline
> stuff, this doesn't seem the easy answer either.
>
> Sorry to take up so much bandwidth, but it's possible another
> Elecrafter type might face a similar issue.  So it seemed worth the
> bits.
>
> BTW, connecting the K3 directly to the 12V SLA battery has none of the
> above effects and works fine.  Please - no flames over the easy
> answer, OK?  :)
>
> 73 and thanks for any thoughts,
>
> matt zilmer, WA6EGJ
> K2 s/n 2810
> K3 s/n 0024
>
>
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