[RFI] Keurig coffee vs Astron Rs35M

[Jim Brown]

On 4/10/2014 10:49 PM, Gary Smith wrote:
> As to the Keurig & the Astron... I do love the Keurig, I use the
> refillable coffee holders and it does make a fine cup but I digress.
> As was mentioned earlier, the Astron was terribly made as regards
> grounding. The wire grounding from the negative DC out was bolted to
> the chassis but it was on paint on both sides and the round plug was
> attached to the chassis exactly the same way. I connected both of
> these ring terminals to one bolt and removed the paint on both sides
> to make a true ground.

The three I opened had exactly that problem, and I did what you did.  
Except that I removed the bond between V- and that terminal strip so 
that V- floats. There are good reasons for doing that.

> I tried figuring out what was making the hum in the Astron when the
> Keurig was on and it seems to be coming from the transformer (which
> is really warmer than I expected). I removed the DC connection and
> just had the Astron plugged into the AC and I still was hearing that
> intermittent humming when the Keurig was on. Nothing I could press on
> in the PS made the noise stop. I finally put three large ferrite
> clamp-ons with three loops each on the power cord to the Astron and
> now I can still hear the hum but now it's greatly reduced and hard to
> hear over the fan in the amp, so that is a real bit of progress.

Hmmmm!  That last sentence strongly suggests an RFI or circuit 
instability issue (oscillation) -- those chokes on the power cable are 
adding some common mode Z above about 10-15 MHz, but only a small amount 
of inductance at power frequencies. Things are getting curiouser!

Something else to look at --it would be interesting to know if that 
Keurig box is connecting neutral and green. It should NOT. Easy way to 
figure out.  Plug the Keurig into a multi-outlet box, and measure AC 
voltage between neutral and ground (the big blade and the round hole) 
with the Keurig running. You SHOULD see some AC voltage between neutral 
and green that is approximately equal to the IR drop from the panel for 
the neutral conductor only (there should be no current on the green wire).

Back to your noisy wall wart problem -- my solution has been to replace 
all the SMPS warts with linear warts that I buy at hamfests and flea 
markets.  Unless I've missed some, the only SMPSs in my home and shack 
are in my Rigol Spectrum Analyzer,  Rigol Scope, and some Thinkpad power 
supplies.  I discovered the Rigol SMPSs when I was using one to probe 
other equipment, and found residual around the back of the Rigol!

73, Jim K9YC