tom,
I'd guess Steve is correct, but it could also just be portions of the case
vibrating in sync with the transformer.  You might just try pressing on the
various joints in the case and see if anything changes. Also check case
screws for tightness.

Tom Fowle WA6IVG
On Fri, Dec 11, 2015 at 09:59:08PM -0500, Steve Dresser wrote:
> Tom,
> 
> I'm pretty sure that the hum is mechanical, caused by vibrations of the 
> transformer inside the supply being amplified by the metal case.  You might 
> try placing the supply on a thick piece of foam, keeping in mind that you 
> don't want the four feet to sink too deeply into the foam, which would block 
> circulation and probably give you a problem that would make you long for the 
> good old days when you were being annoyed by the hum.  I suppose you could 
> open the supply up and try to isolate the transformer from the case, but 
> that sounds like a lot of work, and I don't think you'll get much for your 
> efforts.  Bottom line: don't worry about the hum as long as it's not the 
> result of bad filter caps or poor voltage regulation (and from what you've 
> said, I don't think it is).
> 
> Steve
> 
> ----- Original Message ----- 
> From: "Tom Behler" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: December 11, 2015 20:45
> Subject: Question on New RS35 Metered Power Supply
> 
> 
> > Hi, everyone.
> >
> >
> >
> > Well, I got my new Astron RS35 metered linear power supply yesterday, and
> > set it up tonight here in my home shack.
> >
> >
> >
> > Everything seems to work well, in that I get the output power readings I
> > would expect on resonant HF frequencies via my TS590, and on VHF/UHF
> > frequencies via my TMV71A.
> >
> >
> >
> > I have noticed something, though, that I could use some input on.
> >
> >
> >
> > For some reason, this new RS35 has a bit of a louder hum than I noticed 
> > with
> > my older unit.
> >
> >
> >
> > Has anyone noticed this sort of thing with the newer RS35's, and if so, 
> > were
> > you able to do anything to address the issue?
> >
> >
> >
> > I might consider moving the supply to another location, because right now,
> > it is on a shelf just above my operating position.  However, I will need 
> > to
> > be certain that the newer location gives the supply sufficient space for
> > ventilation.
> >
> >
> >
> > I am not sure the problem is significant enough to return it to HRO, but I
> > would be curious if anyone has had a similar experience, at least 
> > recently.
> >
> >
> >
> > Tom Behler: KB8TYJ
> >
> >