[R-390] R-390A power transformer hum

[Jim Whartenby]

Sounds like (no pun intended) the result of magnetostriction.  I would think that all power transformers have this problem to some degree with florescent ballasts perhaps being the worst.  The hum should be at twice the line frequency.  The easiest cure is to replace the power supply.  Trying to melt the bitumen inside the transformer, to recoat the laminations, will be more trouble then it is worth.  If you go the second route, look for a bump of solder that covers a breathing hole that needs to be removed.  No need to split the transformer case and make a real mess in the wife's oven!

Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Saturday, October 19, 2024 at 09:57:16 AM CDT, Barry Scott <72volkswagon at gmail.com> wrote:   

 I seem to recall that from my R-390A/URRs.  The transformer in the
R-390/URR doesn't seem to do that.  The cost reduction must have taken
its toll there.

Thanks,
Barry - N4BUQ

On Sat, Oct 19, 2024 at 8:21 AM Tom Frobase <tfrobase at gmail.com> wrote:
>
> Some are noisier than others.  I have a large supply here if you want to
> replace it.  Contact me offline ... tom, N3LLL
>
> On Sat, Oct 19, 2024 at 8:18 AM Leo Reynolds via R-390 <
> r-390 at mailman.qth.net> wrote:
>
> > Is it normal for a R-390A power transformer to hum audibly? Not through
> > the speaker, though there is a little there too, but through slight chassis
> > vibration. It is there from the instant of turning on power and changes
> > slightly as the rectifier starts conducting and B+ flows. It is easily
> > audible in the shack, as if it were a small AM broadcast transmitter minus
> > the blower.
> >
> > Thanks,
> >
> > Leo Reynolds
> > K4LDR
> >
> >
> >
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