PDP-8/A transformer hum

Josh Dersch derschjo at gmail.com
Tue Jun 9 16:10:04 CDT 2020

On Tue, Jun 9, 2020 at 7:44 AM Jon Elson <elson at pico-systems.com> wrote:

> On 06/09/2020 01:59 AM, Josh Dersch via cctalk wrote:
> > Hi all --
> >
> > I'm working on a PDP-8/A I picked up at VCF West last summer.  After a
> lot
> > of cleaning and some power supply repair it's showing signs of life (CPU
> > seems to be at least minimally functional, core memory is going to need
> > some debugging.)
> >
> > The transformer in the power supply is humming quite loudly, however, and
> > I'm curious if this is normal for the 8/A (or is typical for an 8/A of
> this
> > vintage).  I'm used to the supplies in various other DEC machines not
> being
> > exactly silent but this is a rather severe 60Hz buzz that you can clearly
> > hear over the fans when the machine is in operation.
> >
> >
> The old PDP machines used ferroresonant transformers to
> provide better voltage regulation.
> These do hum a bit.  The cause is that they run part of the
> iron into the saturation region, and there is a lot of
> leakage magnetic flux.  It may not be the actual transformer
> that is buzzing, but any piece of steel nearby.  You might
> put your hand on various pieces in the area and see if you
> can find one that has developed a loose bolt or whatever.
> Jon

Thanks Jon (and everyone else).  Yes, this is a ferroresonant transformer
(w/matching motor-run cap), I'd just never heard one in a DEC machine that
was so very loud.  I'll see if there's anything that needs to be screwed
down a bit more.

- Josh

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