On Tue, Jun 9, 2020 at 7:44 AM Jon Elson <[email protected]> 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
