PDP-8/A transformer hum

Jon Elson elson at pico-systems.com
Tue Jun 9 09:44:07 CDT 2020

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.


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