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
