PDP-8/A transformer hum

[Ethan Dicks]

On Tue, Jun 9, 2020 at 10:46 AM Jon Elson via cctalk
<cctalk at classiccmp.org> wrote:
> On 06/09/2020 09:27 AM, Anders Nelson via cctalk wrote:
> > Related question: Is there any reason, other than historical accuracy and
> > cost, to not replace the power supplies in vintage computers with modern
> > switching power supplies?
> >
> The old discrete transistor machines often used fairly odd
> voltages, not like 5V and 12V.

DEC's transistorized machines used +10V and -15V supply rails for the
logic, and some other large voltage (+28V?) for the core.

Later machines still used odd voltages.  Lamp-based PDP-8 front panels
used voltages like +8V, and later core was IIRC +20V.

There are a couple versions of the PDP-8/a PSU, for core memory and
for MOS memory.  MOS memory is standard 1970s triple-voltage stuff
(4096s and 4116s) but they pull a lot of +12V compared to later
machines.

It's often easier to just fix the old PSUs than replace them.  At
least linear supplies aren't so hard to debug.

-ethan