PDP-8/A transformer hum

Mattis Lind mattislind at gmail.com
Tue Jun 9 13:54:36 CDT 2020

Den tis 9 juni 2020 kl 19:04 skrev Ethan Dicks via cctalk <
cctalk at classiccmp.org>:

> 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.

The MOS PDP-8/a PSU provide +5V,  +15  and -15V. So no 12 V there.

The Core PDP-8/a has -5V and +20V for the core memory.

To be able to test and debug Omnibus boards in my lab (aka garage) I
converted a MOS 8/a box into switched PSUs. Managed to squeeze five single
output PSus in the box to be able to run both 8/e core memory (which
require quite a lot -15V) and also newer 8/a core memory in it.

It is so much less heavy to move around on the lab bench.



> -ethan

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