PDP-8/A transformer hum

[Glen Slick]

On Tue, Jun 9, 2020 at 3:10 PM Bill Gunshannon via cctalk
<cctalk at classiccmp.org> wrote:
>
> On 6/9/20 2:51 PM, Glen Slick via cctalk wrote:
> > On Tue, Jun 9, 2020 at 9:04 AM Bill Gunshannon via cctalk
> > <cctalk at classiccmp.org> wrote:
> >>
> >> I have a VXT-2000.  It is broke.  If I could find out what the voltage
> >> and current requirements were and their positions on the power connector
> >> I would replace it in a heartbeat.
> >>
> >
> > For the VXT-2000 with an H7109-B the rated voltage and current values
> > are printed right on the power supply label:
> >
> > +5.1V, 7.81A
> > +12.1V, 0.62A
> > -12.1V, 0.46A
> > -9V, 0.2A
> >
> > Fixed width character pinout diagram:
> >
> >        +=================+
> >    -9V | Yellow | Orange | +12.1V
> >        +--------+--------+
> >    ??? | White  | Black  | Gnd
> >        +--------+--------+
> > +5.1V | Red    | Blue   | -12.1V
> >        +--------+--------+
> > +5.1V | Red    | Black  | Gnd
> >        +--------+--------+
> >    Gnd | Black  | Black  | Gnd
> >        +=================+
> >
> > The mystery is the White wire. The power supply label only lists 4
> > output voltages. The White wire appears to be routed to the Ethernet
> > daughter board. The measured voltage appears that it might be floating
> > slightly negative, somewhere around -1.5V when the Ethernet daughter
> > board is installed and around -5V when it is removed. Maybe it is a
> > high impedance earth ground connection? It appears to be connected to
> > the shield of the Ethernet BNC, which measures around 1M-Ohm to the
> > chassis ground when the power supply is disconnected from the main
> > board, and around 0.75M-Ohm when the power supply is connected.
> >
>
>
> Thank you for that.  Mine has no labels or anything.  It is PC Board
> with Digital, Side 1, Side 2 and a number I don't recognize on it.
> I guess the -9V is the only odd one but you could easily get that
> from a -12V line.  I may get to fix it yet.
>
> bill

(I was going to change the subject line for this reply to reflect the
VXT-2000 discussion but maybe it is already too late to do that
without making message threading even worse?)

Does your VXT-2000 look significantly different than the images on
this page? Is the power supply in your VXT-2000 a bare board that is
not fully enclosed?

https://terminals-wiki.org/wiki/index.php/DEC_VXT2000

The power supply in my VXT-2000 matches the fully enclosed H7109-B
power supply as shown in this image:

https://terminals-wiki.org/wiki/images/b/bb/DEC_VXT2000_111543249570-2.jpg