PDP-8/A transformer hum

[Maciej W. Rozycki]

On Tue, 9 Jun 2020, Bill Gunshannon via cctalk wrote:

> > 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.

 For the unlabeled white wire this must be -9V return, such as say with 
the H7109-AA PSU used with the VAXstation 4000 VLC, the H7819-AA used with 
VAXstation 4000 60/90/etc. and storage expansions or the H7821-00 PSU used 
with several systems, where the -9V supply circuit is isolated and used, I 
believe, for serial line drivers/receivers.  It might be worth checking.

  Maciej