PDP-8/A transformer hum

Glen Slick glen.slick at gmail.com
Tue Jun 9 13:51:16 CDT 2020

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.

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