Was: PDP-8/A transformer hum Is: VXT-2000 stuff.

jim stephens jwsmail at jwsss.com
Tue Jun 9 18:57:16 CDT 2020

On 6/9/2020 11:51 AM, Glen Slick via cctalk wrote:
> On Tue, Jun 9, 2020 at 9:04 AM Bill Gunshannon via cctalk
> <cctalk at classiccmp.org> 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.
As long as we are talking VXT-2000, I have this on file, figure it would 
be useful for future googling of the list if
this were here as well.  May be a repeat of info from an earlier post, 
or maybe elsewhere.

I didn't save the source of the info, but looks like someone named Matt 
Millman did it.

The setup is as follows: HP Envy laptop running MOP boot daemon, 
connected to Lantronix LTX-C twisted pair
ethernet to AUI converter (not the same as a twisted pair MAU). This is 
then connected to a cabletron coax
MAU with a 10BASE5 vampire tap. The coax runs over to another MAU, which 
is connected via an AUI cable to
my VXT2000. I didn't think of it, but it would have been nice to have a 
real DEC MAU on the VXT2000, I've
got several DEC h4000's I could have used.



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