PDP-8/A transformer hum

[Bill Gunshannon]

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