PDP-8/A transformer hum

Bill Gunshannon bill.gunshannon at hotmail.com
Fri Jun 12 09:02:38 CDT 2020

On 6/9/20 7:13 PM, Glen Slick via cctalk wrote:
> 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

Sorry for the delay.  Since the lockdown started Verizon DSL has
been a total disaster and I frequently go for days without any

As for the subject under discussion.  You are right mine did have
a metal cover over it with the voltages, (but not the pinouts!)
It's been so long since I tore it apart to fix that I had forgotten
and had to dig the box out that has everything but the PSU Circuit
Board in it to refresh my memory.  Things having changed so much
since then I am going to take another shot at fixing the original
PSU before replacing it with something non-DEC.  But, based on the
voltages used, building a replacement from scratch would be
relatively trivial.


More information about the cctalk mailing list