VAX 4000-500 PSU Overload?

Noel Chiappa jnc at
Fri Dec 11 07:46:50 CST 2015

    > From: Jon Elson

    > You should be able to safely power the machine with only a couple
    > boards at a time to find which one is bad.

For debugging power supplies, a 'load module' (a card with only a bunch of
resistors on it) is invaluable. No trying to figure out where/how to connect
a large load resistor - just plug the board in.

DEC made several:

  M7556		dual - +5V 1A -15V 375mA
  M9060-YA	quad - +5V 5A
  M9713-AA	dual - +5V 2.75A -15V .75A

The M7556 (at least) can be used on both QBUS and UNIBUS backplanes (since
they share the same pins for +5 and -12V (QBUS) and -15V (UNIBUS). Probably
all the others can be used on both buses, too, but check the pins they draw
from first.

The M7556 can be easily modified (with a few etch cuts) into a +5/+12V load
module, but the resultant board can _only_be used in a QBUS.

If you can't find any of these (there are none left on eBay, but other
suppliers still seem to have them), another alternative is to get a blank
prototyping board from Douglas Electronics (,
and add your own resistors.


More information about the cctalk mailing list