I have just discovered something peculiar.

I have checked a number of service manuals for various HP Harrison Power Supplies and they all say the same thing. The unit is shipped with the positive terminal strapped to GND at the back terminal strip. This would make it a negative supply.

Why on earth would they do this? Almost every user would want to remove the strap to GND.

 

This is set up as the negative supply for a split supply. You will need a second one to get the positive rail. On that one, you must move the positive ground to a negative ground. Or, you could remove both and use a floating ground scope.

In this modern era of single supply op amps, you will want it on the negative rail or float your supply.