HP 6129C 6130C Digital Voltage Source Supplies
 

I have a couple of these power supplies -- nice old HP vintage -- couldn't
figure out why, with the same binary code, one would give a negative
voltage, others it would be positive -- the secret is in the jumper W on
Board A2 -- this sets the "sign" signal for the DAC circuitry.

By using two supplies, one with A2 W Jumper at "A", the other with A2 jumper
at "B" you need only one programmer to yield positive output from one supply
and an equal negative voltage on the other -- nice for testing amplifiers.

For $19 the HP6129C (+/- 50VDC, 5 amps) is a real bargain.