HP6111A DC Power Supply

HP 6111A DC Power Supply is a precision power supply for 0-20 volts and 0-1 ampere current. It has a 5 digit digital input to select the precise output voltage. The HP611x is a whole series of such PSUs with different output voltages.

I have also a HP6112A, going upo to 40 volts. See here for details on that other PSU.

This is a device from the 60ies or 70ties. I could not find any date code for my device. So it is about 50 years old. Built completely from discrete components, no integrated circuits.

Front. The digital dials allows to set output voltage down to 1mV resolution in a range of 0.000 .. 20.000 volts.

Backside connectors:

Condition on arrival

The PSU is in a good optical shape. It is clean, inside and outside.

First measurements show that it does not correctly regulate voltage, current limiting seems to work.

Repair work

I was able to repair the PSU. This was done with the great help from HP group at groups.io (https://groups.io/g/HP-Agilent-Keysight-equipment, look for “HP6111A power supply issue” topic).

After measuring at various places, it was found that the voltage was regulated, but there was a large ripple on the output, and, even more bad, the PSU was oscillating. It oscillates with about 3Khz and an AC amplitude of about 17 volts. The AC waveform integrates then to an DC offset of about 4 volts, for the selected 0 volts.

It came out that the output capacitor C23 was dead. It had about 5 uF (from the original 1450) and an ESR value of about 2KOhms.

After replacing the capacitor, the PSU behaved totally normal. The voltage was not 100% bang on, but I could calibrate the output to 1mV precision and also adjusted the analogue meter, because it also was not perfect.

After that, the PSU was in a perfect, usable state. Only thing I plan to add is a LED showing on/off state. Otherwise, it is not visible that the device is switched on, and I forget frequently to switch off these devices.

To have an excellent regulation and no drift over time, the input part of the differential amplifier is contained in a closed oven. This oven is heated up to about 60 degrees. PCB inside oven contains a dual transistor and two other transistors (Q1A/B, Q2, Q3):

Look on transformer side. Large output transistor at left. The most left large blue capacitor is the one found to be faulty.:

Oscillating output, look at AC component I had: 18 volts of amplitude, at 3 Khz:

Defective C23:

Inside C23. I opened the capacitor with a saw. The contacts were totally corrodated, and the paper/metal roll was somewhat dry:


  • Large schematics screenshot