I have a Hickok 440 curve tracer that I am having difficulty troubleshooting, even though it is a relatively simple circuit.

The base generator staircase is not stable, and will not stay in calibration. It looks like noise comes and goes and distorts it. I have replaced most of the semiconductors. The 15v power supply is a bit low at about 13.9v, though the regulator transistor and zeners have been replaced.

I have the manual, schematic, parts and usual test equipment.

Thank you,
Don