What common factor could be causing wrong voltage on both rails of this PSU?
Looks to me as if some of the feedback divider resistors may have drifted in value. Or maybe the transistors fed by these feedback resistors may have changed characteristics with age.

If you look at the snippet below the area in the red box are resistors involved with setting the level of the +43V. The fact that the design shows the 18K and 180K in parallel implies that these may have intended to be hand selected and soldered in to tweak the supply voltage into the +43V level. You could try tweaking these values to raise the positive rail voltage some. It may even be possible to replace the 180K with a trimpot to allow adjustability.



Once you get the positive rail adjusted the negative rail will likely follow along. In case it is still off then the resistor glob in the yellow box is used to adjust the negative rail. Note how the feedback of the negative rail is developed off the positive rail. This is done to allow the two voltages to track. Once again the parallel combinations of the resistors in the upper and lower parts of the divider seem to imply that one or more of these resistors was meant to be hand selected to set the negative rail voltage. You could also try replacing one of the resistors with a trimpot to allow easy adjustment of the negative rail.