Well, I've got to the bottom of this one and it's now in fine fettle
The problem was with the master PSU (the right-hand one): more often than not it would sit flashing the decimal points in the current meter at me no matter what the position of the output switch was, and as a result you could not get an output from it. I should explain that on these PSUs the flashing decimal points indicate that the unit is in current limit mode - and in this case, limiting the current to 0A!!
The cause was the front panel 'current limit set' pot.
If we look at the circuit, VR1 is the aforementioned current limit set pot, and R8 is the current sense resistor. A voltage generated by Z3 and then divided down by the components VR1¦¦R21, R22 and PR3 is compared with the voltage across the 0.5Ω resistor R8 by the comparator IC2 (an LM358). When the voltage across R8 exceeds that at the wiper of VR1, the comparator changes state and puts the PSU into limit. Note that the output of IC2 also controls the decimal points.
So what had happened with mine?
Well, when I got it I did notice that VR1 turned endlessly - because the nut on the control had worked loose and the pot had torn itself away from the wires soldered to it. I refitted it and although I initially though it had survived, when in the fault condition there was 0V at the wiper of the pot in any position
- the top end of the track was now intermittently o/c.
Fitting a new pot - which incidentally, is a logarithmic one - fixed the problem.
Another fault was the decimal points not switching properly between the output 'off' and 'on' states - as ever, this was due to switch oxidation. Working some DeOxit down past the toggle followed by a drop of Electrolube sorted that.
The final job was to check the metering, and on the master PSU the voltage readings were quite wrong. This turned out to be a calibration error, but I did need to change a resistor: R28 in the attached diagram. It was 15kΩ, but is now 10kΩ - in the manual it's an AOT component.
So there we have it - a very useful addition to my workshop.