Confirming PSU spec for steppers.

[Jonathan]

If the rating of your transformer is less than around 600VA, I wouldn't bother with any soft start circuit.
Thermistor is an easy option, as it's resistance decreases with temperature so no real need to take it out of the circuit.
You can use a relay to short circuit the fixed resistor. Essentially you want to switch the power on, wait about a second, then switch the relay on. You've reminded me of a circuit I came up with when I was about 10 - put a largeish capacitor in parallel with the relay coil and charge it via a resistor, then wire the relay output to break the capacitor charging current and you end up with a simple oscillator! In your case you would just use the capacitor and resistor to make a 1 second delay, so calculate it such that it takes 1 second for the capacitor voltage to reach the threshold voltage of the relay coil. Remember to include the resistance of the relay coil in parrallel with the capacitor in your calculation. You can power the relay coil from the 70V output via a potential divider. It's a bit of a crude way to do it, but easy to understand. Bonus points for using a solid state relay.

[Rogue]

If the rating of your transformer is less than around 600VA, I wouldn't bother with any soft start circuit.
Thermistor is an easy option, as it's resistance decreases with temperature so no real need to take it out of the circuit.

It's 625VA. It might not be very useful but it's more useful than detrimental, I suppose.

I understood the problems with thermistors to be the time needed to cool down again if they are not taken out of the circuit - if the PSU is shut off but restarted within quite a short time, the thermistor doesn't offer enough resistance to work as an effective soft start.

...In your case you would just use the capacitor and resistor to make a 1 second delay, so calculate it such that it takes 1 second for the capacitor voltage to reach the threshold voltage of the relay coil. ... You can power the relay coil from the 70V output via a potential divider.

There's 24V available for such electrickery, though I suppose the more self-contained the system the better.