Instrument Voltage Stabiliser Test Rig
The diode is any basic blocking diode, say 100v 1 amp, it prevents the charge on the capacitor leaking away through the stabiliser and gauges. The resistor and capacitor aren't critical, but a higher resistance with an analogue meter will result in the reading being lower than the true average, for reasons I won't go in to (unless asked). Lower values of capacitor will result in more fluctuation of the reading making it harder to see what the true average is. The average should be 10v.