[cctalk] Re: Ferroresonant transformer mystery

Recently I got a nice and complete PDP-8/s from the US. The power supply uses a ferroresonant transformer which in addition to the standard primary and secondary windings has a separate 2.3H winding connected in series to a 2uF 660VAC capacitor forming a resonant "tank" circuit. The transformer's secondary side and the resonant circuit are operated in saturation. There is a magnetic shunt to prevent the primary side going into saturation as well. It accepts a wide input voltage range, but is very sensitive to the input frequency of 60Hz. This is quite a nice if not elegant design for the period in question, but maybe not the most efficient. As I live in Australia I get 240VAC and 50Hz as opposed to the US 115VAC and 60Hz. I can easily convert our 240VAC to 115VAC with a step-down transformer, but cannot easily supply 115VAC at 60Hz. So I was considering using a step-down transformer to get the 115VAC, but modify the resonant "tank circuit" for 50Hz. Unfortunately there is some magic I don't understand. The resonant frequency of a LC circuit with L=2.3H and C=2uF is about 75Hz not the expected 60Hz. Otherwise I could just solve the standard LC resonant circuit formula for C and plug in 50Hz and 2.3H to get the required C. Obviously ferroresonant transformers are more complex than this former software engineer can grasp. Could any experienced EE with relevant transformer knowledge please chime in and help me understand how to redimension the tank circuit to use 50Hz instead of the original 60Hz input. Obviously I could replace the entire power supply with two modern switch mode supplies to create the two rails, but it would be really nice to keep the original supply and just reversibly adapt it for 50Hz.