I traced out the circuitry of the Libby regulator. I may have found a design error. They rectify the power used to run the reference regulator and the unijunction pulser that pulses a pair of SCR's that generate the field current. This rectified supply is pre-regulated with what appears to be a 29 volt Zener diode. The supply doesn't appear to have any filtering. I suspect that the raw full wave rectified is impressed upon the regulation system. This is NOT good engineering practice. I will do some more testing. My regulator blew up the MOV varistor that is across the field. This causes major damage to the board and the voltage drops to around 40 volts instead of 208. Tracing the circuitry was the only way to understand how the regulator works. Essentially a the rate of pulsing the SCR's is done by a unijunction transistor which the timing capacitor is fed from a PNP transistor that is biased on by a pulldown resistor. A rectified sample from the metering source is or'ed into the base of the pnp transistor, turning it off if the sensed voltage is higher than the reference set by an LM723 regulator chip.