Today I lifted the three heater element wires from the terminal strip located within the Regeneration Control Panel and used a "High Potential" tester to apply 1000 volts between each wire and ground.  I varied the current limiter throughout its range (0.3mA - 12mA) while the high voltage was being applied and did not detect a short to ground - good!  Next, I terminated the 4-20mA CDS wires on to the SCR unit and energized the Regeneration Control Panel - no faults.  Finally, and without any GN2 flow, I enabled the CDS PID control with a setpoint of 50C and proportional gain of 7.  The Regeneration Temperature rose -> 24.1C,  24.2C, 24.3C, 24.4C, 24.5C - over the course of 5 minutes or so before the control panel tripped with a "HIGH LIMIT" fault.  This is as expected considering the absence of GN2 flow to remove the heat from the heating elelments.  The thermocouple used for the "HIGH LIMIT" is in contact with the heating elements ballast mass while the thermocouple used for the PID control "Regeneration Temperaure" is several inches downstream of the heating elements and it samples the GN2 flow.  

So, it looks like the Regeneration Control Panel is working again (for the time being!).  We are installing a second in-duct cartridge heater tomorrow and will have to shut down the forced-air heating unit so maybe Monday we can try to start up the regeneration flow again and start dumping some Joules into that CP4 bad boy!