With the use of a PC Based Oscilloscope and a differential probe, it became evident that a problem with the DC Link was present. There was either a misfire or faulty diode in the SCR assembly. As it happens it was misfiring. This was causing an irregularity in the DC Link after the Choke in the magnitude of 20 VDC.
The scope trace determined that the SCR on L1 was causing the irregularity. Further testing of the switching gate showed that it was being misfired due to a mis-wiring in it’s wiring loom. Further investigation showed the Gate Plugs for Thyristor L1 and Thyristor L2 had been incorrectly located. Once the correction was made to the SCR’s Gate connections the Pre-Choke DC Link began to perform as expected. The DC Link after the Choke became very regular in all load conditions.