I think I have found a possible cause, but not too sure what to do about it if I have.
I had to replace the 7812 regulator that supplies Vstart. The new one has an output that seems to vary between 12.2 and 12.4V. The 7812 on the good PSU outputs a steady 12.1V. I think that is enough to cause the E3d comparator to turn the comparator off and allow its output to float high. I suspect I may have used one with a wide tolerance rather than a narrow one, or just have a bad one. It should be a genuine part as it came from CPC Farnell. Not quite sure why the output varies on the bad one though. Regards Rob
