I would go with the "stuck on" scr, but there may (or may not) be something causing the "stuck on" (or more properly, failing to turn off) SCR. likely causes include a pin (or larger) hole in the insulation sheet under the number 1 scr, a bad number 2 or 5 scr or leakage in the 3&4 recs/diodes, and/or the insulation sheet under the heat-sink that the number 3&4 recs are mounted to. This is why the first check should be for leakage to the frame from any of the power cables. any leakage that results in a reading of less than 100k ohms should be repaired FIRST, then go looking for other causes.
when you say "Leads to the SCRs are good" are you saying that you checked them for shorts to the frame and found NONE? (some is normal in the m-ohm scale), especially with a battery installed and plugged in)