[sdiy] 78xx regulator gives negative output half the time

[David G. Dixon]

> Has anyone seen this before? Any common causes for this problem?

No, but I can tell you from experience (from just lastnight, in fact) that a
79L05, when accidentally shorted with a multimeter probe, will output about
0.6V instead of its customary -5V (or -4.94, in the case of my specimen).
I'm assuming that this is the manifestation of some internal protection
circuitry.  From that, might one infer that you are somehow causing the
internal protection circuitry to fire upon fire-up?