Dave, thanks for much for this project, very educational!
I've built it up, hopefully correctly, however there is something I don't get.
Connected to a 8V lab supply, and with a 100 ohm load across the output ...
I am measuring the output voltage and current with two multimeters.
Dialing up 5V output using the 10-turn pot, and cranking up the current pot (so the LED doesn't light) all is well. I read 49.63 mA current which is about according to theory, bearing in mind I didn't take a huge amount of care to check all the resistor values were exactly to spec.
Now this is the funny bit ...
Gradually turning down the current pot, I get this:
48.11 mA gives 4.84V
45.04 mA gives 4.53V
41.09 mA gives 4.15V
38.93 mA gives 3.93V
37.42 mA gives 3.78V
So far so good ...
But now carefully reducing the current very slightly, I get:
0.636 mA giving 0.064V
It's like it has hit some threshold condition, where the voltage regulator doesn't turn on.
Can this be explained? I could accept "construction error" if it wasn't so well behaved up to that point.