John's scope is not set incorrectly. 
The zero is indeed at the bottom of the half-wave trace and centered on the AC waveform.
Using my old 1033 bench transformer, I connected a diode, a 620 ohm resistor for a load, and took some measurements with each on AC and DC.
I don't have an analog meter, but I did a little experiment with two different digital meters, one a Fluke 8012A True RMS model and one a Fluke 77 handheld that does not have True RMS for contrast.
I never get zero volts, but I know that some older meters have capacitive coupling for the AC ranges, perhaps that's what Dale sees.
These findings are consistent with what I see with the smoke triac doing half-wave rectification.
The first two photos are measuring directly across the 16 volts from the 1033 using the AC and DC settings of the bench meter.
AC Setting
DC Setting
The next two photos are the bench meter with True RMS measuring across the 620 ohm resistor in AC and then DC.
AC Setting
DC Setting
The final two photos are the handheld meter without True RMS measuring across the 620 ohm resistor in AC and then DC.
AC Setting
DC Setting
