Arfa:

You hit the nail on the head. The problem appears to be related to
the power supply...or at least to a component downstream from the
power supply.

Right next to the connector that leads to the laser assembly, one of
the contacts is labeled "5V". Measuring this contact gives me less
than half a volt under all circumstances.

This 5V contact leads directly to the collector of a 2SA1270
transistor. The transistor checks out OK, although the same half a
volt reading is measured at the collector. There is a jumper wire
that leads to the emitter of this same transistor. It is also clearly
labeled "5V", and it measures a healthy 5.08V.

The base of this transistor is connected, via a 10k resistor, to the
middle lead of a very small transistor with "C114" on the case. If it
is a 2SC114, I cannot readily find a replacement for this transistor,
and I cannot find a datasheet, either, so I don't know how to check
this transistor. It gives me weird readings whenever I test it in
standard NPN or PNP pin configurations. IT DOES NOT SEEM TO BE
DEFECTIVE, HOWEVER, because there are several "C114" transistors and
they all give me extremely similar readings whenever I attempt to
measure them in standard NPN and PNP pin configurations. There are
also a couple of 100uF/16V electrolytics in this circuit used as
couplings to ground, but they check out fine with my ESR meter and my
analog ohmmeter.

I am going to attempt to keep checking upstream to see where things
break down.