Looking at the schematic for my harbor freight power feed, looks like
universal motor, with SCR control, with no full wave rectifier in
circuit. So it's half wave only control. The SCR is before motor
(which is connected to neutral return), so I believe it is sensing
back EMF from motor which would improve speed control. However it
just does not do a good job of regulating constant feed.

Has anyone seen a simple circuit, or some other off the shelf device
that can replace the internal control board with a much improved
design?

ignator