JB Haskins wrote:
https://i.imgur.com/kSHzB0T.jpg

Unfortunately, I've never seen a schematic of this unit on the web even

Given the seeming simplicity of that board in your image (it appears to
be a single sided board) have you considered investing a moderate
amount of time into reverse engineering a schematic by following which
traces on the underside connect to which components from the top?