I've hunted all over and find decades old information on this being a
typical problem for this unit, but cannot find the parts &
instructions on how to fix it.

I Have The Tools

Anyone familiar with this situation on these units, so I can get mine
working again, please?

In the interim I bought their PCM-M1 in mint condition to get going
again, but it'd be really nice to get my old tried & true TCD-D8
running again as it's in pristine condition other than this failure to
load.

Thanks