I was able to swap the power supply with the spare KSU I have in the attic. I noticed that one of the main caps in this replacement power supply was newer looking, so it looks like perhaps there was a field replacement at some point in this unit's past. (Both KSUs came out of a hospital...I could have had three, but they're bulky).

Once I swapped in the new power supply, I was able to successfully test with a clean talk line and successful tone dialing on the intercom. So, we'll call this one fixed! The malfunctioning power supply will go on the bench for new capacitors when I have time.

Thanks again! Looks like I'll be running wire to the garage this weekend...if the upstate NY weather stays fair.