From Old Coot's advice to look for a low-resistance path to ground I spent some hours checking continuities and retesting several components. Although everything seemed OK, I did a temporary replacement of three components (below) which would give a low-resistance path if they failed. Lo and behold!...the detector plate now gets 27V!!!
Now I have a decently-functioning radio with a moderate hum which the hum adjuster reduces to an acceptable minimum. I'll reconnect the original components one at a time to find the culprit(s), reassemble, and then, if I don't screw up, return it to the owner.
Heartfelt thanks to all my fans out there.