[Heath] IT-28 Cap Checker Problem

[digital-conjurers at roadrunner.com]

Had this thing for decades, never a problem.

Now the bridge section seems inoperable (can't get a shadow/dip on the eye 
tube on any type of cap), but the leakage test section still works fine.

The schematic is deceptively simple, but before I go mucking around in it, 
does anyone have one of these things, and has had the same problem/issue, 
and what was the result?

Barring this, anyone have any idea where in the circuit to start looking?

Thanks,

-Lin/KJ6EF