Sam, your question was:

Q. What does the top of the zener connect to?
A. One end traces to a 25V electrolytic cap--other end traces to a 4
pronged rectangular chip (S1VB ?) and eventually to the 32 pin on a
38870L25 processor.

In-circuit diode test:

1. VOM attached diode leads--ohm meter reading started low and rose to
1.6 meg ohms

2. Leads were reversed and test repeated--initial reading neg number
but rose to 1.38 meg ohms

Diode is made of clear glass, black painted line around the circum at
on end, has markings 180 or 18D on its casing--will have to remove to
get clear image of # markings--

Diodes glass body is cloudy--not shiny like others on the board
Proposed Action: Remove diode and head to electronics supply store