Lee wrote:
How often does a zener have a failure mode where it acts like a good
(not leaky) much lower voltage zener? I usually see leaky or shorted
ones rather than substantial drift...

Fault was 185v line in 'scope low at 170v and slight ripple, failure was
24v zener which was acting like a good 3.3v zener.
'Scope was still working, just had slightly reduced intensity and
thought it better to check voltages before twiddling the sub-intensity
pot

Lee

That is common for a zener.. excessive heat usually does that.

From what I understand the zener is fabbed a little differently so
not to destroy itself when doing the break down. Normal diodes tend
to not like being used as zeners for any length of time. I guess
it could be possible you may had a zener that had some impurities in
the process that could of allowed it to break down like non-zener semi's
or, it just got wacked some how. If this reference is supplying some
out side device as a source of energy or ref on the scope? It's possible
it got whacked.

Also, if only supplies internal references, check to make sure the
connecting circuit is still using it. If not, the zener could be
clamping too much and operating hot..

Normally the evidence is showed with a over heated board that it's
attached to.


Jamie