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