[sdiy] Zener issues (again)

[David G Dixon]

Hello Friends,
 
You may recall a few years ago that I built an interpolating scanner with a
trapezoid generator for controlling linear VCAs.  The trapezoids were
generated with a kind of folding circuit that uses 5.1 zener diodes to limit
the voltage in a couple of places.  As the incoming CV rises linearly, the
circuit output rises sharply from a little less than 0V to 5V, stays there
briefly, and then falls sharply back to a little less than 0V as the CV
continues to rise.
 
So, I built the original circuit with 1N4733 5.1V 1W zener diodes, and it
worked perfectly.  Just the other day, I built a very similar circuit for
someone, and again used 1N4733s.  However, in this circuit, instead of
getting nice well defined trapezoids that peak and ride at 5V, I'm getting
narrow peaks that don't quite rise to 5V before falling again.
 
I have checked and re-checked (and re-checked, and re-checked) the layout
against the schematic, and it is correct.  I went to my simulation, and
nothing I did changed the general robustness of the circuit.  However, then
I tried replacing the simulated 1N4733 diodes with simulated 1N5338, a 5.1V
5W zener, and -- lo and behold -- the simulation generated wimpy little
narrow peaks up to about 4V instead of trapezoids up to 5V.  So, I can only
conclude that the 1N4733s that I bought (from Small Bear) are actually
1N5338s in disguise.  I haven't yet desoldered the old zeners and tried them
in the new circuit, but I'm going to shortly.  I'd bet that this will fix my
circuit.  (And, before you ask, I decreased the current-limiting resistors
from 4.7k in the original circuit to 3.3k in the new circuit, to no avail --
also, the simulation shows no effect of decreasing these resistors.)
 
Has anyone else experienced similar inconsistent behavior with zener diodes?
How likely is it that these 4733s are actually mislabeled 5W zeners?
 
Cheers,
Dave Dixon
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