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Make no mistake, I am not wanting to solder on tunnel diodes unless I absolutely must.
Thanks for the insights!
On Sat, Feb 27, 2021 at 11:37 AM, Ed Breya wrote:
Sean, I don't know about the other diodes discussed earlier, but I have seen
those tiny glob ones with the dot (in the picture) before, in Tek and other
gear. They could be just a different version (stripline) package for regular
TDs, or they could be "back-diodes," which are a close relative. As I recall,
back-diodes are even more heavily doped than regular TDs, so they are already
in forward bias at zero volts, and the interesting part of their behavior
exists around the zero-crossing, and includes that small region that seems
kind of like a crappy TD. Your mention of the weird curve reminded me. So,
don't assume they're bad TDs just yet, until you study it some more. Somewhere
in the circuit description or schematics it may become more clear. I don't
recall what the usual TD part designations are, but somewhere in tekwiki,
there's a nice listing and summary of many TD numbers from the old days. Look
for that - it may explain what TD253 is supposed to be. Also, you can look up
back-diodes, and see if the description matches the curve you saw.
You are right that this unit is a prototype. If the same phase development
stages were used back then, as in more modern times, then "B11" should be very
close to what went into production. The poor solder joints you mentioned,
around those tiny diodes, may result from a couple of considerations. First,
those things are very delicate, so anyone soldering them in or out would know
they have to work fast - very fast - to get that iron tip out of there ASAP,
even if the joint looks crappy, as long as it worked. Second, being a
prototype, these diodes may have been swapped out or had type changes many
times during development, and the same soldering issues apply. This could also
mean that those diodes are not necessarily the same part number that was
ultimately used in production. But, you'd think they would be close at least.
Good luck in figuring it out.