Re: Looking to ID a diode in a 1S1 sampling plug-in


Tom Lee
 

The 8056 is the most transistor-like of all Nuvistors. If you are not insistent on preserving original factory innards, you can replace that Nuvistor with a JFET. You can even plug it directly into the Nuvistor socket, so no mechanical mods are necessary. For details on that, see the tekwiki page on the 422. The MPF102 suggested for the latter is probably not a bad choice here, either. The gm is a little on the low side, but that shouldn't matter much here at all. You will have to readjust the staircase DC level trimmer, but that's par for the course even when replacing a Nuvistor with another Nuvistor.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/1/2021 13:14, cmjones01 wrote:
I've had a look in my 1S1 and the tunnel diodes seem to be a mix of tiny
beads like the ones in the OP's photo, and the gold "top-hat" style package
I've always associated with Tek tunnel diodes.

One explanation for the trace being stuck off the bottom of the screen is
also clear. The Nuvistor and a nearby transistor are entirely missing, so
there will be no staircase being generated. Typically, the Nuvistor isn't
the same as the ones in any of my other plugins so I can't even borrow one
to try. Time to go shopping for an 8056...

Chris

On Sun, 28 Feb 2021, 02:49 Sean Turner, <sdturne@q.com> wrote:

Ed,

Make no mistake, I am not wanting to solder on tunnel diodes unless I
absolutely must.

Thanks for the insights!

Sean

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.

Ed




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