On Thu, Jan 9, 2025 at 09:12 AM, jason neumann-grable wrote:
I have a Tek 321A scope that I am very anxious to get working. However, it
has the "legendary" 2N2207 transistors (lots of 'em) that grow crystals
inside and short themselves out.
I ran into this same problem with several Tek 3S76 plugins. I had the same experience that you had when buying replacements; it's clear that the zinc whiskers grow just as well in storage as in use. I tried the mechanical shock method with little success, but blowing the shorts away electrically worked. Eventually I worked out a method that almost always worked, but the whiskers sometimes returned. The new shorts could be blown away again and the MTBF (Mean Time Between Failure) always seemed to get longer.
I wrote up my experience and method here:
In the 3S76, I found that Tek used Si NPN/Ge PNPs in pairs to make amplifiers that could be analyzed just like an Op-Amp: the gain was solely dependent on the ratio of an input and feedback resistor, plus the biasing of the Ge PNP was "long tailed" so the circuit readily accepted a Si PNP as a replacement with no other component changes. In the case of the 3S76, I found that 2N3906s worked perfectly.
While I started out hoping to maintain the originality of my 3S76s, I slowly realized that any transistor in a whisker-prone TO-7 case was too unreliable in the long term.
--
Jim Adney
Madison, WI USA
