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On Wed, Mar 24, 2021 at 11:01 PM, Jeff Dutky wrote:

I've been trying to find what the selection criteria were for Q1197, which is
Tek part number 151-0209-00, if I am not mistaken. The service manual does say
that it is selected from 2N3442, but when I consult the Common Design Parts
Catalog I do not find 151-0209-00 listed under selected transistors. Further,
in the section that discusses (and tries to dissuade engineers from using)
selected parts it sounds like a selected part should have a part number
starting with 153- rather than 151-. It also sounds like there are a
population of 151-****-89 parts that are still considered "selected" in the
sense that while they have failed the selection criteria, they have been
checked to be free of other faults, and so are considered higher quality
parts.

Could it be that Q1197 is from the latter population of selected parts and
that the -89 suffix was not yet in use when the 454 was designed? Maybe any
old 2N3442 would suffice, so long as it was working?

-- Jeff Dutky

I vaguely seem to recall a discussion from some time ago regarding a similar type of power transistor, in another instrument.. Something about epitaxial (high gain) vs hometaxial (low gain) construction with a lower gain hometaxial construction being the "Desirable" characteristic in the particular application? I just do not remember all the details. Or perhaps I am just wrong?

I see that the TEK Common Parts list calls for an HFE of 20 and the 2N3442 shows an HFE range of 12 to 72. Perhaps this memory above applies and TEK selected those parts with a fairly low HFE?

Just a thought.
--
Michael Lynch
Dardanelle, AR