>I was repairing a piece of old equipment, and needed a replacement RF
>transistor. Checking at Digi-Key, I found very thin pickings, so i checked
>a few other distributors, and found the same situation. They seemed to have
>a modest choice of SMT transistors, but REALLY thin variety in T0-92 and
>similar plastic packages. Not a good sign, as we have a lot of 30 - 40 year
>old nuclear instrumentation here.
Yup. The last five years or so have been dire times for through-hole
semiconductors. A lot of the popular parts are now gone, past the
end of the "lifetime buy" cycle from their original manufacturers.
I've been trying to stock up my own (hobbyist-level) supplies of
useful TO-92 transistors - a mix of general-purpose jellybeans, RF
amps, JFETs, and low-noise audio parts - whenever I can. I figure
that a bag of 100 of any particular type constitutes a "lifetime buy"
for me.
For parts that have only recently gone obsolete, you might want to
check with Rochester Electronics. They seem to have a pretty good
stock of a lot of parts that Digi-Key and Mouser have dropped.
There's a $100 line-order minimum, but it might be worth selecting a
few useful parts and investing some $$ in a lifetime stock.
>Another issue is the circuit is a differential NPN pair set up as a one-
>shot. So, it was running about 23 mA through one transistor at idle, and
>all of these on-at-idle transistors have failed. (The other transistors in
>the pairs seem fine.) The part actually in the unit is an FMT1190, which
>certainly seems like it should have been able to handle that current long-
>term. After replacing it with the best thing I could find, the transistor
>only has about 2V C-E, so the power dissipation is less than 50 mW,
>shouldn't have burned them out. I'm wondering if somehow the startup
>condition exceeded the base ratings.
Maybe a short inverse spike condition during powerup, which
reverse-biases the base and makes it avalanche?
Or, is there any chance that when the one-shot fires, the transitions
are slow enough that the transistor passes outside of its SOA while
turning on or off?