Yes Teach a man to fish... Often the problem has greater scope. Recently I when through a similar situation.. problem was the psrson could solder and somewhat read a schematic.. Terms like breakdown voltage, Ic, Beta and Ft were completely lost as he had no context other than what building a lot of kits taught him. So sometimes the scope is not only teaching fishing, but what a fish is and why one wants to catch one. I was trying to be light but maybe a ;-) may have helped. There are many substitutes possible, likely dozens or more. I picked one that is generally easy to find in many places. The other case was without context I ahd to pick a robust solution rather than one that oen that might work. An example is for simple logic a 2n3904 or the plastic pn2222 should do fine but if it drives a relay or lamp the added current capability if the 2n2222 is a better choice or maybe 2n2219. There were some applications that DEC used transistors that wanted faster transistors, core drivers and head drivers being those that easily come to mind.. Others it was sufficient to insure it was SI or Ge and the proper PNP or NPN. If the design had any vestage of ECL or other non-saturating logic then a lot of parameters become important. Then it is more difficult. SMPSUs are an animal and yes the wrong part there can cause pain or at best end up in a troubleshooting endless loop. Some old logic from the days before silicon devices can be fussy as they use MADT and other generational germainium devices and yes those do not substitute well without a carefull examination as to how they are applied. The gates and flipflops from the TX2 are a good example of early fast logic using germainium transistors. For context fast in early transistor computer designs is anything over a few megahertz (like greater than 1mhz!). Allison