What Use Is An F-call?

What use is an F-call? Recently I received an email from an Amateur who has had their license for over 30 years and been in the electronics field for more than 40. He shared with me an experience that goes to the heart of why it pays to share, even if this particular experience left his mates with sore bellies from laughing so hard. Without going into too much technical background, the devil is in the detail. Our amateur, was working on a project which required a particular part. He was a little short, so after a quick check of the specs, substituted for a part with similar performance. The parts were duly soldered onto the board - bit of a mammoth task, 1.5mm separation, 21 components in a double row next to each other. A little later, more parts were needed. At this time our friend discovered that there was a difference between the two parts, one was an NPN Darlington transistor, the other a PNP Darlington transistor. If you don't know what that means, think of it as two identical devices that work in