[TinCanAlley]

Originally Posted by vol.2

So the thing about transistors is that they have a lot of characteristics, but which ones make a difference entirely depends on your application.

When a manufacturer is deciding which transistor to use in which spot, they have an entirely different set of considerations than a general repair-person does, or in fact you or I have many years later.

Usually, they are working with a bunch of boxes of parts they buy in bulk, and that is all down to contracts and maybe just what's around at the moment. They try to make it as cheap as possible while still doing the job, but that might actually mean using a way over-spec'd part if that's what they have a whole lot of in the back room right now.

The repair manuals are looking at the whole field of parts available and giving you choices because different repair shops have their own stock and their own prices based on the volume of parts they can afford to buy at the same time.

Sometimes (a lot of times) the exact transistor you chose doesn't make a heck of a lot of difference. As long as it sort of fits the voltage and current requirements as the junctions, it's going to do the job.

Other times, the gain characteristics and/or the transition time of the transistor is going to make a big difference, and you have to pay attention to it in the circuit. In these cases, even a single transistor replacement can require reverse engineering the whole portion of the circuit that it is found in.

If you are seeing a single transistor type shown as a replacement for many different transistors in BOM, then it's likely that is a common replacement part used for a variety of transistors from different manufacturers. And so is more of a general replacement than an exact fit. They are usually pretty much okay, but I have seen situations in which Sam's replacements didn't work in a circuit. Not often, but I've seen it.

Got it. Was just wonder as I needed two 695s and found a seller with a lot of 10 for next to nothing. So I bought them and wondered what I could do with the rest. If the ECG123A is a basic replacement for a number of transistors, and one of those is the 695, then if need be, I can use the 695 as a replacement for those other transistors?