Quoted from woody24:
I have to replace one or two TIP122s. I know TIP102s are suggested as a replacement, but after stopping by Radio Shack (yes, there's still one open by me), they only had TIP120s. I picked up 2 just in case. I also have TIP102s on the way from Marcos. But if TIP120s can work as a replacement, I can have my machine up and running tonight. Or I'll have to wait a week.
I saw the only difference is voltage. And the 120's are lower rated. Is it going to be borderline, and I shouldn't mess with it? Or will I be fine, because voltage shouldn't be anywhere near that?
As you already knew and then had repeated back to you a few times, the TIP122s *should* be replaced with TIP122s. But it is still a legitimate question - if you were in a pinch, like the morning the machine was to be used in a tournament, you might still want to do it (at least temporarily). To know if it would be ok to substitute or not we'd need to know what the actual supply voltage is in the circuit and what the load is (probably a solenoid). I don't know what the solenoid supply voltage is inside a Data East.
Also, I'll bet you a day's wages that a TIP122 and a TIP120 from the same manufacturer with a shared datasheet (like http://www.adafruit.com/datasheets/TIP120.pdf) are the exact same silicon (identical parts). It's cheaper to mass produce 3 parts with the exact same flow than to have 3 different designs. So typically you'd design the transistor to yield >99% at 120V, then just stamp a different part number on them at the end depending on how your orders broke down.
The TIP122 will be tested at 100V (actually higher, maybe 110V to have some margin for a 100V guarantee). The TIP120 *might* be tested at a lower voltage (say 70V), but if they've done their job right all the transistors will have >100V breakdowns so you'll save money just having one test program with one set of limits. Anything breaking down below 100V would indicate there is something seriously outside expected performance so you wouldn't ship it even if max VCE was ok at 80V.
So odds are a TIP120 would perform exactly the same as a TIP122 because they would be identical silicon, with the only difference being the marking on the part. I wouldn't lose any sweat over the substitution. But all things being equal I'd use the TIP122 to go from 99.9% certain it's ok to to 100% and to not confuse whoever worked on the machine after I did.
Qualifications: I'm an EE with 25 years in the semiconductor industry.