Electronic – Understanding specs of PNP transistor for replacement

Could be leaky, could be pickup of AC hum from the mains or RF from AM radio etc. as the transistor will act as a detector.

To tell the difference, put a 1uF ceramic (low leakage) capacitor from base to emitter and see if the issue disappears.

In any case, you can reduce the sensitivity simply by connecting a relatively high-value resistor from emitter to base. Something in the range 10K-100K will likely work for you.

Edit: One thing that can cause a great deal of leakage is the use of acid-core (plumber's) solder. If this is accidentally used, clean the board with warm soap and water, preferably followed by isopropanol alcohol. I have seen mains circuits burn up as a result of this. As an aside, the solder used in plumbing was often 50:50 PbSn, so it would not solder as nicely (and will require much higher temperatures) compared to good electronic grade 63:37 solder.

The PNP/NPN combo allows simple drive circuit once you consider the logic states that your transistors need to be in and their base voltages. Prototypical textbook example:

This circuit makes it impossible to give it the wrong input[s] and short the transistors by turning on both transistors on the same side of the H-bridge simultaneously. And this is done simply by wiring the bases of the PNP and NPN transistor on the same side together (well through base resistors) to the same logic input signal. So it's a very simple circuit.

But this circuit only allows locked-antiphase PWM drive. You cannot do sign-magnitude drive with it; for large motors you usually want the latter. But also you want low losses in the transistors then, so BJTs are usually out of the picture in favor of MOSFETs anyway.

Commercial BJT-based intgrated drivers, including all time classics like L293 or L298 do use two NPNs as the final stage of [each of] their half-bridge[s]:

To be more precise however, the bottom pair is a Darlington, and the top one is a "complementary Darlington" aka Sziklai pair. I couldn't find the rest of the L293 circuitry (even at the rough equivalent that datasheets usually have), but it's clear from this bit that the top half is turned on a low logic signal, so you can potentially just wire together the two control signals there as well.