So you use a version with transistor not a chip. Therefore, decoupling capacitor is no need.
I heard that the transistor version doesn't work well on some MSX1s. Choose a version with chip.
I think I am also coming to that conclusion, thanks gdx
It is possible that the used transistor is not suitable.
I use a JoyMega adapter made with a transistor. It works very well on four MSXs tested, two of which are MSX1. When I made it, I didn't have the indicated transistor so I used another one. I don't remember which one though.
You can try replacing yours. Look for one with fast switching. Also check the resistors at the same time.
