Multiplexing problem with Arduino, MAX7219 e BJT
Brhans is right you can't put NPN transistors there you either need a pullup resistor or PNP there but i fail to see the point since inside max u already have a NPN transistor driving that line. I think you will have better luck if you explain what you are trying to accomplish, im guessing some weird dimming or something similar witch wont work anyway. Have in mind that 7219 uses multiplexing as a means to drive more than 8 leds.