4N35 opto-isolator for MIDI input does not work?
The 4N35 is too slow for MIDI.

With a current transfer ratio of 100 % (which is specified for 10 mA, so you get even less for MIDI's 5 mA), you cannot rely on getting more than 5 mA through the output. This means that to be able to drop the full 5 V, you need a pull-up resistor of at least 1 kΩ. And this means that you will not be able to switch at the MIDI baud rate of 31250 Hz (and a digital UART signal requires much more bandwidth than a sine wave):



And with a resistor at pin 6, the CTR would become even worse:



If you really want to use the 4N35, then it is possible to speed it up by adding a transistor as an amplifer with a lower input impedance:



But the simplest method to get the MIDI input to work would be to use a high-speed optocoupler like the H11L1 or 6N137.