Re: MIDI keyboard fault
Yes, if you have a MIDI cable plugged into the keyboard's MIDI OUT and into the MIDI IN of something else then the other device's input optocoupler and its associated series resistor will provide the load through which the open-collector output on the keyboard will sink current. In that case, you don't need a load resistor. But you should be looking at pin 5 on the keyboard, not pin 4 which you were looking at originally, to see the main data output from the keyboard.
That said, the source path from +5V to pin 4 on the MIDI out on the keyboard will be through a low value resistor like it is on that diagram I attached above, so, provided you have a load, you will also see the data at some level on pin 4. But it won't be as 'big' as it will be on pin 5.
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