[sdiy] (OT) Doepfer MIDI-CV oddity

[Colin f]

> I did quite a bit of research looking for MIDI input circuits 
> based on the 6N138/9 and found a large range of resistor 
> values used. 

I've been there too...
Take a look at this image:
http://www.colinfraser.com/images/6n139-fairchild-vs-vishay.jpg

It's a scope trace of the output from a 6N139 in a MIDI input stage.
Top trace is a Fairchild part, bottom is a Vishay.
These two are just swapped over in the same circuit.
All other values are the same.
So clearly, all optos are not.
Fairchild worked perfectly, Vishay was hopelessly unreliable, missing more
bytes than it managed to receive intact.
I've always stuck to Fairchild since.

Cheers,
Colin f