[sdiy] (OT) Doepfer MIDI-CV oddity

[Ingo Debus]

Am 19.01.2011 um 11:45 schrieb Colin f:

>
> 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.

That's funny. In the lower trace, the first pulse comes through just  
fine although it's short. The later pulses look like there's a much  
larger time constant, as if the pullup resistor is too high. Anyone  
have an explanation for this?

Which pullup resistor value did you use?

The ringing at high level at the end (lower trace) and the undershoot  
on both traces at the beginning look funny too. Did you use a 10:1  
probe? Where did you connect the ground of the probe?

Ingo