200e

Firstly, most professional synth builders, and I'm sure the Buchla is
no exception, create systems where you can't damage a modular patching
it to itself. The danger lies in sending a potentially dangerous
voltage from the "outside" world to the Buchla.

One can certainly use a voltmeter and examine the outputs of what you
want to connect once you know the units well enough to set up a max
level and compare it to the levels on your Buchla. One can also use a
simple attenuator to scale down a range.

I guess with Buchla the audio paths use lower voltage ranges than it's
own CV range and systems where audio and CV mix.

You may want to connect the grounds to be common. There should be a
banana for that purpose on the Serge and Buchla for example.

Generally it's a no-no to make a patch that sends an output back into
another output. Within a system you should not do any damage but it's
generally something to be avoided. (An output back into an input for
feedback is always cool though). To a beginner, you might think you
won't connect two outputs that easily but it's tempting to take 2
outputs and patch them to a *single* input. That might seem like
nothing wrong, but you have to consider both outs will be sending
signals to each other, not just the input you were intending. That's
one thing proper mixers are for.

Aside from some sort of freak mislabeling or malfunction of another
instrument (like an input of something outputting high voltage spikes)
it will certainly do the Buchla no harm to send it's output to
something else. Now will the Buchla harm the other instrument... well
check the voltage ranges of where it's going to against the Buchla.