Hello all. Here is some info on Hohner portable reed organs, usually called an Organa or Organetta:

The slider is a volume adjustment of sorts. Really, it adjusts the fan speed, causing more or less air to pass over the reeds. This appears to be more effective on some models than others.

These are a "unique item" of sorts, but there is no demand for them. Because the electric fan is so noisy, recording or amplifying these things is a nightmare (not to mention the tiny, sticky keys), making them pretty much useless to serious musicians. I am a serious musician and a collector. I love my Organa, but I only paid $30 for it, and probably wouldn't pay much more.

I have not tried to tune mine, as it is still in tune for the most part. I imagine the process would be very similar to tuning a melodica (gently filing bits off the reeds), a process for which the internet has lots of advice. Beware! Trying to tune instruments for which there are no replacement parts or technicians is risky business. If you break a reed trying to file it, then the whole thing is pretty much sunk. Better to have a slightly out of tune key than a dead key.

These are very reliable instruments, just not very playable or useful. I play mine a few times a week, but just for fun. I use the digital stuff and a Hammond tonewheel organ for serious keyboard sounds.