I recently stumbled upon this well-used Zeiss Ikon Nettar, which I could purchase for less than the price of a roll of film because it didn't work properly. That would either make it a very affordable way of trying medium format, or a nice piece of decoration in case I couldn't fix it...
It's a 518/16, the 6x6 version with the 75mm f/4.5 Novar lens and Velio shutter.
The problem was quite simple, the double exposure prevention system kept jamming the shutter release.
A quick clean & lubrication of the mechanism got it back in good working order. Bellows are light-tight and all shutter speeds seem to be close enough.
I've put two rolls through it so far and it produces some nice images!
However, the first roll didn't get wound tightly around the take-up spool during film advance in the camera, resulting in light leaks all over the second half of the roll when I took the roll out of the camera.
I've used Fomapan 400 and the empty spool was a Kodak "Spool made in England" I found inside the camera.
Inspection showed that the metal springs of the camera still gave enough tension to the supply spool, so I wound the backing paper back onto the Kodak spool and ran the film through the camera again onto the Foma spool. No problem, tight as it should be.
Then I ran the film through the camera again, from the Foma spool to the Kodak spool. Problems were back.
The second roll I shot was also Foma, and I used my empty Foma spool with it. No problems whatsoever!
Long story short, as I've never shot medium format before in my life, is this a known issue when using 120 spools from different manufacturers, or was it just bad luck with this one (possibly very old) Kodak spool?