I have no experience in trying to solder Aluminum... but the only type of flux I would use on something like this would be rosin, or rosin-core solder.
I would be concerned about acid-flux ( Dutch Boy, Ruby-Fluid, No-Korrode ) eventually eating-away the thin aluminum diaphragm.
I'm glad you have it working... that is a neat machine.
Thanks De Soto Frank. In this case I was soldering the metal stylus bar to the metal rivet and not the aluminum. The solder I used was the type used for electronics, and that is the type of rosin flux I meant. Maybe there is a more preferable solder to use?? The solder that I have probably is rosin core filled. I think that might be sufficient if one would (very carefully) remove the corrosion from the stylus bar and rivet surfaces with a very fine small jewelers file. This was the metal would heat more quickly and take the solder better. A hands free big magnifying glass with lights would help one do this job more effectively.