Determining correct adaptor polarity

I just bought a Sony TCM-121 cassette recorder and am wonder what kind of adaptor would be compatible with this unit. It takes a 6v adaptor, but there is no indication as to the polarity of the adaptor (the manual did not come with it -- I bought it second-hand). Would you have any tips on how to figure out the correct polarity? I'd like to purchase an adaptor but don't want to damage anything by using the wrong kind. Many thanks.

Reply