How (or can) I change the special on a Duotron? I'm assuming it's a jones plug somewhere but a lot of the labels were blown off during cleaning with compressed air, and I can't find a scan of the manual anywhere online. It's currently on extra credit, but I have it set to work on free play and the extra credit it adds messes it up. I'd like it to be extra ball. TIA!

By extra credit, I mean that when special is lit and I hit the appropriate target, the game awards me another credit (as if I put in another coin), and it advances the credit reel. I should have said free credit. It being on free play isn't the problem by itself. It becomes a problem when the game gives me a free credit and increments the credit reel. The way I've set up free play, I have 1 switch permanently closed when the credit reel is at 0 credits. The free credit(s) I get from the special messes this up.

The only radio plug/jones plug I see in the back is the points scoring one (approx. 10 rows and 2 columns), which doesn't have anything to do with special, if I'm not mistaken.