Probably one of those small 2uf caps, have you done a cap kit on it? Usually solves those types of problems:

http://bigdaddy-enterprises.com/repairkits/bally_kits.htm#b-snt1

And honestly if one has failed, another will likely fail soon. They age and sometimes just totally drop out, others just gradually get weak. Replacing them all usually does improve the overall sound too. These are 30 years old, most people agree electrolytic cap life is about 20-25 years.

That's just specifying it's for Medusa, meaning the ROMs and maybe a few other things.
-8 Elektra,
-9 Vector
-10 Spectrum
-11 Rapid Fire
-12 Mr & Mrs PacMan

I don't have the full list handy

You got the polarity right on the 4700uf cap, the "+" on and the ridge indicate the positive side. On the tantalums, I'm looking at two identically marked boards and surprised one has a "+" sigh on the right and one does not on ONE of the caps. Regardless, the cap should be with the plus leg on the right, and the cap itself is marked with a little "-" and ""+", but also (too late now) the + side has the longer lead. I stock another cap that has + + marked on one side, but I think in EITHER case, if you look at the cap head on the + is on the right. While it works, you may want to check that it's installed correctly.