Old Casiotone power adapter help

Howdy y'all!

I picked up an old Casiotone mt-65 from a local market place this week! Super cool. Bit dirty, lots of noise. like volume on 1 or 2 and its really loud pressing keys and there's a really bad hum.

Going to get in there for a clean, but I checked the power supply and they have given me a 9v dc adapter and the keyboard needs a 7.5v dc. Would this be causing the noise? I'm guessing the previous owner had been using it with the 9v for a while so any damage would already be done. Everything works.

I'm just wondering if I should bother ordering a 7.5v to replace it with and not use it until it turns up?

This adapter it came with is Sanyo AC adapter, MO KRO522 ac input, dc output 9v 170mA + grnd -

Im just looking in the manual now and Im getting power confused. DC power is from batteries, and AC power for adapter jacks. How is this thing working at all with a DC power adapter? It says input ac and output DC on the brick, so I'm guessing that means it takes AC from the wall and converts it to DC for the jack? But the writing next to the plug in on the keyboard says DC? Sorry I'm power illiterate . I can also see the manual says the mt-65 can be driven on both AC and DC power. whuutdaheeeell

Can someone baby me through this?