I FINALLY got around to replacing the battery in my very old Bally Video TwentyOne. Thanks to member "amechanic," I got the proper battery. For about two years, I've been getting "low battery" or "bad battery" messages when I turned it on. Repeated resets would clear the error as the battery charged slightly. But it would no longer hold a charge.
I'm always leery about removing an MPU out of an old machine that's essentially working. BUT eventually the battery WILL LEAK so it's best to do this soon as you get the message. You definitely do NOT want battery acid to ruin an MPU board that's probably impossible to replace.
It's a three piece board and was easy to pull out. There's two plugs on the right side to remove and the thing comes out.
I was able to de-solder the old one out and solder the new one in without unscrewing the "sandwich."
Plugged it all back in, said a prayer to the Bally Gods and got a working slot again... with no error!
Pictures show the entire board "sandwich" and the old (white) battery.