You need the memory cap for classic Ballys.

https://www.greatplainselectronics.com/products.asp?cat=165

Here's how to install

http://www.pinwiki.com/wiki/index.php?title=Bally/Stern#Memory_Capacitor

Cr2032 and holder also works well. Change the 270 ohm resistor to a diode with band pointing up. If you go to a non rechargeable battery.

They sell 2032 batteries at any store that has a decent battery selection.

The holders can be found from part suppliers and ebay.

I have used hundreds of the cheap Chinese ones.

Ebay item 321044251460

You can get CR2032s at Walmart, Target, most pharmacies, etc. That's what I used on your BK CPU awhile back.

3 AAs in an external pack would work fine, too, if you've got one of those handy.

If you decide to go with the cap, it was the first one in the link above, the top, little one that looks like a coin cell.

A CR2032 is also a great option. Make sure you put the blocking diode in as mentioned above, if you opt for the CR2032. A CR2032 will hold memory settings for 5 to 10 years, and the game does not need to be periodically powered on.

If you opt for the memory cap, you will not put in a blocking diode, as the memory cap gets charged from the board, just like the original rechargeable battery did. The memory caps hold settings for several months. The game needs to be powered on for about 30 minutes every few months to keep the cap charged.