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I'm new to this site. Have a Bally E series slot that the battery on the MPU leaked and damaged one of the IC's on the board. Does anyone know What the purpose of the battery is? Is it just for logging usage? Also what is the purpose of the IC labeled Q30? it is some sort of timer. I replaced the socket and the IC but not the battery and the machine appears to be working fine. 
Thanks in advance

Good job!!!!! No need for the battery on an E-2000! And a other good job on the IC repair, most would not have even attempted it!  :3- :3- :3-

You may have fat fingered the IC, should be U-30 not Q30

U-30 is a counter circuit that is used in the 'Watchdog' circuit. Yes it is a timer and its purpose is to take corrective action should things go wrong.
The part number is MC14020. The components that make up the watchdog circuit are U-30, U32, SW-8 of the dip-switch and U-21 (pins 1 & 16)
U-21 is a transistor array

I thought the battery for the RAM was for keeping track of payout activity. Sort of, if you have a machine that has a 90% payout, it'd store in the register, how much has been paid out vs. number of coins in. So, to maintain that 90%, you'd need to keep the memory alive. Otherwise, it'd reset every time you shut the machine off. Resetting the register would prevent the payouts from being consistent with the percentage.

Or, I could be entirely wrong.

Inquiring minds want to know.

The battery holds bookkeeping data in memory, odds are based on the chip being used. For these particular machines the last 2 digits of the game eprom tell you what the pay out percentage would be. As an example the chip 186583 the payout would be 83%.

Casinos would need to know how many coins were last played if a customer were to hit the attendant button because of a tilt and they might also want to know how many times a particular set of tests were run. This info is stored in the ram chips.

I really don't think having a batter is necessary unless you happen to be running a casino where bookkeeping would be needed.

That makes sense. No audits when the battery is out of the machine. Home use, you really don't need the audits anyway.

I read somewhere that the payback is calculated based on a 7 year average. Does this apply to the E2000 series machines? If so, it would seem that the history data stored would be needed though. Very unclear to me about those percentages and how the PROMs are programmed. And, I'd love to find someone who actually programmed the CPU's for Bally back in the day.

Typically the odds are based off of a random number generator. Each reel is assigned a number the moment you pull the handle and then the reels will stop at the assigned number.

Years ago (1975) I had an Imsai computer and a friend that worked for Signetics. He had given me a copy of a 2650 disassembler that would run on the 8080. I wish I could find one today that would run under Windows.

Having that would make me drag my ancient MS DOS computer out of hibernation and fire it up. Although, what would really be useful is a copy of the original cal docs with footnotes and equations...