Newsletter Signup
Stay informed with the
NEW Casino City Times newsletter!
Recent Articles
Best of John Robison

Gaming Guru

author's picture

Randomness in Electro-Mechanical Slot Machines

10 July 2006

By John Robison

I was thinking about the whole RNG issue the other day and thought of the old slot machines "electro-mechanical" of the 60s and 70s.

A few questions...

*Since the older slots were "electro-mechanical" wouldn't the slots been more predictable in their payouts or was the "randomness" generated?

*If the "mechanical" machines had a "PAR sheet" (or percentage of payout) how did the slot manufacturers "sell" a particular schedule to a casino/gaming board as a "random guarantee" of payout, if it wasn't really random but mechanical?

*When did the change and or the reason(s) why the move from electro to a motherboard with a chip and "randomness" to the possible long term "guarantee of payout?"

*In the movie CASINO where the BIG machine is "hit" 3 times in 15 minutes and the boss is questioning the slot manager if he was in it or not..... How would the bigger "jackpot/top award" of say $7,000 be "programmed" or "set" into the "electro-mechanical" machine other than entering the machine and or with magnets?

*Have you heard of slots out of a box or on opening night hit for the "top award" within 24 hrs?

*Do slot managers track the number "top award" payouts on machines or does it not matter to them as long as the machine is within its "PAR sheet"?

Just curious about the old days and how new slot machines have evolved from the old machines.

Thanks for the insight!


Dear bryoninvegas,

I'll answer your questions in order.

Electro-mechanical machines, as the name suggests, used a mechanism to stop the reels at random. I'm not an engineer, so I can't describe -- heck, I don't understand -- exactly how the mechanism operates. Suffice it to say that however the mechanism transferred the energy from pulling the handle to spinning and stopping reels, the results were random enough that each stop was equally likely to land on the payline and, thus, in the long term the machine's actual payback approached its long-term payback.

Electro-mechanical slots weren't any more predictable than today's slots, but they were less volatile because they didn't have the rare, high-dollar jackpots some of today's machines have.

Electro-mechanical slots had PAR sheets too. The long-term payback was calculated by using the number of times each symbol appeared on each reel. We do the same thing today, only we use the number of times each symbol appears on the virtual reels. Electro-mechanical machines weren't necessarily any less random than today's machines. Manufacturers sold paytables the same way they do today, when results aren't really random but mathematical.

I don't know how one might force an electro-mechanical machine to spin to a jackpot, and most of the slot cheats of that era are now playing that big slot in the sky, so there are few people left to ask. Operators could cheat customers by putting "bugs" between the teeth on a reel. The reel could not stop on the symbol at that stop when there was a bug in place. I suppose the slot manager could have put bugs in the machine to make it more likely to land on the jackpot. Or the player could have figured out some way to manipulate the reels to move them to the jackpot combination.

Jackpots are hit on the first 24 hours of operation every time a new casino opens. In fact, I hit a video poker progressive at Bellagio about 12 hours after it opened -- and I wasn't the first one to have hit it!

I don't think a slot manager cares how many times a machine has paid its top award as long as its actual payback is in the range expected for the amount of play the machine has received. But I admit that I've never asked.

Best of luck in and out of the casinos,

Hello, John,

I have been researching slot machines for approximately 12 years. I have a strong math background and presently teach a variety of courses at several local colleges. I've tried numerous systems (you name it, I've probably tried it). Most gave me big loses, some moderate losses, and a few minimized my losses (these are the ones I usually use).

Being a math person, I realize that the odds are against all players at a casino (with the possible exception of poker) and slots do not compare very well with some other casino games. However, the others (craps, blackjack, and other card games) are very boring for me and to hit big, you have to bet big, which I refuse to do.

Slots, however, give you the chance of a big hit with a very small amount of money (however unlikely)...the thrill of that big jackpot with a few quarters or dollars is quite a high. I play on the premise that anything can happen in the short term, knowing well that in the long run the casino wins.

While in Vegas, I played in several slot tournaments. The machines used for these were the same ones that I would have played if there were no tourney. They were just banked together in a special section. Well, normally, they would behave like regular slots...most give nothing or little...move on searching for the hot one.

But in slot tournaments, they're all hot...jackpots come fast and furious. My question to you John is, if the casino can adjust certain machines to make them looser for the tournaments, they can adjust any machine in the casino.

I know this is illegal...however, how would the control agencies check on this? How do they make these tourney machines loose? A turn of a screw? Replacing the RNG? Or what? Can you give me some insight into this matter when you get a chance? It would be greatly appreciated.

Thanks much,

Dear Sal,

You mentioned an important aspect of the appeal of slot machines. No other casino game gives you the opportunity to win a huge amount of money with a very small bet.

Tournament machines usually run a special tournament program and have special tournament virtual reel layouts that are, as you pointed out, much more generous than the normal virtual reel layouts. The machines are changed from normal game mode to tournament mode either by changing chips in the machine or by using a configuration menu. Another option that will be available in many casinos soon is downloading the tournament game program from the casino's central game server.

Casino control agencies know the game program and virtual reel layout that is supposed to be in each machine. They can remove the chips in the machine and compare them against reference chips to ensure that the data stored in the chips from the machine is identical to that in the reference chips.

Best of luck in and out of the casinos,

Send your slot and video poker questions to John Robison, Slot Expert, at Because of the volume of mail I receive, I regret that I can't send a reply to every question. Also be advised that it may take several months for your question to appear in my column.

John Robison
John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

Books by John Robison:

The Slot Expert's Guide to Playing Slots