Here is a Strike Zone shuffle alley with a display issue. A few digits were locked on and it was displaying some garbage. Before switching to the LED’s the master display was showing nothing. Since that can cause some of the issues I switched it with two other ones we had laying around. Neither of them fixed the issues. The plan was to switch to LED displays so I decided to wait till they came in before pulling to troubleshoot and service the logic board.
After installing the Pin Score displays I found the master display now works but I still have the issue of the thousands digits in players one and four were still locked on. The problem with the other master displays was the glasses were burned out. Unfortunately it’s happening so frequently that it’s becoming necessary to just upgrade to LED displays as part of the overhaul process. I understand the purist mindset but as these glasses are failing and becoming harder to come by there isn’t much alternative. Even NOS glasses either aren’t working off the shelf or failing shortly after installation.
I pulled the logic board to do the usual service and install my battery board. I wanted to diagnose the display issue first so I didn’t waste any time on a board that isn’t fixable. This shouldn’t be the case since the display circuit on the Williams System 9 logic board is pretty simple with the only major components being a 6821 PIA and a 74154 decoder/demultiplexer chip. Basically what the 74154 does is it takes a 4 bit input and provides 16 mutually exclusive outputs as described in the datasheet. If you want to learn more about what this chip does or how its used in greater detail I encourage you to download the datasheet here.
One thing I want to make you aware of is that the 74154 in its 24 pin dip through hole package is being discontinued. The only new ones being produced are surface mount. I was able to find some on Jameco’s website for about $4 each as of this writing. I can always get them off of junk boards but you may want to get one or two to keep on hand just in case. The Jameco part number is 49568 and you can get them here.
A quick and easy way to find a faulty chip is to simply put your meter in diode test and check the chip in question by placing the red lead on a ground point somewhere on the board and check each leg of the chip with the black lead. See below.
Now of course this test isn’t always conclusive but I’ve saved a ton of time by finding bad chips this way. If you look closely at the picture you’ll notice that pin one has a dead short. I got real lucky by finding the problem on the first pin I checked. I should caution you though that sometimes a pin (other than the ground pin) will be tied to ground. On this particular circuit pins 18 and 19 are tied to ground. Refer to the pic of the schematic above and you will see what I mean.
Also there could be something else on that line that is shorted to ground. It could be anywhere. Right next to the 74154 is a resistor pack, since that is easier to remove than the 74154 I pulled that first even though I was certain the ‘154 was bad. Resistors never go short, they always go open. Sometimes those SIP packs have capacitors as well as resistors and capacitors do go short when they fail, so be sure to check them when in doubt.
In the above picture you will see as I move to the next pin I’m getting .601v which is about what all the other pins show except the ground pin which is pin 12 and also pins 18 and 19 which should show a short according to the schematic. Pin 24 is the +5v pin and it reads about .225v or so. If you have access to another board you can always compare readings to see if they are the same if you’re not sure. I do this occasionally when working on an unfamiliar board.
It’s always recommended to use a socket when installing a new chip. I usually don’t, it depends on how cocky I’m feeling about my soldering skills or if its a common chip. In this case the 74154’s are harder to come by so I used a socket.
After I installed a new chip I rechecked it for good measure. Actually this chip isn’t new, its a pull. When buying chips, especially EPROMS, you may notice the word pull in the description, this of course lets you know that it’s a used component.
To further illustrate the different readings between a faulty chip and a good one I connected my meter with the chips out of circuit so you can see the differences in the readings. See below.
After the socket and replacement 74154 are installed I verified the display problems were corrected.
The last step is to finish servicing the board which includes reheating the connector pins, replacing the electrolytic caps and installing a battery board. Now this logic board will provide years of trouble free service to the customer.
Using your meter in diode test to check for shorted chips can save you a lot of time. If you find a pin that’s shorted that shouldn’t be then its most likely the cause of your problem. I use this a lot on the Williams systems 3-7 to find faults in the switch lines, but can be used in most other situations. Remember this test isn’t fool proof, no test is really. I say it all the time a part can test good but fail under a load. You should also be aware that other problems can exist. The bad chip you find could have failed on its own or something could have blown it up. If your replacement fails with the same problem then more troubleshooting is required.
Thank you for checking out my tech notes. I hope you find these articles useful and educational. I’m trying to post a new one each Sunday night. If you have a topic you’d like me to cover feel free to mention it in the comments below.
You can get your battery boards in my store, your purchase helps support Project Pinball, I donate $2 from every battery board sold. Be sure to check them out, they are a great organization that helps sick kids by placing pinball machines in children’s hospitals around the country.