Williams Decoder Rom
by Bob Roberts
Williams used the Harris 7641 proms for decoder chips on the main board for their games such as Defender & many others of that era. Right from the get go the blanks were hard to find & very expensive. I still have a few of the leftover blanks that I bought in at around the $18 apiece range that were really just too expensive to even program according to ops who wanted to keep all their quarters. In looking for a way around these proms, I believe it was Joe Guidry, a great local tech, who suggested dumping the files into 2716 eproms. After a few play sessions this was accomplished & used routinely on route machines... a much cheaper method with readily available eproms.
Skipping ahead to around 9 years ago... long after these games were on the street... hobbyists were asking for WMs decoders & since I had blank 7641s to get rid of before my retirement in 2000 I began programming them once again. They went so quickly that I held out a few in case an occasion arose where I might need them. Once I stopped selling them many of the guys were asking about alternate chips for the decoders, so I went back & found my files on an old 5.25" floppy disk... which I can now say is not a very good way to store them, but it was all we had available at the time, without killing a hard drive... and I tried programming them as I had many years before, but I gave up after several attempts without getting them to work. Old age is a terrible thing.... I had forgotten how to make them work, right along with thousands of other things that I like to think I had to forget in order to make room for all the fast paced new technology in games, monitors, power supplies, gaming machines & even vending machines. Just when you learn enough to get repairs done in a profitable amount of time & think you know everything there is to know about a game, something new & very different has come along to take it's place & you have to clear your mind & start all over again :-( What! It's not hard wired! It has solid state devices... huh... what's that?
Well... a few of y'all kept after me to get the eproms working & most persistent was Mike Ranger & a friend of his. Finally, he said... Bob, just tell me that it can be done for sure & we'll figure out how to do it! I believe they did just that, too, although I don't know for sure. In any event, sometime in 2000 I had a few minutes to call my own & I was looking for some things for another Help Page project when I stumbled onto a box of zero ohm resistors & before I could even wonder about them, it just popped into my mind where I had used them & what for. Yes... with the decoders & later on pinball machines & poker machines. Since I hate hacked bds I always did my best to use wire wizardry for rerouting & the zero ohm resistors helped to do that with eproms. In the case of the decoders it was just a matter of grounding the chip select line which was held high on the WMs bd, so lifting pin 18 from the socket, & running a zero ohm resistor to pin 12 grounded the select line without any bd hack. This is how we had done it on the streets. F1 or faster eproms work best, but eproms are less forgiving, so everything else on the bd must be up to par including the supply voltage or they may not work where a 7641 would.
Later on I found that since pin 19 of the eprom is a "don't care" pin it can be lifted as well & provides a closer ground via pin 20 eliminating the need for the zero ohm resistor. The chips can be verified by a programmer in their Frankenstein state, as well, making it easier to t-shoot down the road.
I removed the decoder sets from the Parts Page last August. I had one complaint of the sets not working in 2001 & 2 complaints in 2003... I now know one of these to be an installation error (4 & 6 swapped as erroneously pic'd in the Joust manual) & one with a bad 74xx... and in 2004 I got a complaint from a tech experienced with WMs bds saying that they did not work in all bds as they should, so I pulled them until I could look into it further. The complaintant added... "I had a problem with another pair in that the pair worked on a Joust CPU board but would not work 100% on a Robotron CPU board."
In the interim I have been selling sets to regulars with the understanding that they may not work in all cases & I have been pulling old bd sets & jigging them up with chip sets burned into various eprom brands for trials. I've not had anyone say that their set did not function for them & I have tried the chips in Stargates, Sinistars, Robotrons, Jousts and a half dozen Defender bds without incident, so I will be adding them back to the Parts Page, but with this disclaimer that they may not work for you. I've got so many already made up from the experimenting that I'll lower the price on them, as well.
I feel that these are necessary to keep around for a couple reasons. The proms are sometimes pretty cooked & even though they may be working in general, there are 2 problems that they can be the source of. The first is erroneous ram error reports... I hear this many times... I've changed over to a switcher & replaced all the rams & still get a ram error (131 most common one) and it almost always is a decoder fault at this stage. The second one is HS save failing intermittently even after changing over to a lithium battery & changing the CMOS ram & at this point it is often a decoder fault. Lastly, it's the least expensive way to keep the CPUs running.
The 100ns & 150ns eproms are getting nearly as expensive as the proms and in my searching for the eproms I came across a cache of the proms at a reasonable price, so I am going to go back to using them for as long as the supply last.