Great write up, Kevin! I've been testing some of my extra boards as well, but I don't install them until the tantalums are replaced.Here are my observations and thoughts.... YMMV
I have repaired over 200 of these board sets (Gorf, WoW, Robby, Space Zap, and Extra Bases). The tantalums fail often at this point, but it is almost always the caps on the 12v rail that fail. This happens with both the original PS as well as switchers. On my bench supply the voltage is set precisely at 12.00v, and it still happens.
It's gotten to the point where I will not even test a board set anymore on my bench without first replacing the caps on the 12v rail. It's just not worth the headache for if the rupture of the cap is pointed down towards the board it can actually do some significant damage to the board and/or traces. I'm too old for fireworks on my bench (it reminds me of a "flower" firework).
I've replaced all the tants (16 on Gorf, for example, not including the one on the Amp board), on every set I've done. I replace them with electrolytics and have never had an issue nor complaint. I'm sure we could argue the original intent of why the engineers chose the tants for this application, and many other Bally Midway applications, and there probably was a valid reason, but that would be out of my knowledge area or interest quite frankly. But again, the electrolytics work fine.
As for the customs, I'm not sure I am a big believer of the mildly "over-voltage" theory. I'm sure if the 12v is cranked way up over 12.00v then it can cook them, but it's my experience that switchers do not typically run super high on the 12v as long as the 5v is dialed in. The on-board regulation circuit is very crude, but it usually holds fine. Even when the customs die the voltage is still at the prescribed setting if the 12v input is accurate.
Moreover, the single I/O applications (Space Zap, Extra Bases) run that I/O custom at 10v whereas the dual I/O' applications (Gorf, WoW, Robby) pull that down to 8.5v. As such, I suspect the VGG voltage on these customs have an acceptable range and are a bit more lenient then we give them credit for.
That all said, I am a firm believer in heat killing these customs (and, yes, I do understand that heat can be related to voltage), and especially a killer for the Data custom. It's odd that the earliest production CPU board version ("C") and every later release has the through-holes. Could that have been for a heat sink to be zip-tied to the Data custom? or maybe that was for a daughter board instead of a custom?? but either way Bally did not install a heat sink on the Data custom until the later production run of the later games (Gorf). Either they got lazy or cheap but it eventually caught up to them. Of all the dead Data customs I have seen (probably at least 80-100 of that variety), I think only two had the factory heat sink installed. So, that heat sink really did/does make a difference.
Since there are almost as many failures on the other two chip varieties (Address and I/O), I also have been wondering lately if the fact that AMI was the partner here (and not another vendor) also contributes to the failure rate. This is pure speculation but if I use the high failure rate of AMI's 6821 PIA chips as compared to other vendors, perhaps they just didn't have the best quality in their 40-pin DIP manufacturing process?? Again, pure speculation here but it does make you ponder. Maybe if another partner was chosen by Bally we would have less failures..... maybe....
On the topic of heatsinks, do you suggest all 4 customs should have heatsinks or just the data custom on the CPU board?
Thanks,
Jason

