Does anyone know what purpose of the 220ohm resistors on the switch matrix strobes in Gottlieb System 3's are serving? These resistors are found on the diode board under the playfield.

Looking at manuals for the very early System 3 games, I don't these resistors listed on the switch matrix chart or the component listing for the diode board. This would be "Lights, Camera, Action!" and "Silver Slugger". A game or two later, and these 220ohm resistors added on the strobe lines start showing up. At first I thought possibly something to do with the smart switches or optical switches later games used, but I don't think that's it either. Trying to figure it out from the schematic.. but with Gottlieb System 3's using 20v for the lamp/switch circuitry and using comparators and some other fun stuff, it's making it a bit difficult to get at the purpose of these 220ohm resistors.

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These aren't pull-ups though. The 220ohm resistors are in-line with the strobe signal data as if they're limiting current.. or part of "switch matrix input protection" mentioned in the manual.. or somehow involved with the LM339 comparators.. or possibly the smart switches. I just haven't figured it out yet =) Gottlieb did some weird things with the switch matrix on System 3's.. like having it share strobes with the lamp strobe signals and the voltage on those strobes is 20v.

I saw these mentioned in Clay's System 3 guide.. as burning up from time to time.. and also an old RGP post where someone had to replace one because it went open or burnt up. But no information on what purpose they serve. I get if they're limiting current going through a switch.. but there are 4.7k resistors on the return lines prior to the LM339's and like I said, likely some voltage divider stuff happening because I *think* the LM339's are using VRef=1.6v. So the 20v is getting knocked down somewhere.

Maybe they're there so if a technician is poking around on the strobe connector they can't just create a dead short on the 12P06 mosfet and blow it up.

Was hoping someone else out there knew definitively what the purpose was. Or that there was some Gottlieb System 3 architecture document out there that documented it.

The 220ohm resistors (R1-R4) are on the left-side of the switch matrix chart below (from Cue Ball Wizard).

switchmatrix.jpg

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Kits, upgrades and test equipment for pinball machines

Bump for the weekend crowd

One more bump just in case someone knows something