What would be the likely culprit for a relatively slow (about 1/2 second), smooth horizontal strobing of brightness on all displays? Not bad, just noticeable if you stare at them. I replaced both big electrolytics in the cabinet and the two axial caps on the power board and did the ground wire mods, all about 15-20 years ago but don't really remember when I first noticed this. No other issues. Hoping it is just one of the caps I replaced now getting old or maybe some other cap I need to change. Again not a big deal, just noticeable and for all I know a little bit of this might be considered 'normal' and I'm nitpicking. Thanks!

Thanks, I'll pop one in and see if that does the trick, I'll order order a power supply cap kit as I don't have any of those values.

Aw heck I ain't bought nuthin' new for this game in 30 years besides rubber rings and balls so I ordered a brand new power supply board for 60 clams. Will report back when I get it in there.

I did all the external wire additions to the correct spots on the boards way back when but I don't remember if I did any other mods to the board traces themselves, maybe a few. But haven't had any typical ground-related glitches like things failing or coils being locked on since I did it. That original power board is pretty beat from all the hot parts on it anyway so the game deserves a new 'heart' as it were. Will see how it goes. Ordered five 1uf 100v electrolytics to replace the little ones on each display board too just in case since they handle the 60v.. From my other hobby of fixing transistor radios I don't trust those little buggers when they get old.

No all of mine are very bright, they just have the slow strobing in the brightness like an A/C ripple is leaking in there. Similar to what you might see if you watched a video of pinball displays or a TV screen.

No change with the new $60 p.s. board including attaching my previous 'ground mod' wires to its ground lug, but hey at least I don't have to worry about the old board anymore. Doesn't seem to be coming from the light chaser board, the sound board or the aux. power board, I tried disconnecting them and no change to the displays. Still waiting for the little electrolytics on the display boards. There is one original electrolytic cap on the CPU board so I'll replace that one. If those caps have no effect I'll just live with it.
Ordered both big caps for the cabinet too, they could be 25+ years old, I just don't remember when I changed them. I'm not even 100% sure I replaced the biggest one but it is a blue Sprague and not orange so I was assuming I must have put it in.

Replaced the 10,000uf (w/ a 12,000uf), 4800uf and 100uf electrolytics on the cabinet mech board and the one small cap on the mpu board, no change. I'm putting off replacing the small one on each display board for now because even when I unplug all six displays in the game but one, the one left still strobes. But again it's is almost imperceptible unless you are specifically looking for it. Maybe they all do it a tiny bit to one degree or another or it's just that the displays are 40 years old although still seem plenty bright overall. I might convert it to the newer LEDs at some point in the future.

I put in a whole new power supply board since they're only $60, the original was looking a bit toasty from the hot resistors and power transistor. No change. All the other boards are original but I have no other issues. I measure solid ground continuity on all the display's ground pins.

I added add'l ground wires to all the boards, they are definitely grounded. Plus it can't be the fault of the sound/speech, sound p.s. or the chaser board because with those totally disconnected it still does it. Cleaned the male pins on the mpu that go to the displays, no help. I'll change the cap on each display board anyway since I bought 'em already but I can't see it doing anything. I've run out of caps to change so I'll do some more checking on connectors.