Have a 3-bank board that's begun acting weirdly as follows:

During a game and test, every target drops smoothly and registers properly. The targets reset as they should.

Problem is if the left one drops first, the bank might go a bit crazy registering multiple hits from other targets, usually the right one (as identified in test mode). If the middle or right are dropped first, in any order, this doesn't seem to happen. If that left target is hit last, nothing seems amiss because the bank resets. But if that left one drops first... you might be in for lots of distraction.

Sounds weird but wait, there's more! The left target MIGHT drop "clean" (without triggering its neighbors) for quite a while. But then it starts for no apparent reason. I can wiggle the board and get that behavior to happen, or not. With the left drop down, I can hold the board absolutely still and it might start fluttering phantom switch hits with no apparent movement. When it drops clean, I can bang on the playfield and get phantom hits... or maybe not.

(If I remove the board and manually trigger the left optos, I can't seem to get the multiple / other-target hits to happen. It only seems to occur when the board is on the mech. Which frankly confuses the heck out of me because that shouldn't make a difference... so maybe my manual triggering is not effective, or there is too much ambient light interference when the board is not mounted, or...??? I've parenthesized this part in case it's a red herring distraction).

Troubleshooting: I reflowed solder on the optos and diodes. Cleaned the optos. Everything else looks solid and shiny clean like new. Replaced the rubber grommets to fully dampen vibrations. Moved the board to another drop bank in the game, and the problem followed the board. So the mechs, wiring, MPU, nearby parts, etc are apparently (thankfully) not causing the problem.

So the problem's gotta be on the drop board... but what could be so intermittent about it?

Where did you get the board from? I haven't looked for replacements, but I know various options have been made over the years.

I might try replacing the diodes to see if that fixes it. They're cheap and on-hand. I'm just really perplexed by the behavior and hope it's not the IC.

At any rate thank you for confirming the behavior has been seen before. I hope someone else has seen and has a solution.

Well I might have fixed it... but it wasn't the diodes. I replaced all three and that made no difference, sigh.

So nothing to lose, I took the board out again and retouched the connector pins (which looked really freakin nice even under magnification) and IC mounts (one of which looked slightly less nice). And then it worked in test mode, even with me banging. And then through 4 played games! I've been fooled before but am somehow feeling pretty good. We'll see over the next few days.

UPDATE: Started getting less-disruptive signs of weirdness a couple days ago so I replaced the LM339 (IC). BE VERY CAREFUL with the traces on this drop board: having never done one before WOW are the topside traces and pads (hidden under the IC because of course) offensively thin! Had to do some unplanned repair there. But the game is working with the new chip, I wailed on the pf trying to get it to trigger and nothing happened so was the IC the issue? As I said above, "I've been fooled before" so not gonna hold my breath. But I'll know soon enough.

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