If the ball hitting the flipper causes it to re-engage the full-strength winding on the coil you see that sort of behaviour. I assume it's the EOS switch tripping or the hold coil (there is one?) being weak. You see this all time on Gottlieb flippers, but I haven't actually had to fix any Williams games that old.

I had that problem on a Firepower, and it ended up being the coil stop if you can believe that. On firepower they're molded into the damn base plate too so I had to swap the whole assembly. Might try swapping your two coil stops if only 1 side is doing it (yours are removeable!)

Hmm...I'm experiencing this on my Gorgar as well. I was just planning on rebuilding the whole thing and upgrading to the WPC style setup like Vid has in his flipper thread. This might be a lot simpler though...shotgun it or sniper it....tough call. Good lead though

It can be several things mentioned above, bushing as well......it's time to buy a flipper rebuild kit

Instead of starting a new a new thread I'll just revive a 4 year old one! Having this exact same issue with my MET Pro. Just had both Q15 and Q16 transistors replaced by someone that does board repair. My flippers are strong, and appear to work fine but if they are held in the upward position then the ball will bounce back off of them, like they get energized for a split second. It's not just when the ball is rolling fast, it is any speed. Any ideas appreciated. Thanks.

A while back I tried putting silicone rubber on the coil stops of Iron Maiden and got the same behavior - the EOS switches weren't adjusted for the new stroke length, so any pushing back would re-energize the power windings. I'd start looking there.

So the rubber was a test to lengthen the coil stop? I can try that tonight. Thanks.

Make sure the eos switch is opening at full flipper stroke. Also, make sure the eos actuator didn’t snap off the flipper crank. Sterns are prone to that.

Increase the eos gap with flipper held fully up just a bit (power off of course).

It was a failed test in trying to quiet down the flippers. IMDN is a loud machine.

I’ve never seen this on a modern Stern, even though I’ve had plenty of EOS and actuator issues.

Do you have the right length coil sleeves? If the sleeve is too long and it prevents the plunger from going all the way in, this could happen as the plunger linkage is getting choked by the end of the sleeve. At least, this happened to me recently on my Old Chicago and I imagine could happen on a modern Stern as well.

So here is the story. Everything with the flippers is still original. Just got the board back from having Q15 and Q16 transistors replaced on the power board. The issue didn't exist before this so I think it has something to do with the new transistors, I had them replaced with STP40NF10L. Now I'm wondering if they should have been replaced with IRL540N instead? Does any of this make sense to anyone or am I chasing the wrong issue?

The left flipper had quit working completely before, that's why I had the transistors replaced. The right flipper worked fine but I had the transistor replaced anyway so they were both new again and they are very cheap to buy. Now both flippers have this weird bounce back issue. Keep in mind since the transistor replacement the flippers work fine otherwise, they are strong and responsive.

IRL540N rings a bell to me more than that other code. If nothing has changed aside from the transistors, then it does seem pretty likely that’s the issue...

I called Stern today. He didn't really know what the cause could be. He said the one that was put in was one they've used before. I might buy the IRL540N and try it, kind of scratching my head on this, no clue why the transistor would cause this behavior. It's really unplayable because you can't trap a ball, if you try to hold up the flipper the ball just bounces every time.

If anyone near me has a Stern power driver board I could pop into this and just try it out that would be greatly appreciated. That would help me narrow down if the power driver board is causing this or not.

Did you confirm that the eos switch actuators didn’t snap off and that the eos switch opens when the flipper is engaged?

Those pics are with the flipper open all the way. I think everything looks ok.

Yep agreed. I’m officially out of ideas. Haha

Yea, this one is frustrating. If I had another Stern board in another machine that I could swap in then at least I could verify the problem is the power driver board. Unfortunately I only have one machine! Those power driver boards are really expensive on the Marco or Pinball Life site, I hate to have to blindly buy one not knowing if it is the issue.

Here is a link to a short video showing it...

Try adjusting your EOS so that the gap is wider when the flipper is fully extended. Those look like they could be too close...a tiny bump on the flipper might be closing the switch, which causes it to re-engage.

After trying that adjustment...

Another thought...is it possible you have the wrong coil stops? When the flipper is engaged, the plunger should be making contact with the coil stop on the other end. I'm wondering if the coil stop is too short....maybe the plunger can't go all the way, so it's in a state of hovering/suspension that re-engages when slight pressure is put onto the flipper?

What exactly does a coil stop do? You have the plunger that goes into the coil and hits the stop, I thought that it just prevented it from going to far. Does it have another purpose?

That’s the propose. What Ryan is suggesting is that your flipper is being stopped by the flipper linkage, not the stop. Meaning the stop is too short. Meaning the flipper plunger never touches the stop.

.

They seem ok.

Okay, if the plunger is hitting the stop before the linkage hits the coil, then it's probably not the stop.

Did you adjust the EOS switches like I suggested?

The stops look ok to me. The plunger goes into the coil all the way, but I can't tell if it is hitting the stop or not, I think the linkage does touch the coil, but it's going in 100%. How could that cause my issue?

Yes, eos gaps are plenty good. I can see when I test that there is a gap even when it bounces the ball back. I can hit the flipper gently with the ball 2x times with the flipper up, then it goes down completely on it's own. Weird?

Just to clarify. When I hold the flipper up I can take the ball and drop it on the flipper. It bounces off each time and then after the 2nd time the flipper just goes down even though I'm still holding the flipper button.

Put the game into switch test while you’re doing this and see if you can see any odd switch behavior.

Good suggestion. Make sure all the switches register correctly in switch test mode and there are no switches that are unexpectedly triggering other switches. Pound around on the playfield to make sure vibrations aren't triggering anything.

Still, it sounds like it's not the switches. But if you wanted to be 1000% certain, you could just set them so they are completely open or completely closed at all times and see if the problem persists. In either case, you would not see this behavior on a modern Stern. (Open at all times would be better because a closed switch might actually be opening and closing if the contacts are bad).

(But it sounds like this has been tested. You can see the switch is still open, even when the problem is happening. Please be *certain* that this is the case, because what you're experiencing is exactly what could happen if the EOS gap was too small).

If it were me I would just replace the transistors because that was the only thing that was changed. But that sucks if you have to send away the board to get it done.

I found this quote on another thread on a different website, the aussie arcade.

"Just read a deleted thread on an OP havng a problem with a blown mosfet that drives the flipper coil on Sam system Sterns, if this happens on your machine dont replace the mosfet with the factory fitted 22NE10L fit an IRL540 in its place,these mosfets are a bit more robust and in future the fuse will blow instead of the mosfet. Dont be inclined to fit the higher spec 40NE10L used to drive other 50v coils on the machine as the turn on voltage is to high and will interfere with the flipper hold system on Sam Sterns."

Looks like the 40NE10L is the wrong choice for a Stern power driver board. I ordered 4 of the IRL540 and 4 of the BUK9529. Thought I'd get some as backups, they are inexpensive. Not sure if I'll use the IRL540s or the BUK9529. The BUK9529 was what was in there from the factor at Stern. When I called Stern yesterday the guy said they were putting in the BUK9529 from the factory at some point because they thought they might hold up better. If anyone has any thoughts as to which might be better I'd gladly take the input. They should just put a socket on the Q15 and Q16 from the factory so you can just quickly pop a new one in as needed.

Just replaced both flipper transistors with BUK9529 and everything is working perfectly!