Quoted from guitarded:
How do I fix knockdowns? My Right Flipper (Brand New Coil) suffers terribly from knockdowns on incoming/multiple ball strikes.
OK for knockdowns... This is basically the ball traveling at full speed at the flippers and the held up flippers. This can cause a ball to knock back the flipper unexpectedly due to the hold power of the flipper mech. This does not hurt your game, or your flipper mechanism as it is just the flipper falling. What I did on TNA to combat this was made the hold stronger in the software by pulsing the main winding (this is no secret). This is the buzzing sound that you hear once in a while. This is not a one size fits all type thing though as people in different parts of the globe have different incoming voltages to their systems. TNA does not have EOS switches, if it did, I could send a "reflip" signal to the coil like other manufacturers do, but honestly, I am not a fan of this. This would cause the ball to be flipped away upon knockdown. The good news is that you can do a few things that will stop these knockdowns from happening.
1: If you have an early original run TNA, within the first 100 or so, swap out your coil stops with the A-12111 coil stops. It helps with the knockdowns.
https://www.pinballlife.com/williamsbally-flipper-coil-stop-a-12111.html. You could also rebuild your flippers if your game has a ton of plays on it.
2: Replace your flipper return springs with the Williams ones. This helps significantly. Part number 10-364.
3: There is a setting called "Flipper Hold Boost Level" under the "Machine (Standard)" menu. Turn it up 1 notch. Keep in mind, this WILL make your coils run warmer, but will reduce the knockdowns. Just watch your coil heat with this one turned up. IF YOUR GAME IS WORKING FINE DO NOT TOUCH THIS SETTING. The flipper hold boost should be on the lowest number that your game plays well with.
4: If all else fails, slightly lower the left scoop power and flipper coil power for all flippers. I am basically pushing this design to the edge of failure to give it a fun experience with the default coil settings. I honestly may have to reduce this default setting in a future update so people do not have to tweak stuff.
I hope this helps to clarify what the knockdowns are, why they happen, and what can be done to fix it.