Quoted from fuko:

I have odd behavior with my Defender, and am wondering if others are experiencing the same. If I lock a ball in one of the 3 slots at the lock trough and then drain the current ball in play, when it goes to kick a new ball into the shooter lane it ALSO drops the locked ball into play. Almost like I get a free multiball... except that I shouldn't be rewarded for draining a ball. It will also drop the locked ball when feeding the shooter lane on starting a new game (locked balls are not cleared between games for some reason). These are the only times I've seen locked balls get dropped at the wrong time. Starting a true multiball works fine and all 3 balls get dumped as expected.
The lock lane switches appear to be working fine, because the green inserts properly show the state of locked balls in the trough (flashing for empty, solid for locked... and when the locked ball drops the insert starts flashing again indicating the slot is open)
I read elsewhere that this game has more solenoids than drivers, so a relay is used to switch between A and B coils. Well, the coil to feed the shooter lane is shared with the coil that kicks out from the lock trough. I located the relay and cleaned the contacts, tested the coil, etc, and I think it's fine. MPU and power boards are modern day repros so I don't suspect them. Not sure what to check next.
Anyone else ever seen this behavior?

Quoted from snyper2099:

It's a switch issue. The game is losing track of the balls.
Go into switch test.
First, confirm the ball in play/credit display shows NOTHING and that NO SWITCHES ARE CLOSED. If any switch numbers are shown (closed), you will need to find them and open/gap them.
Now the game is ready for you to test the switches.
Remove the apron and the plastic covering the ball lock.
Move balls trough those areas, testing all 6 switches. As you go, also check for balls hanging up. Also test the outhole switch and the shooter lane switch. What did you find?

Quoted from Defender:

Nope, not a switch issue. It is the A/B solenoid relay (fired as solenoid 11) that resides on the underside of the playfield in the back right corner.