At first I’d say check diode on shooter lane switch but sounds like you changed all that.

Check for magnetized balls or trough with divots not allowing balls to feed properly

Does it kick the first ball out before the 2nd is put in the launcher or does it stack them?

I have never owned a Dr. Who. I worked on a 1993 Indiana Jones and I eneded up having to pull out the two opto boards in the ball return lane and replace several Of the diodes that both sent and received.

The divisors in the trough are also symptomatic of this.

Only other time I saw something like this was when a game had too many balls in it.

There are no divots or optos on this era of trough so it's not any of that stuff.

It's almost surely the trough switches themselves, or possibly the lock switches in the moving playfield (does the game think you have a lock?).

It's also possible it's a "mechanical" issue, in that maybe there's something in the wood / trough area that's causing the balls to sit funny on the switches. A good way to figure out if it's this kind of problem...does it only happen when two balls are in the trough? Like, you just locked a ball and now two get kicked out? If so it's probably this.

Perhaps take off the apron and carefully watch how it's operating when balls move around in there.

Beyond that I'd carefully check the MPU board for acid damage.

I would double down on these sentiments. My feeling is that it's a switch.

Break the opto beam(VUK) as soon as you launch the ball and see if another ball if released in the outlane. Eliminate any opto issues

are you sure you are seeing the trough switch closing in switch test? Have you buzzed it with a meter?

May have to repin the connector .

Yes, the troughs are mechanical and are very sensitive and if adjustments are made with PF up in the air, sometimes the game will react differently when PF is in lower position.

You indicate it happens after you lock a ball.

So pay VERY close attention to what happens (with apron off) when you do this.

I can almost guarantee that with 2 balls in trough something weird is happening that isn't happening when you have three balls in there. Could be because of the position of the switches or wear in the wood path from the troughs to the shooter lane. I know you replaced switches etc but this is very sensitive stuff if something is just a little off this could happen. Could even have something to do with the positioning of the lever that kicks the ball into shooter lane.

Yeah that sucks but look at it this way - maybe it has something to do with the apron!

You can't discount ANYTHING when it comes to annoying shit like this.

I know you checked it but it sure sounds like a problem with the trough switch for ball 1 or 2.
My theory.. it happens when a ball is locked. 2 balls in trough. 1 kicks out. The machine isn't seeing a ball missing so kicks out another. Perhaps the switch for ball 1 isn't registering if another ball isn't pushing it???

Take a pic of the apron, lets see how the metal rod is mounted, maybe the ball is making contact as it ejects into the outlane.

The only thing that could be interfering with the ball is the metal rod attached to the apron. If it mounted too high/low, it may be impeding the ball. Otherwise, there is nothing on the apron that would cause your issue, that I can think of.

Sounds like a mechanical issue not an electrical issue especially when it doesn’t happen with the apron off

Play for an hour or so with the apron off, if everything works OK then play with apron on. If problem happens again then you know it has something physical to do with the apron and can focus on that.

I think I know what is happening. When you put your apron back on, the ball is hitting the ball guide. See my pic. Bend that ball guide towards the coin door and see if that helps.

outlane (resized).jpg

I would take the apron off and keep messing around with it.

Yep this is what I'm thinking too.

It looks like when you put the apron back on, the small bracket is interfering with that ball guide from my perspective and when u remove the apron, the problem stops.

Take a video: one with apron on and one without

This^^^^^
Or take the apron off and play until it happens again or until you're convinced the apron might have something to do with it.

I know this is an older post although I would like to know if this was resolved as I am seeing the same issue on a Dr Who machine.

Have you checked for broken/loose wire at J206-2 on the MPU?

Bad solder connections on the outhole switch itself or wrong gap.

Perhaps the outhole switch is just failing and is sticking closed for too long and releasing a 2nd ball bcuz it doesn't close fast enough. What have you tried to remedy the situation?

Agree with Eye that it is probably a switch in the ball trough or in the shooter lane. Those wires that stick up through the PF can get stuck in the down (closed position or UP by being too close the wood they protrude through. The pivot point on those switch wires also crack with age/use. Because it is occasional, I would bet it is not a broken switch but a sticking switch.

Several things happen in sequence, and it is quite complicated. There are the 4 switches in the ball trough. The first one with the wide blade and hook on the end, detects that a ball has drained. The next 3 detect how many balls are in the trough at any moment in the game. The switch in the shooter lane tells the brain that a ball was launched. If that switch is stuck down, it will try to launch another ball.

Hah. I randomly read this thread the other night, and now my Doctor Who is doing the same thing! The thread must be cursed.

I haven't spent any time trying to figure it out yet (other than everything looks fine in switch tests, etc).

Other possible clues I'll add to the discussion that's already here: In my machine, I've noticed that the launch button might be a little 'sticky' - staying closed after launching the ball, intermittently. Additionally, the shooter solenoid seems to be behaving weirdly and leaves the plunger in the extended position. It seems to do it every time in the solenoid test, while it usually behaves properly in-game.

Your shooter rod issue is either a spring, liner or both issue.

Launch button also might be a spring issue or someone spilled a drink in there. Launch buttons are cheap and easy to replace.

The random shoot 2 balls issue happens even on new games. Some say it is because of magnetized balls and some say it is a bad trough switch. I have new balls, and new switches. It still happens occasionally.

The spring was my first thought, but it looks fine. Haven't inspected the coil sleeve yet but will take a look. With power off, the plunger moves freely and the spring pushes it back to the normal resting position effortlessly.

Yep, annoying when those microswitches fail. Easy to swap out though and see if it fixes anything.

Interesting. I hadn't heard of issues like that. And don't recall ever having a problem in my first several years of ownership. (I bought the DW probably ~12 years ago, but it's been essentially powered off for the last ~8). I've certainly seen weird, schizo behavior in games like STTNG when the ball trough optos get flaky - so I'll be scrutinizing the ball trough switches for sure. But so far they seem fine...

I had the same probkem with my Terminator 2. Same trough system. The problem was not any switch but the mechanism that kick the ball in the shooter lane. Sometimes it was sticking up too high and was blocking the next ball to activate the first switch. Plus I had some wear in the ball trough so the balls were stopping in some divots. In the end the problem was mechanical and not electrical.

Interesting - thanks for the reply! Mine hasn't misbehaved in a week now; when it starts happening again I'll take a closer look at the eject mechanism. I didn't have any reason to suspect any issues there.