Wednesday, December 16, 2015

Apollo 13: My Moon, My Magnet

So the moon has issues.  We know this.
And if you google "apollo 13 moon magnet" you'll discover that many people also have problems.
Sorry moon, you are the weakest link.


moon with the blue moon ramp removed. 

Based on what I've read, common things going down when the moon isn't behaving:
fuse blown (located at the far back, underneath the playfield)
transistor wonky on the moon board (under the playfield, on the right hand side, partway near the front)
moon coil issue
moon magnet issue

So quick recap:
The fuse list at the front of the manual says 4A.  The schematics say 3A, and apparently there was a service bulletin (can't find it, just mentions on forums) that it should indeed be 3A SB.
What did I find under the playfield?  6A.
The transistor on my board was probably bad, the fuse didn't blow, and the magnet stayed on for long periods of time, giving a slight melt to some of the nearby plastics, melting itself a bit, and making the coil toasty.

Old magnet vs new magnet:


There are small E-Clips on either side of the moon assembly, where it rotates.  Get those out of there with whatever magic you can muster.
Then use a small screw driver to remove the screws going in to the moon on the right side.
You should then be able to force the moon out of the bracket holding it.  Be careful not to stress the two wires going up inside the moon and connecting to the coil.
Then the moon opens like a Kinder Egg, revealing the glorious toys: a coil and a magnet. 
The magnet has a threaded hole/bum and is sitting on a screw on an internal bracket.  I had to use a wrench to loosen it, but then it screws off and it and the coil are ready to be stared at in horror.

your prize inside is a fried coil and a potpourri of burnt plastic smell!

Do note that for something like that coil the price at Pinball Life is $23.50 and at Marco it is $39.95.


to get started, you can grab this wonderful service bulletin (#80) which includes a "theory of operation" of some of the moon board logic, as well as a great quality schematic.
(I also found another service bulletin that reminds us to change F21 in the head from a 5A to a 3A, which I did)

Magnet was changed, as the old one was smashed.
Coil was changed, as the old one was crispy and shorted.  It read just 0.3 ohms, while the new coil read an appropriate 4.7 ohms.
There was an internal break in one of the wires to the moon, and that was fixed, providing proper continuity.
And after all that?  We go in to solenoid test, run MOON GRAB, and still barely any effect.  Something is happening, but certainly not enough to stop the ball as it is whipping around the ramp.

I initially had some confusion about the tiny board, but I found this excellent video showing a test on the board:

This illustrates how the board itself  just creates an ON/OFF condition for the moon magnet.

It is the preceding logic (which is on a board with the 7-segment display, under the playfield, under the red subway,) which controls if the board get an ON signal (which is what GRAB is,) or a 10% duty cycle ON signal (which is what the HOLD is)

We figured something was wrong on the aux board, and rebuild it with all new components.   This did not solve the issue.
But one thing to note:  There are two components on the moon board that are not in the schematic.  Based on what I can see from others on pinside, this is a common fix that must have been done at the factory line.  There is a resistor from pin 1 to 5 and a diode from pin 5 to ground.

So that board was rebuild.  The board that precedes it was looked over by a tech and some solder was reheated, but nothing obvious.  Still no solution.

We ran a test to verify that the magnet + coil are indeed OK:  We bypassed the moon board and put 50V to the coil and the magnet + moon kicked like a mule.  It was unmistakable.

Then with the board in we tested the voltage drop across the magnet:  It was going 76V to 66V, so only 10V across the magnet, which is why the grab was so gosh darn weak.
(while the schematic shows +50V, I was told seeing 76V isn't out of the ordinary for the high power.  An initial guess was that the transistor was out of spec at 76V, but I guess not)

At this point I am wondering if the control signal that comes in from the computer?
Or something else that is causing the voltage drop to be so inadequately small?

click to embiggen!


  1. Hello Friend, solved the problem with apollo 13 ... I have a similar situation, mosfet short when grab is triggered, already checked magnet, board drive and board pulse seems to me everything working correctly.

    1. turns out in our initial attempt we used the wrong class of transistor on the aux board, and that's why the output wasn't proper.