Make sure you haven't grounded one of the solder connections to the metal bracket holding the coil. Had the same issue about a week ago.

If you wired it backwards (power to the anode side of the diode) then you more than likely blew the drive transistor Q13 and its now permanently shorted causing the coil to active all the time. You can verify this by disconnecting either side of the coil and it should not blow the fuse.

You can also verify this is the case by using an ohm meter between the return side of the coil to ground, if it reads a really low resistance that's another good indication that its the drive transistor; if you then remove J9 from power drive board and the resistance to ground goes away, then you know the short is on the driver board.

^^^^ Wow, great description right there.

For the "coil to ground" part, are you telling him to put one of the DMM leads on the return part of the coil, and the other one on the ground braid? If so, how do you know which side of the coil is the "return" side?

Asking for my own edification. Thanks!

You blew the drive transistor. Replace it. The diode is fine.

Sadly, I did the same thing earlier this week.

Diode could certainly be bad too, at least test it.

If testing on the coil, you should at least see a few Ohms from the coil windings across the diode. If the meter shows a direct short, the diode is bad.

--
Rob Anthony
Pinball Classics
http://LockWhenLit.com
Quality Board Work - In Home Service
borygard at gmail dot com

After you validate your wiring connections and confirm there is no short circuirts (including solder "whiskers") and proper continuity, keep this small important tip in mind.

If you burned out the PCB drive transistor, you MUST test the associated coil diode, along with all upstream electrical components when repairing. They can all be damaged simultaneously all the way to the IC logic chip. If a blown transistor is the cause, I always replace diodes, as even testing cannot be 100% reliable.

If you do not, you may be chasing circles forever.

Yes the ground braid, the "return side" goes to the drive transistor as opposed to the "power side" which is connected to 50VDC; its the Blu-Grn wire, and should be on the anode side of the diode

There is a small chance that the diode may have blown, but this would be a secondary failure from the blown drive transistor.

Simpsons (resized).jpg

+1 Check the diode on the coil for a short.

I believe this means the transistor is blown. You can test it with your dmm set to continuity, one probe to the transistor tab, other probe to game ground. If you get no reistance/ buzz,, the transistor is shorted.

Can you post a picture of your coil wiring? Maybe you wired it wrong again. I'm not in front of my game, but does the coil require a diode? If so, is there one on it?

Disconnect J9, if it still does it with this cable off, then you have a short in the return wire (BLU-GRN), fix the short.

If the problem doesn't happen with J9 disconnected then the problem is on your new board, Q13 would be the most likely issue.